From 5febf432df1cfa5b25d99b54c32103fafbdd0eb9 Mon Sep 17 00:00:00 2001 From: Miguel Ojeda Date: Sun, 16 Aug 2026 15:32:33 +0200 Subject: kbuild: rust: keep Rust objects out of Clang LTO with inline helpers Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit `objtool` errors such as: vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a: can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd The reason is that in such builds, the Clang invocation that compiles the combined Rust plus helpers bitcode emits LLVM bitcode (again) -- the final code generation happens in the linker's LTO step, which the `-mllvm` trap options passed to Clang do not reach. This, in turn, means that unreachable traps are missing, and the impossible paths do not merely fallthrough to the next symbol, but past the end of their own section, since LTO builds place each function in its own section. Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always emits machine code directly, with the traps in place. Assisted-by: LLM Cc: Gary Guo Cc: Boqun Feng Cc: Alice Ryhl Cc: Matthew Maurer Cc: Josh Poimboeuf Cc: Peter Zijlstra Cc: stable@vger.kernel.org Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust") Acked-by: Gary Guo Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org Signed-off-by: Miguel Ojeda --- Documentation/admin-guide/LSM/LoadPin.rst | 31 + Documentation/admin-guide/LSM/SELinux.rst | 44 ++ Documentation/admin-guide/LSM/SafeSetID.rst | 118 ++++ Documentation/admin-guide/LSM/Smack.rst | 871 ++++++++++++++++++++++++++++ Documentation/admin-guide/LSM/Yama.rst | 75 +++ Documentation/admin-guide/LSM/apparmor.rst | 54 ++ Documentation/admin-guide/LSM/index.rst | 51 ++ Documentation/admin-guide/LSM/ipe.rst | 835 ++++++++++++++++++++++++++ Documentation/admin-guide/LSM/landlock.rst | 194 +++++++ Documentation/admin-guide/LSM/tomoyo.rst | 52 ++ 10 files changed, 2325 insertions(+) create mode 100644 Documentation/admin-guide/LSM/LoadPin.rst create mode 100644 Documentation/admin-guide/LSM/SELinux.rst create mode 100644 Documentation/admin-guide/LSM/SafeSetID.rst create mode 100644 Documentation/admin-guide/LSM/Smack.rst create mode 100644 Documentation/admin-guide/LSM/Yama.rst create mode 100644 Documentation/admin-guide/LSM/apparmor.rst create mode 100644 Documentation/admin-guide/LSM/index.rst create mode 100644 Documentation/admin-guide/LSM/ipe.rst create mode 100644 Documentation/admin-guide/LSM/landlock.rst create mode 100644 Documentation/admin-guide/LSM/tomoyo.rst (limited to 'Documentation/admin-guide/LSM') diff --git a/Documentation/admin-guide/LSM/LoadPin.rst b/Documentation/admin-guide/LSM/LoadPin.rst new file mode 100644 index 000000000..dd3ca68b5 --- /dev/null +++ b/Documentation/admin-guide/LSM/LoadPin.rst @@ -0,0 +1,31 @@ +======= +LoadPin +======= + +LoadPin is a Linux Security Module that ensures all kernel-loaded files +(modules, firmware, etc) all originate from the same filesystem, with +the expectation that such a filesystem is backed by a read-only device +such as dm-verity or CDROM. This allows systems that have a verified +and/or unchangeable filesystem to enforce module and firmware loading +restrictions without needing to sign the files individually. + +The LSM is selectable at build-time with ``CONFIG_SECURITY_LOADPIN``, and +can be controlled at boot-time with the kernel command line option +"``loadpin.enforce``". By default, it is enabled, but can be disabled at +boot ("``loadpin.enforce=0``"). + +LoadPin starts pinning when it sees the first file loaded. If the +block device backing the filesystem is not read-only, a sysctl is +created to toggle pinning: ``/proc/sys/kernel/loadpin/enabled``. (Having +a mutable filesystem means pinning is mutable too, but having the +sysctl allows for easy testing on systems with a mutable filesystem.) + +It's also possible to exclude specific file types from LoadPin using kernel +command line option "``loadpin.exclude``". By default, all files are +included, but they can be excluded using kernel command line option such +as "``loadpin.exclude=kernel-module,kexec-image``". This allows to use +different mechanisms such as ``CONFIG_MODULE_SIG`` and +``CONFIG_KEXEC_VERIFY_SIG`` to verify kernel module and kernel image while +still use LoadPin to protect the integrity of other files kernel loads. The +full list of valid file types can be found in ``kernel_read_file_str`` +defined in ``include/linux/kernel_read_file.h``. diff --git a/Documentation/admin-guide/LSM/SELinux.rst b/Documentation/admin-guide/LSM/SELinux.rst new file mode 100644 index 000000000..cdd65164c --- /dev/null +++ b/Documentation/admin-guide/LSM/SELinux.rst @@ -0,0 +1,44 @@ +======= +SELinux +======= + +Information about the SELinux kernel subsystem can be found at the +following links: + + https://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/selinux.git/tree/README.md + + https://github.com/selinuxproject/selinux-kernel/wiki + +Information about the SELinux userspace can be found at: + + https://github.com/SELinuxProject/selinux/wiki + +If you want to use SELinux, chances are you will want +to use the distro-provided policies, or install the +latest reference policy release from + + https://github.com/SELinuxProject/refpolicy + +However, if you want to install a dummy policy for +testing, you can do using ``mdp`` provided under +scripts/selinux. Note that this requires the selinux +userspace to be installed - in particular you will +need checkpolicy to compile a kernel, and setfiles and +fixfiles to label the filesystem. + + 1. Compile the kernel with selinux enabled. + 2. Type ``make`` to compile ``mdp``. + 3. Make sure that you are not running with + SELinux enabled and a real policy. If + you are, reboot with selinux disabled + before continuing. + 4. Run install_policy.sh:: + + cd scripts/selinux + sh install_policy.sh + +Step 4 will create a new dummy policy valid for your +kernel, with a single selinux user, role, and type. +It will compile the policy, will set your ``SELINUXTYPE`` to +``dummy`` in ``/etc/selinux/config``, install the compiled policy +as ``dummy``, and relabel your filesystem. diff --git a/Documentation/admin-guide/LSM/SafeSetID.rst b/Documentation/admin-guide/LSM/SafeSetID.rst new file mode 100644 index 000000000..6d439c987 --- /dev/null +++ b/Documentation/admin-guide/LSM/SafeSetID.rst @@ -0,0 +1,118 @@ +========= +SafeSetID +========= +SafeSetID is an LSM module that gates the setid family of syscalls to restrict +UID/GID transitions from a given UID/GID to only those approved by a +system-wide allowlist. These restrictions also prohibit the given UIDs/GIDs +from obtaining auxiliary privileges associated with CAP_SET{U/G}ID, such as +allowing a user to set up user namespace UID/GID mappings. + + +Background +========== +In absence of file capabilities, processes spawned on a Linux system that need +to switch to a different user must be spawned with CAP_SETUID privileges. +CAP_SETUID is granted to programs running as root or those running as a non-root +user that have been explicitly given the CAP_SETUID runtime capability. It is +often preferable to use Linux runtime capabilities rather than file +capabilities, since using file capabilities to run a program with elevated +privileges opens up possible security holes since any user with access to the +file can exec() that program to gain the elevated privileges. + +While it is possible to implement a tree of processes by giving full +CAP_SET{U/G}ID capabilities, this is often at odds with the goals of running a +tree of processes under non-root user(s) in the first place. Specifically, +since CAP_SETUID allows changing to any user on the system, including the root +user, it is an overpowered capability for what is needed in this scenario, +especially since programs often only call setuid() to drop privileges to a +lesser-privileged user -- not elevate privileges. Unfortunately, there is no +generally feasible way in Linux to restrict the potential UIDs that a user can +switch to through setuid() beyond allowing a switch to any user on the system. +This SafeSetID LSM seeks to provide a solution for restricting setid +capabilities in such a way. + +The main use case for this LSM is to allow a non-root program to transition to +other untrusted uids without full blown CAP_SETUID capabilities. The non-root +program would still need CAP_SETUID to do any kind of transition, but the +additional restrictions imposed by this LSM would mean it is a "safer" version +of CAP_SETUID since the non-root program cannot take advantage of CAP_SETUID to +do any unapproved actions (e.g. setuid to uid 0 or create/enter new user +namespace). The higher level goal is to allow for uid-based sandboxing of system +services without having to give out CAP_SETUID all over the place just so that +non-root programs can drop to even-lesser-privileged uids. This is especially +relevant when one non-root daemon on the system should be allowed to spawn other +processes as different uids, but it's undesirable to give the daemon a +basically-root-equivalent CAP_SETUID. + + +Other Approaches Considered +=========================== + +Solve this problem in userspace +------------------------------- +For candidate applications that would like to have restricted setid capabilities +as implemented in this LSM, an alternative option would be to simply take away +setid capabilities from the application completely and refactor the process +spawning semantics in the application (e.g. by using a privileged helper program +to do process spawning and UID/GID transitions). Unfortunately, there are a +number of semantics around process spawning that would be affected by this, such +as fork() calls where the program doesn't immediately call exec() after the +fork(), parent processes specifying custom environment variables or command line +args for spawned child processes, or inheritance of file handles across a +fork()/exec(). Because of this, as solution that uses a privileged helper in +userspace would likely be less appealing to incorporate into existing projects +that rely on certain process-spawning semantics in Linux. + +Use user namespaces +------------------- +Another possible approach would be to run a given process tree in its own user +namespace and give programs in the tree setid capabilities. In this way, +programs in the tree could change to any desired UID/GID in the context of their +own user namespace, and only approved UIDs/GIDs could be mapped back to the +initial system user namespace, affectively preventing privilege escalation. +Unfortunately, it is not generally feasible to use user namespaces in isolation, +without pairing them with other namespace types, which is not always an option. +Linux checks for capabilities based off of the user namespace that "owns" some +entity. For example, Linux has the notion that network namespaces are owned by +the user namespace in which they were created. A consequence of this is that +capability checks for access to a given network namespace are done by checking +whether a task has the given capability in the context of the user namespace +that owns the network namespace -- not necessarily the user namespace under +which the given task runs. Therefore spawning a process in a new user namespace +effectively prevents it from accessing the network namespace owned by the +initial namespace. This is a deal-breaker for any application that expects to +retain the CAP_NET_ADMIN capability for the purpose of adjusting network +configurations. Using user namespaces in isolation causes problems regarding +other system interactions, including use of pid namespaces and device creation. + +Use an existing LSM +------------------- +None of the other in-tree LSMs have the capability to gate setid transitions, or +even employ the security_task_fix_setuid hook at all. SELinux says of that hook: +"Since setuid only affects the current process, and since the SELinux controls +are not based on the Linux identity attributes, SELinux does not need to control +this operation." + + +Directions for use +================== +This LSM hooks the setid syscalls to make sure transitions are allowed if an +applicable restriction policy is in place. Policies are configured through +securityfs by writing to the safesetid/uid_allowlist_policy and +safesetid/gid_allowlist_policy files at the location where securityfs is +mounted. The format for adding a policy is ':' or ':', +using literal numbers, and ending with a newline character such as '123:456\n'. +Writing an empty string "" will flush the policy. Again, configuring a policy +for a UID/GID will prevent that UID/GID from obtaining auxiliary setid +privileges, such as allowing a user to set up user namespace UID/GID mappings. + +Note on GID policies and setgroups() +==================================== +In v5.9 we are adding support for limiting CAP_SETGID privileges as was done +previously for CAP_SETUID. However, for compatibility with common sandboxing +related code conventions in userspace, we currently allow arbitrary +setgroups() calls for processes with CAP_SETGID restrictions. Until we add +support in a future release for restricting setgroups() calls, these GID +policies add no meaningful security. setgroups() restrictions will be enforced +once we have the policy checking code in place, which will rely on GID policy +configuration code added in v5.9. diff --git a/Documentation/admin-guide/LSM/Smack.rst b/Documentation/admin-guide/LSM/Smack.rst new file mode 100644 index 000000000..c5ed775f2 --- /dev/null +++ b/Documentation/admin-guide/LSM/Smack.rst @@ -0,0 +1,871 @@ +===== +Smack +===== + + + "Good for you, you've decided to clean the elevator!" + - The Elevator, from Dark Star + +Smack is the Simplified Mandatory Access Control Kernel. +Smack is a kernel based implementation of mandatory access +control that includes simplicity in its primary design goals. + +Smack is not the only Mandatory Access Control scheme +available for Linux. Those new to Mandatory Access Control +are encouraged to compare Smack with the other mechanisms +available to determine which is best suited to the problem +at hand. + +Smack consists of three major components: + + - The kernel + - Basic utilities, which are helpful but not required + - Configuration data + +The kernel component of Smack is implemented as a Linux +Security Modules (LSM) module. It requires netlabel and +works best with file systems that support extended attributes, +although xattr support is not strictly required. +It is safe to run a Smack kernel under a "vanilla" distribution. + +Smack kernels use the CIPSO IP option. Some network +configurations are intolerant of IP options and can impede +access to systems that use them as Smack does. + +Smack is used in the Tizen operating system. Please +go to http://wiki.tizen.org for information about how +Smack is used in Tizen. + +The current git repository for Smack user space is: + + git://github.com/smack-team/smack.git + +This should make and install on most modern distributions. +There are five commands included in smackutil: + +chsmack: + display or set Smack extended attribute values + +smackctl: + load the Smack access rules + +smackaccess: + report if a process with one label has access + to an object with another + +These two commands are obsolete with the introduction of +the smackfs/load2 and smackfs/cipso2 interfaces. + +smackload: + properly formats data for writing to smackfs/load + +smackcipso: + properly formats data for writing to smackfs/cipso + +In keeping with the intent of Smack, configuration data is +minimal and not strictly required. The most important +configuration step is mounting the smackfs pseudo filesystem. +If smackutil is installed the startup script will take care +of this, but it can be manually as well. + +Add this line to ``/etc/fstab``:: + + smackfs /sys/fs/smackfs smackfs defaults 0 0 + +The ``/sys/fs/smackfs`` directory is created by the kernel. + +Smack uses extended attributes (xattrs) to store labels on filesystem +objects. The attributes are stored in the extended attribute security +name space. A process must have ``CAP_MAC_ADMIN`` to change any of these +attributes. + +The extended attributes that Smack uses are: + +SMACK64 + Used to make access control decisions. In almost all cases + the label given to a new filesystem object will be the label + of the process that created it. + +SMACK64EXEC + The Smack label of a process that execs a program file with + this attribute set will run with this attribute's value. + +SMACK64MMAP + Don't allow the file to be mmapped by a process whose Smack + label does not allow all of the access permitted to a process + with the label contained in this attribute. This is a very + specific use case for shared libraries. + +SMACK64TRANSMUTE + Can only have the value "TRUE". If this attribute is present + on a directory when an object is created in the directory and + the Smack rule (more below) that permitted the write access + to the directory includes the transmute ("t") mode the object + gets the label of the directory instead of the label of the + creating process. If the object being created is a directory + the SMACK64TRANSMUTE attribute is set as well. + +SMACK64IPIN + This attribute is only available on file descriptors for sockets. + Use the Smack label in this attribute for access control + decisions on packets being delivered to this socket. + +SMACK64IPOUT + This attribute is only available on file descriptors for sockets. + Use the Smack label in this attribute for access control + decisions on packets coming from this socket. + +There are multiple ways to set a Smack label on a file:: + + # attr -S -s SMACK64 -V "value" path + # chsmack -a value path + +A process can see the Smack label it is running with by +reading ``/proc/self/attr/current``. A process with ``CAP_MAC_ADMIN`` +can set the process Smack by writing there. + +Most Smack configuration is accomplished by writing to files +in the smackfs filesystem. This pseudo-filesystem is mounted +on ``/sys/fs/smackfs``. + +access + Provided for backward compatibility. The access2 interface + is preferred and should be used instead. + This interface reports whether a subject with the specified + Smack label has a particular access to an object with a + specified Smack label. Write a fixed format access rule to + this file. The next read will indicate whether the access + would be permitted. The text will be either "1" indicating + access, or "0" indicating denial. + +access2 + This interface reports whether a subject with the specified + Smack label has a particular access to an object with a + specified Smack label. Write a long format access rule to + this file. The next read will indicate whether the access + would be permitted. The text will be either "1" indicating + access, or "0" indicating denial. + +ambient + This contains the Smack label applied to unlabeled network + packets. + +change-rule + This interface allows modification of existing access control rules. + The format accepted on write is:: + + "%s %s %s %s" + + where the first string is the subject label, the second the + object label, the third the access to allow and the fourth the + access to deny. The access strings may contain only the characters + "rwxat-". If a rule for a given subject and object exists it will be + modified by enabling the permissions in the third string and disabling + those in the fourth string. If there is no such rule it will be + created using the access specified in the third and the fourth strings. + +cipso + Provided for backward compatibility. The cipso2 interface + is preferred and should be used instead. + This interface allows a specific CIPSO header to be assigned + to a Smack label. The format accepted on write is:: + + "%24s%4d%4d"["%4d"]... + + The first string is a fixed Smack label. The first number is + the level to use. The second number is the number of categories. + The following numbers are the categories:: + + "level-3-cats-5-19 3 2 5 19" + +cipso2 + This interface allows a specific CIPSO header to be assigned + to a Smack label. The format accepted on write is:: + + "%s%4d%4d"["%4d"]... + + The first string is a long Smack label. The first number is + the level to use. The second number is the number of categories. + The following numbers are the categories:: + + "level-3-cats-5-19 3 2 5 19" + +direct + This contains the CIPSO level used for Smack direct label + representation in network packets. + +doi + This contains the CIPSO domain of interpretation used in + network packets. + +ipv6host + This interface allows specific IPv6 internet addresses to be + treated as single label hosts. Packets are sent to single + label hosts only from processes that have Smack write access + to the host label. All packets received from single label hosts + are given the specified label. The format accepted on write is:: + + "%h:%h:%h:%h:%h:%h:%h:%h label" or + "%h:%h:%h:%h:%h:%h:%h:%h/%d label". + + The "::" address shortcut is not supported. + If label is "-DELETE" a matched entry will be deleted. + +load + Provided for backward compatibility. The load2 interface + is preferred and should be used instead. + This interface allows access control rules in addition to + the system defined rules to be specified. The format accepted + on write is:: + + "%24s%24s%5s" + + where the first string is the subject label, the second the + object label, and the third the requested access. The access + string may contain only the characters "rwxat-", and specifies + which sort of access is allowed. The "-" is a placeholder for + permissions that are not allowed. The string "r-x--" would + specify read and execute access. Labels are limited to 23 + characters in length. + +load2 + This interface allows access control rules in addition to + the system defined rules to be specified. The format accepted + on write is:: + + "%s %s %s" + + where the first string is the subject label, the second the + object label, and the third the requested access. The access + string may contain only the characters "rwxat-", and specifies + which sort of access is allowed. The "-" is a placeholder for + permissions that are not allowed. The string "r-x--" would + specify read and execute access. + +load-self + Provided for backward compatibility. The load-self2 interface + is preferred and should be used instead. + This interface allows process specific access rules to be + defined. These rules are only consulted if access would + otherwise be permitted, and are intended to provide additional + restrictions on the process. The format is the same as for + the load interface. + +load-self2 + This interface allows process specific access rules to be + defined. These rules are only consulted if access would + otherwise be permitted, and are intended to provide additional + restrictions on the process. The format is the same as for + the load2 interface. + +logging + This contains the Smack logging state. + +mapped + This contains the CIPSO level used for Smack mapped label + representation in network packets. + +netlabel + This interface allows specific internet addresses to be + treated as single label hosts. Packets are sent to single + label hosts without CIPSO headers, but only from processes + that have Smack write access to the host label. All packets + received from single label hosts are given the specified + label. The format accepted on write is:: + + "%d.%d.%d.%d label" or "%d.%d.%d.%d/%d label". + + If the label specified is "-CIPSO" the address is treated + as a host that supports CIPSO headers. + +onlycap + This contains labels processes must have for CAP_MAC_ADMIN + and ``CAP_MAC_OVERRIDE`` to be effective. If this file is empty + these capabilities are effective at for processes with any + label. The values are set by writing the desired labels, separated + by spaces, to the file or cleared by writing "-" to the file. + +ptrace + This is used to define the current ptrace policy + + 0 - default: + this is the policy that relies on Smack access rules. + For the ``PTRACE_READ`` a subject needs to have a read access on + object. For the ``PTRACE_ATTACH`` a read-write access is required. + + 1 - exact: + this is the policy that limits ``PTRACE_ATTACH``. Attach is + only allowed when subject's and object's labels are equal. + ``PTRACE_READ`` is not affected. Can be overridden with ``CAP_SYS_PTRACE``. + + 2 - draconian: + this policy behaves like the 'exact' above with an + exception that it can't be overridden with ``CAP_SYS_PTRACE``. + +revoke-subject + Writing a Smack label here sets the access to '-' for all access + rules with that subject label. + +unconfined + If the kernel is configured with ``CONFIG_SECURITY_SMACK_BRINGUP`` + a process with ``CAP_MAC_ADMIN`` can write a label into this interface. + Thereafter, accesses that involve that label will be logged and + the access permitted if it wouldn't be otherwise. Note that this + is dangerous and can ruin the proper labeling of your system. + It should never be used in production. + +relabel-self + This interface contains a list of labels to which the process can + transition to, by writing to ``/proc/self/attr/current``. + Normally a process can change its own label to any legal value, but only + if it has ``CAP_MAC_ADMIN``. This interface allows a process without + ``CAP_MAC_ADMIN`` to relabel itself to one of labels from predefined list. + A process without ``CAP_MAC_ADMIN`` can change its label only once. When it + does, this list will be cleared. + The values are set by writing the desired labels, separated + by spaces, to the file or cleared by writing "-" to the file. + +If you are using the smackload utility +you can add access rules in ``/etc/smack/accesses``. They take the form:: + + subjectlabel objectlabel access + +access is a combination of the letters rwxatb which specify the +kind of access permitted a subject with subjectlabel on an +object with objectlabel. If there is no rule no access is allowed. + +Look for additional programs on http://schaufler-ca.com + +The Simplified Mandatory Access Control Kernel (Whitepaper) +=========================================================== + +Casey Schaufler +casey@schaufler-ca.com + +Mandatory Access Control +------------------------ + +Computer systems employ a variety of schemes to constrain how information is +shared among the people and services using the machine. Some of these schemes +allow the program or user to decide what other programs or users are allowed +access to pieces of data. These schemes are called discretionary access +control mechanisms because the access control is specified at the discretion +of the user. Other schemes do not leave the decision regarding what a user or +program can access up to users or programs. These schemes are called mandatory +access control mechanisms because you don't have a choice regarding the users +or programs that have access to pieces of data. + +Bell & LaPadula +--------------- + +From the middle of the 1980's until the turn of the century Mandatory Access +Control (MAC) was very closely associated with the Bell & LaPadula security +model, a mathematical description of the United States Department of Defense +policy for marking paper documents. MAC in this form enjoyed a following +within the Capital Beltway and Scandinavian supercomputer centers but was +often sited as failing to address general needs. + +Domain Type Enforcement +----------------------- + +Around the turn of the century Domain Type Enforcement (DTE) became popular. +This scheme organizes users, programs, and data into domains that are +protected from each other. This scheme has been widely deployed as a component +of popular Linux distributions. The administrative overhead required to +maintain this scheme and the detailed understanding of the whole system +necessary to provide a secure domain mapping leads to the scheme being +disabled or used in limited ways in the majority of cases. + +Smack +----- + +Smack is a Mandatory Access Control mechanism designed to provide useful MAC +while avoiding the pitfalls of its predecessors. The limitations of Bell & +LaPadula are addressed by providing a scheme whereby access can be controlled +according to the requirements of the system and its purpose rather than those +imposed by an arcane government policy. The complexity of Domain Type +Enforcement and avoided by defining access controls in terms of the access +modes already in use. + +Smack Terminology +----------------- + +The jargon used to talk about Smack will be familiar to those who have dealt +with other MAC systems and shouldn't be too difficult for the uninitiated to +pick up. There are four terms that are used in a specific way and that are +especially important: + + Subject: + A subject is an active entity on the computer system. + On Smack a subject is a task, which is in turn the basic unit + of execution. + + Object: + An object is a passive entity on the computer system. + On Smack files of all types, IPC, and tasks can be objects. + + Access: + Any attempt by a subject to put information into or get + information from an object is an access. + + Label: + Data that identifies the Mandatory Access Control + characteristics of a subject or an object. + +These definitions are consistent with the traditional use in the security +community. There are also some terms from Linux that are likely to crop up: + + Capability: + A task that possesses a capability has permission to + violate an aspect of the system security policy, as identified by + the specific capability. A task that possesses one or more + capabilities is a privileged task, whereas a task with no + capabilities is an unprivileged task. + + Privilege: + A task that is allowed to violate the system security + policy is said to have privilege. As of this writing a task can + have privilege either by possessing capabilities or by having an + effective user of root. + +Smack Basics +------------ + +Smack is an extension to a Linux system. It enforces additional restrictions +on what subjects can access which objects, based on the labels attached to +each of the subject and the object. + +Labels +~~~~~~ + +Smack labels are ASCII character strings. They can be up to 255 characters +long, but keeping them to twenty-three characters is recommended. +Single character labels using special characters, that being anything +other than a letter or digit, are reserved for use by the Smack development +team. Smack labels are unstructured, case sensitive, and the only operation +ever performed on them is comparison for equality. Smack labels cannot +contain unprintable characters, the "/" (slash), the "\" (backslash), the "'" +(quote) and '"' (double-quote) characters. +Smack labels cannot begin with a '-'. This is reserved for special options. + +There are some predefined labels:: + + _ Pronounced "floor", a single underscore character. + ^ Pronounced "hat", a single circumflex character. + * Pronounced "star", a single asterisk character. + ? Pronounced "huh", a single question mark character. + @ Pronounced "web", a single at sign character. + +Every task on a Smack system is assigned a label. The Smack label +of a process will usually be assigned by the system initialization +mechanism. + +Access Rules +~~~~~~~~~~~~ + +Smack uses the traditional access modes of Linux. These modes are read, +execute, write, and occasionally append. There are a few cases where the +access mode may not be obvious. These include: + + Signals: + A signal is a write operation from the subject task to + the object task. + + Internet Domain IPC: + Transmission of a packet is considered a + write operation from the source task to the destination task. + +Smack restricts access based on the label attached to a subject and the label +attached to the object it is trying to access. The rules enforced are, in +order: + + 1. Any access requested by a task labeled "*" is denied. + 2. A read or execute access requested by a task labeled "^" + is permitted. + 3. A read or execute access requested on an object labeled "_" + is permitted. + 4. Any access requested on an object labeled "*" is permitted. + 5. Any access requested by a task on an object with the same + label is permitted. + 6. Any access requested that is explicitly defined in the loaded + rule set is permitted. + 7. Any other access is denied. + +Smack Access Rules +~~~~~~~~~~~~~~~~~~ + +With the isolation provided by Smack access separation is simple. There are +many interesting cases where limited access by subjects to objects with +different labels is desired. One example is the familiar spy model of +sensitivity, where a scientist working on a highly classified project would be +able to read documents of lower classifications and anything she writes will +be "born" highly classified. To accommodate such schemes Smack includes a +mechanism for specifying rules allowing access between labels. + +Access Rule Format +~~~~~~~~~~~~~~~~~~ + +The format of an access rule is:: + + subject-label object-label access + +Where subject-label is the Smack label of the task, object-label is the Smack +label of the thing being accessed, and access is a string specifying the sort +of access allowed. The access specification is searched for letters that +describe access modes: + + a: indicates that append access should be granted. + r: indicates that read access should be granted. + w: indicates that write access should be granted. + x: indicates that execute access should be granted. + t: indicates that the rule requests transmutation. + b: indicates that the rule should be reported for bring-up. + +Uppercase values for the specification letters are allowed as well. +Access mode specifications can be in any order. Examples of acceptable rules +are:: + + TopSecret Secret rx + Secret Unclass R + Manager Game x + User HR w + Snap Crackle rwxatb + New Old rRrRr + Closed Off - + +Examples of unacceptable rules are:: + + Top Secret Secret rx + Ace Ace r + Odd spells waxbeans + +Spaces are not allowed in labels. Since a subject always has access to files +with the same label specifying a rule for that case is pointless. Only +valid letters (rwxatbRWXATB) and the dash ('-') character are allowed in +access specifications. The dash is a placeholder, so "a-r" is the same +as "ar". A lone dash is used to specify that no access should be allowed. + +Applying Access Rules +~~~~~~~~~~~~~~~~~~~~~ + +The developers of Linux rarely define new sorts of things, usually importing +schemes and concepts from other systems. Most often, the other systems are +variants of Unix. Unix has many endearing properties, but consistency of +access control models is not one of them. Smack strives to treat accesses as +uniformly as is sensible while keeping with the spirit of the underlying +mechanism. + +File system objects including files, directories, named pipes, symbolic links, +and devices require access permissions that closely match those used by mode +bit access. To open a file for reading read access is required on the file. To +search a directory requires execute access. Creating a file with write access +requires both read and write access on the containing directory. Deleting a +file requires read and write access to the file and to the containing +directory. It is possible that a user may be able to see that a file exists +but not any of its attributes by the circumstance of having read access to the +containing directory but not to the differently labeled file. This is an +artifact of the file name being data in the directory, not a part of the file. + +If a directory is marked as transmuting (SMACK64TRANSMUTE=TRUE) and the +access rule that allows a process to create an object in that directory +includes 't' access the label assigned to the new object will be that +of the directory, not the creating process. This makes it much easier +for two processes with different labels to share data without granting +access to all of their files. + +IPC objects, message queues, semaphore sets, and memory segments exist in flat +namespaces and access requests are only required to match the object in +question. + +Process objects reflect tasks on the system and the Smack label used to access +them is the same Smack label that the task would use for its own access +attempts. Sending a signal via the kill() system call is a write operation +from the signaler to the recipient. Debugging a process requires both reading +and writing. Creating a new task is an internal operation that results in two +tasks with identical Smack labels and requires no access checks. + +Sockets are data structures attached to processes and sending a packet from +one process to another requires that the sender have write access to the +receiver. The receiver is not required to have read access to the sender. + +Setting Access Rules +~~~~~~~~~~~~~~~~~~~~ + +The configuration file /etc/smack/accesses contains the rules to be set at +system startup. The contents are written to the special file +/sys/fs/smackfs/load2. Rules can be added at any time and take effect +immediately. For any pair of subject and object labels there can be only +one rule, with the most recently specified overriding any earlier +specification. + +Task Attribute +~~~~~~~~~~~~~~ + +The Smack label of a process can be read from ``/proc//attr/current``. A +process can read its own Smack label from ``/proc/self/attr/current``. A +privileged process can change its own Smack label by writing to +``/proc/self/attr/current`` but not the label of another process. + +Format of writing is : only the label or the label followed by one of the +3 trailers: ``\n`` (by common agreement for ``/proc/...`` interfaces), +``\0`` (because some applications incorrectly include it), +``\n\0`` (because we think some applications may incorrectly include it). + +File Attribute +~~~~~~~~~~~~~~ + +The Smack label of a filesystem object is stored as an extended attribute +named SMACK64 on the file. This attribute is in the security namespace. It can +only be changed by a process with privilege. + +Privilege +~~~~~~~~~ + +A process with CAP_MAC_OVERRIDE or CAP_MAC_ADMIN is privileged. +CAP_MAC_OVERRIDE allows the process access to objects it would +be denied otherwise. CAP_MAC_ADMIN allows a process to change +Smack data, including rules and attributes. + +Smack Networking +~~~~~~~~~~~~~~~~ + +As mentioned before, Smack enforces access control on network protocol +transmissions. Every packet sent by a Smack process is tagged with its Smack +label. This is done by adding a CIPSO tag to the header of the IP packet. Each +packet received is expected to have a CIPSO tag that identifies the label and +if it lacks such a tag the network ambient label is assumed. Before the packet +is delivered a check is made to determine that a subject with the label on the +packet has write access to the receiving process and if that is not the case +the packet is dropped. + +CIPSO Configuration +~~~~~~~~~~~~~~~~~~~ + +It is normally unnecessary to specify the CIPSO configuration. The default +values used by the system handle all internal cases. Smack will compose CIPSO +label values to match the Smack labels being used without administrative +intervention. Unlabeled packets that come into the system will be given the +ambient label. + +Smack requires configuration in the case where packets from a system that is +not Smack that speaks CIPSO may be encountered. Usually this will be a Trusted +Solaris system, but there are other, less widely deployed systems out there. +CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level, +and a category set with each packet. The DOI is intended to identify a group +of systems that use compatible labeling schemes, and the DOI specified on the +Smack system must match that of the remote system or packets will be +discarded. The DOI is 3 by default. The value can be read from +/sys/fs/smackfs/doi and can be changed by writing to /sys/fs/smackfs/doi. + +The label and category set are mapped to a Smack label as defined in +/etc/smack/cipso. + +A Smack/CIPSO mapping has the form:: + + smack level [category [category]*] + +Smack does not expect the level or category sets to be related in any +particular way and does not assume or assign accesses based on them. Some +examples of mappings:: + + TopSecret 7 + TS:A,B 7 1 2 + SecBDE 5 2 4 6 + RAFTERS 7 12 26 + +The ":" and "," characters are permitted in a Smack label but have no special +meaning. + +The mapping of Smack labels to CIPSO values is defined by writing to +/sys/fs/smackfs/cipso2. + +In addition to explicit mappings Smack supports direct CIPSO mappings. One +CIPSO level is used to indicate that the category set passed in the packet is +in fact an encoding of the Smack label. The level used is 250 by default. The +value can be read from /sys/fs/smackfs/direct and changed by writing to +/sys/fs/smackfs/direct. + +Socket Attributes +~~~~~~~~~~~~~~~~~ + +There are two attributes that are associated with sockets. These attributes +can only be set by privileged tasks, but any task can read them for their own +sockets. + + SMACK64IPIN: + The Smack label of the task object. A privileged + program that will enforce policy may set this to the star label. + + SMACK64IPOUT: + The Smack label transmitted with outgoing packets. + A privileged program may set this to match the label of another + task with which it hopes to communicate. + +UNIX domain socket (UDS) with a BSD address functions both as a file in a +filesystem and as a socket. As a file, it carries the SMACK64 attribute. This +attribute is not involved in Smack security enforcement and is immutably +assigned the label "*". + +Smack Netlabel Exceptions +~~~~~~~~~~~~~~~~~~~~~~~~~ + +You will often find that your labeled application has to talk to the outside, +unlabeled world. To do this there's a special file /sys/fs/smackfs/netlabel +where you can add some exceptions in the form of:: + + @IP1 LABEL1 or + @IP2/MASK LABEL2 + +It means that your application will have unlabeled access to @IP1 if it has +write access on LABEL1, and access to the subnet @IP2/MASK if it has write +access on LABEL2. + +Entries in the /sys/fs/smackfs/netlabel file are matched by longest mask +first, like in classless IPv4 routing. + +A special label '@' and an option '-CIPSO' can be used there:: + + @ means Internet, any application with any label has access to it + -CIPSO means standard CIPSO networking + +If you don't know what CIPSO is and don't plan to use it, you can just do:: + + echo 127.0.0.1 -CIPSO > /sys/fs/smackfs/netlabel + echo 0.0.0.0/0 @ > /sys/fs/smackfs/netlabel + +If you use CIPSO on your 192.168.0.0/16 local network and need also unlabeled +Internet access, you can have:: + + echo 127.0.0.1 -CIPSO > /sys/fs/smackfs/netlabel + echo 192.168.0.0/16 -CIPSO > /sys/fs/smackfs/netlabel + echo 0.0.0.0/0 @ > /sys/fs/smackfs/netlabel + +Writing Applications for Smack +------------------------------ + +There are three sorts of applications that will run on a Smack system. How an +application interacts with Smack will determine what it will have to do to +work properly under Smack. + +Smack Ignorant Applications +--------------------------- + +By far the majority of applications have no reason whatever to care about the +unique properties of Smack. Since invoking a program has no impact on the +Smack label associated with the process the only concern likely to arise is +whether the process has execute access to the program. + +Smack Relevant Applications +--------------------------- + +Some programs can be improved by teaching them about Smack, but do not make +any security decisions themselves. The utility ls(1) is one example of such a +program. + +Smack Enforcing Applications +---------------------------- + +These are special programs that not only know about Smack, but participate in +the enforcement of system policy. In most cases these are the programs that +set up user sessions. There are also network services that provide information +to processes running with various labels. + +File System Interfaces +---------------------- + +Smack maintains labels on file system objects using extended attributes. The +Smack label of a file, directory, or other file system object can be obtained +using getxattr(2):: + + len = getxattr("/", "security.SMACK64", value, sizeof (value)); + +will put the Smack label of the root directory into value. A privileged +process can set the Smack label of a file system object with setxattr(2):: + + len = strlen("Rubble"); + rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0); + +will set the Smack label of /foo to "Rubble" if the program has appropriate +privilege. + +Socket Interfaces +----------------- + +The socket attributes can be read using fgetxattr(2). + +A privileged process can set the Smack label of outgoing packets with +fsetxattr(2):: + + len = strlen("Rubble"); + rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0); + +will set the Smack label "Rubble" on packets going out from the socket if the +program has appropriate privilege:: + + rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0); + +will set the Smack label "*" as the object label against which incoming +packets will be checked if the program has appropriate privilege. + +Administration +-------------- + +Smack supports some mount options: + + smackfsdef=label: + specifies the label to give files that lack + the Smack label extended attribute. + + smackfsroot=label: + specifies the label to assign the root of the + file system if it lacks the Smack extended attribute. + + smackfshat=label: + specifies a label that must have read access to + all labels set on the filesystem. Not yet enforced. + + smackfsfloor=label: + specifies a label to which all labels set on the + filesystem must have read access. Not yet enforced. + + smackfstransmute=label: + behaves exactly like smackfsroot except that it also + sets the transmute flag on the root of the mount + +These mount options apply to all file system types. + +Smack auditing +-------------- + +If you want Smack auditing of security events, you need to set CONFIG_AUDIT +in your kernel configuration. +By default, all denied events will be audited. You can change this behavior by +writing a single character to the /sys/fs/smackfs/logging file:: + + 0 : no logging + 1 : log denied (default) + 2 : log accepted + 3 : log denied & accepted + +Events are logged as 'key=value' pairs, for each event you at least will get +the subject, the object, the rights requested, the action, the kernel function +that triggered the event, plus other pairs depending on the type of event +audited. + +Bringup Mode +------------ + +Bringup mode provides logging features that can make application +configuration and system bringup easier. Configure the kernel with +CONFIG_SECURITY_SMACK_BRINGUP to enable these features. When bringup +mode is enabled accesses that succeed due to rules marked with the "b" +access mode will logged. When a new label is introduced for processes +rules can be added aggressively, marked with the "b". The logging allows +tracking of which rules actual get used for that label. + +Another feature of bringup mode is the "unconfined" option. Writing +a label to /sys/fs/smackfs/unconfined makes subjects with that label +able to access any object, and objects with that label accessible to +all subjects. Any access that is granted because a label is unconfined +is logged. This feature is dangerous, as files and directories may +be created in places they couldn't if the policy were being enforced. diff --git a/Documentation/admin-guide/LSM/Yama.rst b/Documentation/admin-guide/LSM/Yama.rst new file mode 100644 index 000000000..d9cd937eb --- /dev/null +++ b/Documentation/admin-guide/LSM/Yama.rst @@ -0,0 +1,75 @@ +==== +Yama +==== + +Yama is a Linux Security Module that collects system-wide DAC security +protections that are not handled by the core kernel itself. This is +selectable at build-time with ``CONFIG_SECURITY_YAMA``, and can be controlled +at run-time through sysctls in ``/proc/sys/kernel/yama``: + +ptrace_scope +============ + +As Linux grows in popularity, it will become a larger target for +malware. One particularly troubling weakness of the Linux process +interfaces is that a single user is able to examine the memory and +running state of any of their processes. For example, if one application +(e.g. Pidgin) was compromised, it would be possible for an attacker to +attach to other running processes (e.g. Firefox, SSH sessions, GPG agent, +etc) to extract additional credentials and continue to expand the scope +of their attack without resorting to user-assisted phishing. + +This is not a theoretical problem. `SSH session hijacking +`_ +and `arbitrary code injection +`_ attacks already +exist and remain possible if ptrace is allowed to operate as before. +Since ptrace is not commonly used by non-developers and non-admins, system +builders should be allowed the option to disable this debugging system. + +For a solution, some applications use ``prctl(PR_SET_DUMPABLE, ...)`` to +specifically disallow such ptrace attachment (e.g. ssh-agent), but many +do not. A more general solution is to only allow ptrace directly from a +parent to a child process (i.e. direct "gdb EXE" and "strace EXE" still +work), or with ``CAP_SYS_PTRACE`` (i.e. "gdb --pid=PID", and "strace -p PID" +still work as root). + +In mode 1, software that has defined application-specific relationships +between a debugging process and its inferior (crash handlers, etc), +``prctl(PR_SET_PTRACER, pid, ...)`` can be used. An inferior can declare which +other process (and its descendants) are allowed to call ``PTRACE_ATTACH`` +against it. Only one such declared debugging process can exists for +each inferior at a time. For example, this is used by KDE, Chromium, and +Firefox's crash handlers, and by Wine for allowing only Wine processes +to ptrace each other. If a process wishes to entirely disable these ptrace +restrictions, it can call ``prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, ...)`` +so that any otherwise allowed process (even those in external pid namespaces) +may attach. + +The sysctl settings (writable only with ``CAP_SYS_PTRACE``) are: + +0 - classic ptrace permissions: + a process can ``PTRACE_ATTACH`` to any other + process running under the same uid, as long as it is dumpable (i.e. + did not transition uids, start privileged, or have called + ``prctl(PR_SET_DUMPABLE...)`` already). Similarly, ``PTRACE_TRACEME`` is + unchanged. + +1 - restricted ptrace: + a process must have a predefined relationship + with the inferior it wants to call ``PTRACE_ATTACH`` on. By default, + this relationship is that of only its descendants when the above + classic criteria is also met. To change the relationship, an + inferior can call ``prctl(PR_SET_PTRACER, debugger, ...)`` to declare + an allowed debugger PID to call ``PTRACE_ATTACH`` on the inferior. + Using ``PTRACE_TRACEME`` is unchanged. + +2 - admin-only attach: + only processes with ``CAP_SYS_PTRACE`` may use ptrace, either with + ``PTRACE_ATTACH`` or through children calling ``PTRACE_TRACEME``. + +3 - no attach: + no processes may use ptrace with ``PTRACE_ATTACH`` nor via + ``PTRACE_TRACEME``. Once set, this sysctl value cannot be changed. + +The original children-only logic was based on the restrictions in grsecurity. diff --git a/Documentation/admin-guide/LSM/apparmor.rst b/Documentation/admin-guide/LSM/apparmor.rst new file mode 100644 index 000000000..47939ee89 --- /dev/null +++ b/Documentation/admin-guide/LSM/apparmor.rst @@ -0,0 +1,54 @@ +======== +AppArmor +======== + +What is AppArmor? +================= + +AppArmor is MAC style security extension for the Linux kernel. It implements +a task centered policy, with task "profiles" being created and loaded +from user space. Tasks on the system that do not have a profile defined for +them run in an unconfined state which is equivalent to standard Linux DAC +permissions. + +How to enable/disable +===================== + +set ``CONFIG_SECURITY_APPARMOR=y`` + +If AppArmor should be selected as the default security module then set:: + + CONFIG_DEFAULT_SECURITY_APPARMOR=y + +The CONFIG_LSM parameter manages the order and selection of LSMs. +Specify apparmor as the first "major" module (e.g. AppArmor, SELinux, Smack) +in the list. + +Build the kernel + +If AppArmor is not the default security module it can be enabled by passing +``security=apparmor`` on the kernel's command line. + +If AppArmor is the default security module it can be disabled by passing +``apparmor=0, security=XXXX`` (where ``XXXX`` is valid security module), on the +kernel's command line. + +For AppArmor to enforce any restrictions beyond standard Linux DAC permissions +policy must be loaded into the kernel from user space (see the Documentation +and tools links). + +Documentation +============= + +Documentation can be found on the wiki, linked below. + +Links +===== + +Mailing List - apparmor@lists.ubuntu.com + +Wiki - http://wiki.apparmor.net + +User space tools - https://gitlab.com/apparmor + +Kernel module - git://git.kernel.org/pub/scm/linux/kernel/git/jj/linux-apparmor diff --git a/Documentation/admin-guide/LSM/index.rst b/Documentation/admin-guide/LSM/index.rst new file mode 100644 index 000000000..b44ef68f6 --- /dev/null +++ b/Documentation/admin-guide/LSM/index.rst @@ -0,0 +1,51 @@ +=========================== +Linux Security Module Usage +=========================== + +The Linux Security Module (LSM) framework provides a mechanism for +various security checks to be hooked by new kernel extensions. The name +"module" is a bit of a misnomer since these extensions are not actually +loadable kernel modules. Instead, they are selectable at build-time via +CONFIG_DEFAULT_SECURITY and can be overridden at boot-time via the +``"security=..."`` kernel command line argument, in the case where multiple +LSMs were built into a given kernel. + +The primary users of the LSM interface are Mandatory Access Control +(MAC) extensions which provide a comprehensive security policy. Examples +include SELinux, Smack, Tomoyo, and AppArmor. In addition to the larger +MAC extensions, other extensions can be built using the LSM to provide +specific changes to system operation when these tweaks are not available +in the core functionality of Linux itself. + +The Linux capabilities modules will always be included. This may be +followed by any number of "minor" modules and at most one "major" module. +For more details on capabilities, see ``capabilities(7)`` in the Linux +man-pages project. + +A list of the active security modules can be found by reading +``/sys/kernel/security/lsm``. This is a comma separated list, and +will always include the capability module. The list reflects the +order in which checks are made. The capability module will always +be first, followed by any "minor" modules (e.g. Yama) and then +the one "major" module (e.g. SELinux) if there is one configured. + +Process attributes associated with "major" security modules should +be accessed and maintained using the special files in ``/proc/.../attr``. +A security module may maintain a module specific subdirectory there, +named after the module. ``/proc/.../attr/smack`` is provided by the Smack +security module and contains all its special files. The files directly +in ``/proc/.../attr`` remain as legacy interfaces for modules that provide +subdirectories. + +.. toctree:: + :maxdepth: 1 + + apparmor + LoadPin + SELinux + Smack + tomoyo + Yama + SafeSetID + ipe + landlock diff --git a/Documentation/admin-guide/LSM/ipe.rst b/Documentation/admin-guide/LSM/ipe.rst new file mode 100644 index 000000000..a756d8158 --- /dev/null +++ b/Documentation/admin-guide/LSM/ipe.rst @@ -0,0 +1,835 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Integrity Policy Enforcement (IPE) +================================== + +.. NOTE:: + + This is the documentation for admins, system builders, or individuals + attempting to use IPE. If you're looking for more developer-focused + documentation about IPE please see :doc:`the design docs `. + +Overview +-------- + +Integrity Policy Enforcement (IPE) is a Linux Security Module that takes a +complementary approach to access control. Unlike traditional access control +mechanisms that rely on labels and paths for decision-making, IPE focuses +on the immutable security properties inherent to system components. These +properties are fundamental attributes or features of a system component +that cannot be altered, ensuring a consistent and reliable basis for +security decisions. + +To elaborate, in the context of IPE, system components primarily refer to +files or the devices these files reside on. However, this is just a +starting point. The concept of system components is flexible and can be +extended to include new elements as the system evolves. The immutable +properties include the origin of a file, which remains constant and +unchangeable over time. For example, IPE policies can be crafted to trust +files originating from the initramfs. Since initramfs is typically verified +by the bootloader, its files are deemed trustworthy; "file is from +initramfs" becomes an immutable property under IPE's consideration. + +The immutable property concept extends to the security features enabled on +a file's origin, such as dm-verity or fs-verity, which provide a layer of +integrity and trust. For example, IPE allows the definition of policies +that trust files from a dm-verity protected device. dm-verity ensures the +integrity of an entire device by providing a verifiable and immutable state +of its contents. Similarly, fs-verity offers filesystem-level integrity +checks, allowing IPE to enforce policies that trust files protected by +fs-verity. These two features cannot be turned off once established, so +they are considered immutable properties. These examples demonstrate how +IPE leverages immutable properties, such as a file's origin and its +integrity protection mechanisms, to make access control decisions. + +For the IPE policy, specifically, it grants the ability to enforce +stringent access controls by assessing security properties against +reference values defined within the policy. This assessment can be based on +the existence of a security property (e.g., verifying if a file originates +from initramfs) or evaluating the internal state of an immutable security +property. The latter includes checking the roothash of a dm-verity +protected device, determining whether dm-verity possesses a valid +signature, assessing the digest of a fs-verity protected file, or +determining whether fs-verity possesses a valid built-in signature. This +nuanced approach to policy enforcement enables a highly secure and +customizable system defense mechanism, tailored to specific security +requirements and trust models. + +To enable IPE, ensure that ``CONFIG_SECURITY_IPE`` (under +:menuselection:`Security -> Integrity Policy Enforcement (IPE)`) config +option is enabled. + +Use Cases +--------- + +IPE works best in fixed-function devices: devices in which their purpose +is clearly defined and not supposed to be changed (e.g. network firewall +device in a data center, an IoT device, etcetera), where all software and +configuration is built and provisioned by the system owner. + +IPE is a long-way off for use in general-purpose computing: the Linux +community as a whole tends to follow a decentralized trust model (known as +the web of trust), which IPE has no support for it yet. Instead, IPE +supports PKI (public key infrastructure), which generally designates a +set of trusted entities that provide a measure of absolute trust. + +Additionally, while most packages are signed today, the files inside +the packages (for instance, the executables), tend to be unsigned. This +makes it difficult to utilize IPE in systems where a package manager is +expected to be functional, without major changes to the package manager +and ecosystem behind it. + +The digest_cache LSM [#digest_cache_lsm]_ is a system that when combined with IPE, +could be used to enable and support general-purpose computing use cases. + +Known Limitations +----------------- + +IPE cannot verify the integrity of anonymous executable memory, such as +the trampolines created by gcc closures and libffi (<3.4.2), or JIT'd code. +Unfortunately, as this is dynamically generated code, there is no way +for IPE to ensure the integrity of this code to form a trust basis. + +IPE cannot verify the integrity of programs written in interpreted +languages when these scripts are invoked by passing these program files +to the interpreter. This is because the way interpreters execute these +files; the scripts themselves are not evaluated as executable code +through one of IPE's hooks, but they are merely text files that are read +(as opposed to compiled executables). However, with the introduction of the +``AT_EXECVE_CHECK`` flag (:doc:`AT_EXECVE_CHECK `), +interpreters can use it to signal the kernel that a script file will be executed, +and request the kernel to perform LSM security checks on it. + +IPE's EXECUTE operation enforcement differs between compiled executables and +interpreted scripts: For compiled executables, enforcement is triggered +automatically by the kernel during ``execve()``, ``execveat()``, ``mmap()`` +and ``mprotect()`` syscalls when loading executable content. For interpreted +scripts, enforcement requires explicit interpreter integration using +``execveat()`` with ``AT_EXECVE_CHECK`` flag. Unlike exec syscalls that IPE +intercepts during the execution process, this mechanism needs the interpreter +to take the initiative, and existing interpreters won't be automatically +supported unless the signal call is added. + +Threat Model +------------ + +IPE specifically targets the risk of tampering with user-space executable +code after the kernel has initially booted, including the kernel modules +loaded from userspace via ``modprobe`` or ``insmod``. + +To illustrate, consider a scenario where an untrusted binary, possibly +malicious, is downloaded along with all necessary dependencies, including a +loader and libc. The primary function of IPE in this context is to prevent +the execution of such binaries and their dependencies. + +IPE achieves this by verifying the integrity and authenticity of all +executable code before allowing them to run. It conducts a thorough +check to ensure that the code's integrity is intact and that they match an +authorized reference value (digest, signature, etc) as per the defined +policy. If a binary does not pass this verification process, either +because its integrity has been compromised or it does not meet the +authorization criteria, IPE will deny its execution. Additionally, IPE +generates audit logs which may be utilized to detect and analyze failures +resulting from policy violation. + +Tampering threat scenarios include modification or replacement of +executable code by a range of actors including: + +- Actors with physical access to the hardware +- Actors with local network access to the system +- Actors with access to the deployment system +- Compromised internal systems under external control +- Malicious end users of the system +- Compromised end users of the system +- Remote (external) compromise of the system + +IPE does not mitigate threats arising from malicious but authorized +developers (with access to a signing certificate), or compromised +developer tools used by them (i.e. return-oriented programming attacks). +Additionally, IPE draws hard security boundary between userspace and +kernelspace. As a result, kernel-level exploits are considered outside +the scope of IPE and mitigation is left to other mechanisms. + +Policy +------ + +IPE policy is a plain-text [#devdoc]_ policy composed of multiple statements +over several lines. There is one required line, at the top of the +policy, indicating the policy name, and the policy version, for +instance:: + + policy_name=Ex_Policy policy_version=0.0.0 + +The policy name is a unique key identifying this policy in a human +readable name. This is used to create nodes under securityfs as well as +uniquely identify policies to deploy new policies vs update existing +policies. + +The policy version indicates the current version of the policy (NOT the +policy syntax version). This is used to prevent rollback of policy to +potentially insecure previous versions of the policy. + +The next portion of IPE policy are rules. Rules are formed by key=value +pairs, known as properties. IPE rules require two properties: ``action``, +which determines what IPE does when it encounters a match against the +rule, and ``op``, which determines when the rule should be evaluated. +The ordering is significant, a rule must start with ``op``, and end with +``action``. Thus, a minimal rule is:: + + op=EXECUTE action=ALLOW + +This example will allow any execution. Additional properties are used to +assess immutable security properties about the files being evaluated. +These properties are intended to be descriptions of systems within the +kernel that can provide a measure of integrity verification, such that IPE +can determine the trust of the resource based on the value of the property. + +Rules are evaluated top-to-bottom. As a result, any revocation rules, +or denies should be placed early in the file to ensure that these rules +are evaluated before a rule with ``action=ALLOW``. + +IPE policy supports comments. The character '#' will function as a +comment, ignoring all characters to the right of '#' until the newline. + +The default behavior of IPE evaluations can also be expressed in policy, +through the ``DEFAULT`` statement. This can be done at a global level, +or a per-operation level:: + + # Global + DEFAULT action=ALLOW + + # Operation Specific + DEFAULT op=EXECUTE action=ALLOW + +A default must be set for all known operations in IPE. If you want to +preserve older policies being compatible with newer kernels that can introduce +new operations, set a global default of ``ALLOW``, then override the +defaults on a per-operation basis (as above). + +With configurable policy-based LSMs, there's several issues with +enforcing the configurable policies at startup, around reading and +parsing the policy: + +1. The kernel *should* not read files from userspace, so directly reading + the policy file is prohibited. +2. The kernel command line has a character limit, and one kernel module + should not reserve the entire character limit for its own + configuration. +3. There are various boot loaders in the kernel ecosystem, so handing + off a memory block would be costly to maintain. + +As a result, IPE has addressed this problem through a concept of a "boot +policy". A boot policy is a minimal policy which is compiled into the +kernel. This policy is intended to get the system to a state where +userspace is set up and ready to receive commands, at which point a more +complex policy can be deployed via securityfs. The boot policy can be +specified via ``SECURITY_IPE_BOOT_POLICY`` config option, which accepts +a path to a plain-text version of the IPE policy to apply. This policy +will be compiled into the kernel. If not specified, IPE will be disabled +until a policy is deployed and activated through securityfs. + +Deploying Policies +~~~~~~~~~~~~~~~~~~ + +Policies can be deployed from userspace through securityfs. These policies +are signed through the PKCS#7 message format to enforce some level of +authorization of the policies (prohibiting an attacker from gaining +unconstrained root, and deploying an "allow all" policy). These +policies must be signed by a certificate that chains to the +``SYSTEM_TRUSTED_KEYRING``, or to the secondary and/or platform keyrings if +``CONFIG_IPE_POLICY_SIG_SECONDARY_KEYRING`` and/or +``CONFIG_IPE_POLICY_SIG_PLATFORM_KEYRING`` are enabled, respectively. +With openssl, the policy can be signed by:: + + openssl smime -sign \ + -in "$MY_POLICY" \ + -signer "$MY_CERTIFICATE" \ + -inkey "$MY_PRIVATE_KEY" \ + -noattr \ + -nodetach \ + -nosmimecap \ + -outform der \ + -out "$MY_POLICY.p7b" + +Deploying the policies is done through securityfs, through the +``new_policy`` node. To deploy a policy, simply cat the file into the +securityfs node:: + + cat "$MY_POLICY.p7b" > /sys/kernel/security/ipe/new_policy + +Upon success, this will create one subdirectory under +``/sys/kernel/security/ipe/policies/``. The subdirectory will be the +``policy_name`` field of the policy deployed, so for the example above, +the directory will be ``/sys/kernel/security/ipe/policies/Ex_Policy``. +Within this directory, there will be seven files: ``pkcs7``, ``policy``, +``name``, ``version``, ``active``, ``update``, and ``delete``. + +The ``pkcs7`` file is read-only. Reading it returns the raw PKCS#7 data +that was provided to the kernel, representing the policy. If the policy being +read is the boot policy, this will return ``ENOENT``, as it is not signed. + +The ``policy`` file is read only. Reading it returns the PKCS#7 inner +content of the policy, which will be the plain text policy. + +The ``active`` file is used to set a policy as the currently active policy. +This file is rw, and accepts a value of ``"1"`` to set the policy as active. +Since only a single policy can be active at one time, all other policies +will be marked inactive. The policy being marked active must have a policy +version greater or equal to the currently-running version. + +The ``update`` file is used to update a policy that is already present +in the kernel. This file is write-only and accepts a PKCS#7 signed +policy. Two checks will always be performed on this policy: First, the +``policy_names`` must match with the updated version and the existing +version. Second the updated policy must have a policy version greater than +the currently-running version. This is to prevent rollback attacks. + +The ``delete`` file is used to remove a policy that is no longer needed. +This file is write-only and accepts a value of ``1`` to delete the policy. +On deletion, the securityfs node representing the policy will be removed. +However, delete the current active policy is not allowed and will return +an operation not permitted error. + +Similarly, writing to both ``update`` and ``new_policy`` could result in +bad message(policy syntax error) or file exists error. The latter error happens +when trying to deploy a policy with a ``policy_name`` while the kernel already +has a deployed policy with the same ``policy_name``. + +Deploying a policy will *not* cause IPE to start enforcing the policy. IPE will +only enforce the policy marked active. Note that only one policy can be active +at a time. + +Once deployment is successful, the policy can be activated, by writing file +``/sys/kernel/security/ipe/policies/$policy_name/active``. +For example, the ``Ex_Policy`` can be activated by:: + + echo 1 > "/sys/kernel/security/ipe/policies/Ex_Policy/active" + +From above point on, ``Ex_Policy`` is now the enforced policy on the +system. + +IPE also provides a way to delete policies. This can be done via the +``delete`` securityfs node, +``/sys/kernel/security/ipe/policies/$policy_name/delete``. +Writing ``1`` to that file deletes the policy:: + + echo 1 > "/sys/kernel/security/ipe/policies/$policy_name/delete" + +There is only one requirement to delete a policy: the policy being deleted +must be inactive. + +.. NOTE:: + + If a traditional MAC system is enabled (SELinux, apparmor, smack), all + writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``. + +Modes +~~~~~ + +IPE supports two modes of operation: permissive (similar to SELinux's +permissive mode) and enforced. In permissive mode, all events are +checked and policy violations are logged, but the policy is not really +enforced. This allows users to test policies before enforcing them. + +The default mode is enforce, and can be changed via the kernel command +line parameter ``ipe.enforce=(0|1)``, or the securityfs node +``/sys/kernel/security/ipe/enforce``. + +.. NOTE:: + + If a traditional MAC system is enabled (SELinux, apparmor, smack, etcetera), + all writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``. + +Audit Events +~~~~~~~~~~~~ + +1420 AUDIT_IPE_ACCESS +^^^^^^^^^^^^^^^^^^^^^ +Event Examples:: + + type=1420 audit(1653364370.067:61): ipe_op=EXECUTE ipe_hook=MMAP enforcing=1 pid=2241 comm="ld-linux.so" path="/deny/lib/libc.so.6" dev="sda2" ino=14549020 rule="DEFAULT action=DENY" + type=1300 audit(1653364370.067:61): SYSCALL arch=c000003e syscall=9 success=no exit=-13 a0=7f1105a28000 a1=195000 a2=5 a3=812 items=0 ppid=2219 pid=2241 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="ld-linux.so" exe="/tmp/ipe-test/lib/ld-linux.so" subj=unconfined key=(null) + type=1327 audit(1653364370.067:61): 707974686F6E3300746573742F6D61696E2E7079002D6E00 + + type=1420 audit(1653364735.161:64): ipe_op=EXECUTE ipe_hook=MMAP enforcing=1 pid=2472 comm="mmap_test" path=? dev=? ino=? rule="DEFAULT action=DENY" + type=1300 audit(1653364735.161:64): SYSCALL arch=c000003e syscall=9 success=no exit=-13 a0=0 a1=1000 a2=4 a3=21 items=0 ppid=2219 pid=2472 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="mmap_test" exe="/root/overlake_test/upstream_test/vol_fsverity/bin/mmap_test" subj=unconfined key=(null) + type=1327 audit(1653364735.161:64): 707974686F6E3300746573742F6D61696E2E7079002D6E00 + +This event indicates that IPE made an access control decision; the IPE +specific record (1420) is always emitted in conjunction with a +``AUDITSYSCALL`` record. + +Determining whether IPE is in permissive or enforced mode can be derived +from ``success`` property and exit code of the ``AUDITSYSCALL`` record. + + +Field descriptions: + ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| Field | Value Type | Optional? | Description of Value | ++===========+============+===========+=================================================================================+ +| ipe_op | string | No | The IPE operation name associated with the log | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| ipe_hook | string | No | The name of the LSM hook that triggered the IPE event | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| enforcing | integer | No | The current IPE enforcing state 1 is in enforcing mode, 0 is in permissive mode | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| pid | integer | No | The pid of the process that triggered the IPE event. | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| comm | string | No | The command line program name of the process that triggered the IPE event | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| path | string | Yes | The absolute path to the evaluated file | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| ino | integer | Yes | The inode number of the evaluated file | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| dev | string | Yes | The device name of the evaluated file, e.g. vda | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| rule | string | No | The matched policy rule | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ + +1421 AUDIT_IPE_CONFIG_CHANGE +^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Event Example:: + + type=1421 audit(1653425583.136:54): old_active_pol_name="Allow_All" old_active_pol_version=0.0.0 old_policy_digest=sha256:E3B0C44298FC1C149AFBF4C8996FB92427AE41E4649B934CA495991B7852B855 new_active_pol_name="boot_verified" new_active_pol_version=0.0.0 new_policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB auid=4294967295 ses=4294967295 lsm=ipe res=1 + type=1300 audit(1653425583.136:54): SYSCALL arch=c000003e syscall=1 success=yes exit=2 a0=3 a1=5596fcae1fb0 a2=2 a3=2 items=0 ppid=184 pid=229 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="python3" exe="/usr/bin/python3.10" key=(null) + type=1327 audit(1653425583.136:54): PROCTITLE proctitle=707974686F6E3300746573742F6D61696E2E7079002D66002E2 + +This event indicates that IPE switched the active poliy from one to another +along with the version and the hash digest of the two policies. +Note IPE can only have one policy active at a time, all access decision +evaluation is based on the current active policy. +The normal procedure to deploy a new policy is loading the policy to deploy +into the kernel first, then switch the active policy to it. + +This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall. + +Field descriptions: + ++------------------------+------------+-----------+---------------------------------------------------+ +| Field | Value Type | Optional? | Description of Value | ++========================+============+===========+===================================================+ +| old_active_pol_name | string | Yes | The name of previous active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| old_active_pol_version | string | Yes | The version of previous active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| old_policy_digest | string | Yes | The hash of previous active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| new_active_pol_name | string | No | The name of current active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| new_active_pol_version | string | No | The version of current active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| new_policy_digest | string | No | The hash of current active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| auid | integer | No | The login user ID | ++------------------------+------------+-----------+---------------------------------------------------+ +| ses | integer | No | The login session ID | ++------------------------+------------+-----------+---------------------------------------------------+ +| lsm | string | No | The lsm name associated with the event | ++------------------------+------------+-----------+---------------------------------------------------+ +| res | integer | No | The result of the audited operation(success/fail) | ++------------------------+------------+-----------+---------------------------------------------------+ + +1422 AUDIT_IPE_POLICY_LOAD +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Event Example:: + + type=1422 audit(1653425529.927:53): policy_name="boot_verified" policy_version=0.0.0 policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB auid=4294967295 ses=4294967295 lsm=ipe res=1 errno=0 + type=1300 audit(1653425529.927:53): arch=c000003e syscall=1 success=yes exit=2567 a0=3 a1=5596fcae1fb0 a2=a07 a3=2 items=0 ppid=184 pid=229 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="python3" exe="/usr/bin/python3.10" key=(null) + type=1327 audit(1653425529.927:53): PROCTITLE proctitle=707974686F6E3300746573742F6D61696E2E7079002D66002E2E + +This record indicates a new policy has been loaded into the kernel with the policy name, policy version and policy hash. + +This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall. + +Field descriptions: + ++----------------+------------+-----------+-------------------------------------------------------------+ +| Field | Value Type | Optional? | Description of Value | ++================+============+===========+=============================================================+ +| policy_name | string | Yes | The policy_name | ++----------------+------------+-----------+-------------------------------------------------------------+ +| policy_version | string | Yes | The policy_version | ++----------------+------------+-----------+-------------------------------------------------------------+ +| policy_digest | string | Yes | The policy hash | ++----------------+------------+-----------+-------------------------------------------------------------+ +| auid | integer | No | The login user ID | ++----------------+------------+-----------+-------------------------------------------------------------+ +| ses | integer | No | The login session ID | ++----------------+------------+-----------+-------------------------------------------------------------+ +| lsm | string | No | The lsm name associated with the event | ++----------------+------------+-----------+-------------------------------------------------------------+ +| res | integer | No | The result of the audited operation(success/fail) | ++----------------+------------+-----------+-------------------------------------------------------------+ +| errno | integer | No | Error code from policy loading operations (see table below) | ++----------------+------------+-----------+-------------------------------------------------------------+ + +Policy error codes (errno): + +The following table lists the error codes that may appear in the errno field while loading or updating the policy: + ++----------------+--------------------------------------------------------+ +| Error Code | Description | ++================+========================================================+ +| 0 | Success | ++----------------+--------------------------------------------------------+ +| -EPERM | Insufficient permission | ++----------------+--------------------------------------------------------+ +| -EEXIST | Same name policy already deployed | ++----------------+--------------------------------------------------------+ +| -EBADMSG | Policy is invalid | ++----------------+--------------------------------------------------------+ +| -ENOMEM | Out of memory (OOM) | ++----------------+--------------------------------------------------------+ +| -ERANGE | Policy version number overflow | ++----------------+--------------------------------------------------------+ +| -EINVAL | Policy version parsing error | ++----------------+--------------------------------------------------------+ +| -ENOKEY | Key used to sign the IPE policy not found in keyring | ++----------------+--------------------------------------------------------+ +| -EKEYREJECTED | Policy signature verification failed | ++----------------+--------------------------------------------------------+ +| -ESTALE | Attempting to update an IPE policy with older version | ++----------------+--------------------------------------------------------+ +| -ENOENT | Policy was deleted while updating | ++----------------+--------------------------------------------------------+ + +1404 AUDIT_MAC_STATUS +^^^^^^^^^^^^^^^^^^^^^ + +Event Examples:: + + type=1404 audit(1653425689.008:55): enforcing=0 old_enforcing=1 auid=4294967295 ses=4294967295 enabled=1 old-enabled=1 lsm=ipe res=1 + type=1300 audit(1653425689.008:55): arch=c000003e syscall=1 success=yes exit=2 a0=1 a1=55c1065e5c60 a2=2 a3=0 items=0 ppid=405 pid=441 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=) + type=1327 audit(1653425689.008:55): proctitle="-bash" + + type=1404 audit(1653425689.008:55): enforcing=1 old_enforcing=0 auid=4294967295 ses=4294967295 enabled=1 old-enabled=1 lsm=ipe res=1 + type=1300 audit(1653425689.008:55): arch=c000003e syscall=1 success=yes exit=2 a0=1 a1=55c1065e5c60 a2=2 a3=0 items=0 ppid=405 pid=441 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=) + type=1327 audit(1653425689.008:55): proctitle="-bash" + +This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall. + +Field descriptions: + ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| Field | Value Type | Optional? | Description of Value | ++===============+============+===========+=================================================================================================+ +| enforcing | integer | No | The enforcing state IPE is being switched to, 1 is in enforcing mode, 0 is in permissive mode | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| old_enforcing | integer | No | The enforcing state IPE is being switched from, 1 is in enforcing mode, 0 is in permissive mode | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| auid | integer | No | The login user ID | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| ses | integer | No | The login session ID | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| enabled | integer | No | The new TTY audit enabled setting | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| old-enabled | integer | No | The old TTY audit enabled setting | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| lsm | string | No | The lsm name associated with the event | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| res | integer | No | The result of the audited operation(success/fail) | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ + + +Success Auditing +^^^^^^^^^^^^^^^^ + +IPE supports success auditing. When enabled, all events that pass IPE +policy and are not blocked will emit an audit event. This is disabled by +default, and can be enabled via the kernel command line +``ipe.success_audit=(0|1)`` or +``/sys/kernel/security/ipe/success_audit`` securityfs file. + +This is *very* noisy, as IPE will check every userspace binary on the +system, but is useful for debugging policies. + +.. NOTE:: + + If a traditional MAC system is enabled (SELinux, apparmor, smack, etcetera), + all writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``. + +Properties +---------- + +As explained above, IPE properties are ``key=value`` pairs expressed in IPE +policy. Two properties are built-into the policy parser: 'op' and 'action'. +The other properties are used to restrict immutable security properties +about the files being evaluated. Currently those properties are: +'``boot_verified``', '``dmverity_signature``', '``dmverity_roothash``', +'``fsverity_signature``', '``fsverity_digest``'. A description of all +properties supported by IPE are listed below: + +op +~~ + +Indicates the operation for a rule to apply to. Must be in every rule, +as the first token. IPE supports the following operations: + + ``EXECUTE`` + + Pertains to any file attempting to be executed, or loaded as an + executable. + + ``FIRMWARE``: + + Pertains to firmware being loaded via the firmware_class interface. + This covers both the preallocated buffer and the firmware file + itself. + + ``KMODULE``: + + Pertains to loading kernel modules via ``modprobe`` or ``insmod``. + + ``KEXEC_IMAGE``: + + Pertains to kernel images loading via ``kexec``. + + ``KEXEC_INITRAMFS`` + + Pertains to initrd images loading via ``kexec --initrd``. + + ``POLICY``: + + Controls loading policies via reading a kernel-space initiated read. + + An example of such is loading IMA policies by writing the path + to the policy file to ``$securityfs/ima/policy`` + + ``X509_CERT``: + + Controls loading IMA certificates through the Kconfigs, + ``CONFIG_IMA_X509_PATH`` and ``CONFIG_EVM_X509_PATH``. + +action +~~~~~~ + + Determines what IPE should do when a rule matches. Must be in every + rule, as the final clause. Can be one of: + + ``ALLOW``: + + If the rule matches, explicitly allow access to the resource to proceed + without executing any more rules. + + ``DENY``: + + If the rule matches, explicitly prohibit access to the resource to + proceed without executing any more rules. + +boot_verified +~~~~~~~~~~~~~ + + This property can be utilized for authorization of files from initramfs. + The format of this property is:: + + boot_verified=(TRUE|FALSE) + + + .. WARNING:: + + This property will trust files from initramfs(rootfs). It should + only be used during early booting stage. Before mounting the real + rootfs on top of the initramfs, initramfs script will recursively + remove all files and directories on the initramfs. This is typically + implemented by using switch_root(8) [#switch_root]_. Therefore the + initramfs will be empty and not accessible after the real + rootfs takes over. It is advised to switch to a different policy + that doesn't rely on the property after this point. + This ensures that the trust policies remain relevant and effective + throughout the system's operation. + +dmverity_roothash +~~~~~~~~~~~~~~~~~ + + This property can be utilized for authorization or revocation of + specific dm-verity volumes, identified via their root hashes. It has a + dependency on the DM_VERITY module. This property is controlled by + the ``IPE_PROP_DM_VERITY`` config option, it will be automatically + selected when ``SECURITY_IPE`` and ``DM_VERITY`` are all enabled. + The format of this property is:: + + dmverity_roothash=DigestName:HexadecimalString + + The supported DigestNames for dmverity_roothash are [#dmveritydigests]_ + + + blake2b-512 + + blake2s-256 + + sha256 + + sha384 + + sha512 + + sha3-224 + + sha3-256 + + sha3-384 + + sha3-512 + + sm3 + + rmd160 + +dmverity_signature +~~~~~~~~~~~~~~~~~~ + + This property can be utilized for authorization of all dm-verity + volumes that have a signed roothash that validated by a keyring + specified by dm-verity's configuration, either the system trusted + keyring, or the secondary keyring. It depends on + ``DM_VERITY_VERIFY_ROOTHASH_SIG`` config option and is controlled by + the ``IPE_PROP_DM_VERITY_SIGNATURE`` config option, it will be automatically + selected when ``SECURITY_IPE``, ``DM_VERITY`` and + ``DM_VERITY_VERIFY_ROOTHASH_SIG`` are all enabled. + The format of this property is:: + + dmverity_signature=(TRUE|FALSE) + +fsverity_digest +~~~~~~~~~~~~~~~ + + This property can be utilized for authorization of specific fsverity + enabled files, identified via their fsverity digests. + It depends on ``FS_VERITY`` config option and is controlled by + the ``IPE_PROP_FS_VERITY`` config option, it will be automatically + selected when ``SECURITY_IPE`` and ``FS_VERITY`` are all enabled. + The format of this property is:: + + fsverity_digest=DigestName:HexadecimalString + + The supported DigestNames for fsverity_digest are [#fsveritydigest]_ + + + sha256 + + sha512 + +fsverity_signature +~~~~~~~~~~~~~~~~~~ + + This property is used to authorize all fs-verity enabled files that have + been verified by fs-verity's built-in signature mechanism. The signature + verification relies on a key stored within the ".fs-verity" keyring. It + depends on ``FS_VERITY_BUILTIN_SIGNATURES`` config option and + it is controlled by the ``IPE_PROP_FS_VERITY`` config option, + it will be automatically selected when ``SECURITY_IPE``, ``FS_VERITY`` + and ``FS_VERITY_BUILTIN_SIGNATURES`` are all enabled. + The format of this property is:: + + fsverity_signature=(TRUE|FALSE) + +Policy Examples +--------------- + +Allow all +~~~~~~~~~ + +:: + + policy_name=Allow_All policy_version=0.0.0 + DEFAULT action=ALLOW + +Allow only initramfs +~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=Allow_Initramfs policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE boot_verified=TRUE action=ALLOW + +Allow any signed and validated dm-verity volume and the initramfs +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=Allow_Signed_DMV_And_Initramfs policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE boot_verified=TRUE action=ALLOW + op=EXECUTE dmverity_signature=TRUE action=ALLOW + +Prohibit execution from a specific dm-verity volume +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=Deny_DMV_By_Roothash policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE dmverity_roothash=sha256:cd2c5bae7c6c579edaae4353049d58eb5f2e8be0244bf05345bc8e5ed257baff action=DENY + + op=EXECUTE boot_verified=TRUE action=ALLOW + op=EXECUTE dmverity_signature=TRUE action=ALLOW + +Allow only a specific dm-verity volume +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=Allow_DMV_By_Roothash policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE dmverity_roothash=sha256:401fcec5944823ae12f62726e8184407a5fa9599783f030dec146938 action=ALLOW + +Allow any fs-verity file with a valid built-in signature +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=Allow_Signed_And_Validated_FSVerity policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE fsverity_signature=TRUE action=ALLOW + +Allow execution of a specific fs-verity file +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=ALLOW_FSV_By_Digest policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE fsverity_digest=sha256:fd88f2b8824e197f850bf4c5109bea5cf0ee38104f710843bb72da796ba5af9e action=ALLOW + +Additional Information +---------------------- + +- `Github Repository `_ +- :doc:`Developer and design docs for IPE ` + +FAQ +--- + +Q: + What's the difference between other LSMs which provide a measure of + trust-based access control? + +A: + + In general, there's two other LSMs that can provide similar functionality: + IMA, and Loadpin. + + IMA and IPE are functionally very similar. The significant difference between + the two is the policy. [#devdoc]_ + + Loadpin and IPE differ fairly dramatically, as Loadpin only covers the IPE's + kernel read operations, whereas IPE is capable of controlling execution + on top of kernel read. The trust model is also different; Loadpin roots its + trust in the initial super-block, whereas trust in IPE is stemmed from kernel + itself (via ``SYSTEM_TRUSTED_KEYS``). + +----------- + +.. [#digest_cache_lsm] https://lore.kernel.org/lkml/20240415142436.2545003-1-roberto.sassu@huaweicloud.com/ + +.. [#devdoc] Please see :doc:`the design docs ` for more on + this topic. + +.. [#switch_root] https://man7.org/linux/man-pages/man8/switch_root.8.html + +.. [#dmveritydigests] These hash algorithms are based on values accepted by + the Linux crypto API; IPE does not impose any + restrictions on the digest algorithm itself; + thus, this list may be out of date. + +.. [#fsveritydigest] These hash algorithms are based on values accepted by the + kernel's fsverity support; IPE does not impose any + restrictions on the digest algorithm itself; + thus, this list may be out of date. diff --git a/Documentation/admin-guide/LSM/landlock.rst b/Documentation/admin-guide/LSM/landlock.rst new file mode 100644 index 000000000..314052bbe --- /dev/null +++ b/Documentation/admin-guide/LSM/landlock.rst @@ -0,0 +1,194 @@ +.. SPDX-License-Identifier: GPL-2.0 +.. Copyright © 2025 Microsoft Corporation + +================================ +Landlock: system-wide management +================================ + +:Author: Mickaël Salaün +:Date: June 2026 + +Landlock can leverage the audit framework to log events. + +User space documentation can be found here: +Documentation/userspace-api/landlock.rst. + +Audit +===== + +Denied access requests are logged by default for a sandboxed program if `audit` +is enabled. This default behavior can be changed with the +sys_landlock_restrict_self() flags (cf. +Documentation/userspace-api/landlock.rst), or suppressed on a per-object +basis by using ``LANDLOCK_ADD_RULE_QUIET`` (ABI 10+). Landlock logs can +also be masked thanks to audit rules. Landlock can generate 2 audit +record types. + +Record types +------------ + +AUDIT_LANDLOCK_ACCESS + This record type identifies a denied access request to a kernel resource. + The ``domain`` field indicates the ID of the domain that blocked the + request. The ``blockers`` field indicates the cause(s) of this denial + (separated by a comma), and the following fields identify the kernel object + (similar to SELinux). There may be more than one of this record type per + audit event. + + Example with a file link request generating two records in the same event:: + + domain=195ba459b blockers=fs.refer path="/usr/bin" dev="vda2" ino=351 + domain=195ba459b blockers=fs.make_reg,fs.refer path="/usr/local" dev="vda2" ino=365 + + + The ``blockers`` field uses dot-separated prefixes to indicate the type of + restriction that caused the denial: + + **fs.*** - Filesystem access rights (ABI 1+): + - fs.execute, fs.write_file, fs.read_file, fs.read_dir + - fs.remove_dir, fs.remove_file + - fs.make_char, fs.make_dir, fs.make_reg, fs.make_sock + - fs.make_fifo, fs.make_block, fs.make_sym + - fs.refer (ABI 2+) + - fs.truncate (ABI 3+) + - fs.ioctl_dev (ABI 5+) + + **net.*** - Network access rights (ABI 4+): + - net.bind_tcp - TCP port binding was denied + - net.connect_tcp - TCP connection was denied + - net.bind_udp - UDP port binding was denied + - net.connect_send_udp - UDP connection and send was denied + + **scope.*** - IPC scoping restrictions (ABI 6+): + - scope.abstract_unix_socket - Abstract UNIX socket connection denied + - scope.signal - Signal sending denied + + Multiple blockers can appear in a single event (comma-separated) when + multiple access rights are missing. For example, creating a regular file + in a directory that lacks both ``make_reg`` and ``refer`` rights would show + ``blockers=fs.make_reg,fs.refer``. + + The object identification fields (path, dev, ino for filesystem; opid, + ocomm for signals) depend on the type of access being blocked and provide + context about what resource was involved in the denial. + + +AUDIT_LANDLOCK_DOMAIN + This record type describes the status of a Landlock domain. The ``status`` + field can be either ``allocated`` or ``deallocated``. + + The ``allocated`` status is part of the same audit event and follows + the first logged ``AUDIT_LANDLOCK_ACCESS`` record of a domain. It identifies + Landlock domain information at the time of the sys_landlock_restrict_self() + call with the following fields: + + - the ``domain`` ID + - the enforcement ``mode`` + - the domain creator's ``pid`` + - the domain creator's ``uid`` + - the domain creator's executable path (``exe``) + - the domain creator's command line (``comm``) + + Example:: + + domain=195ba459b status=allocated mode=enforcing pid=300 uid=0 exe="/root/sandboxer" comm="sandboxer" + + The ``deallocated`` status is an event on its own and it identifies a + Landlock domain release. After such event, it is guarantee that the + related domain ID will never be reused during the lifetime of the system. + The ``domain`` field indicates the ID of the domain which is released, and + the ``denials`` field indicates the total number of denied access request, + which might not have been logged according to the audit rules and + sys_landlock_restrict_self()'s flags. + + Example:: + + domain=195ba459b status=deallocated denials=3 + + +Event samples +-------------- + +Here are two examples of log events (see serial numbers). + +In this example a sandboxed program (``kill``) tries to send a signal to the +init process, which is denied because of the signal scoping restriction +(``LL_SCOPED=s``):: + + $ LL_FS_RO=/ LL_FS_RW=/ LL_SCOPED=s LL_FORCE_LOG=1 ./sandboxer kill 1 + +This command generates two events, each identified with a unique serial +number following a timestamp (``msg=audit(1729738800.268:30)``). The first +event (serial ``30``) contains 4 records. The first record +(``type=LANDLOCK_ACCESS``) shows an access denied by the domain `1a6fdc66f`. +The cause of this denial is signal scoping restriction +(``blockers=scope.signal``). The process that would have receive this signal +is the init process (``opid=1 ocomm="systemd"``). + +The second record (``type=LANDLOCK_DOMAIN``) describes (``status=allocated``) +domain `1a6fdc66f`. This domain was created by process ``286`` executing the +``/root/sandboxer`` program launched by the root user. + +The third record (``type=SYSCALL``) describes the syscall, its provided +arguments, its result (``success=no exit=-1``), and the process that called it. + +The fourth record (``type=PROCTITLE``) shows the command's name as an +hexadecimal value. This can be translated with ``python -c +'print(bytes.fromhex("6B696C6C0031"))'``. + +Finally, the last record (``type=LANDLOCK_DOMAIN``) is also the only one from +the second event (serial ``31``). It is not tied to a direct user space action +but an asynchronous one to free resources tied to a Landlock domain +(``status=deallocated``). This can be useful to know that the following logs +will not concern the domain ``1a6fdc66f`` anymore. This record also summarize +the number of requests this domain denied (``denials=1``), whether they were +logged or not. + +.. code-block:: + + type=LANDLOCK_ACCESS msg=audit(1729738800.268:30): domain=1a6fdc66f blockers=scope.signal opid=1 ocomm="systemd" + type=LANDLOCK_DOMAIN msg=audit(1729738800.268:30): domain=1a6fdc66f status=allocated mode=enforcing pid=286 uid=0 exe="/root/sandboxer" comm="sandboxer" + type=SYSCALL msg=audit(1729738800.268:30): arch=c000003e syscall=62 success=no exit=-1 [..] ppid=272 pid=286 auid=0 uid=0 gid=0 [...] comm="kill" [...] + type=PROCTITLE msg=audit(1729738800.268:30): proctitle=6B696C6C0031 + type=LANDLOCK_DOMAIN msg=audit(1729738800.324:31): domain=1a6fdc66f status=deallocated denials=1 + +Here is another example showcasing filesystem access control:: + + $ LL_FS_RO=/ LL_FS_RW=/tmp LL_FORCE_LOG=1 ./sandboxer sh -c "echo > /etc/passwd" + +The related audit logs contains 8 records from 3 different events (serials 33, +34 and 35) created by the same domain `1a6fdc679`:: + + type=LANDLOCK_ACCESS msg=audit(1729738800.221:33): domain=1a6fdc679 blockers=fs.write_file path="/dev/tty" dev="devtmpfs" ino=9 + type=LANDLOCK_DOMAIN msg=audit(1729738800.221:33): domain=1a6fdc679 status=allocated mode=enforcing pid=289 uid=0 exe="/root/sandboxer" comm="sandboxer" + type=SYSCALL msg=audit(1729738800.221:33): arch=c000003e syscall=257 success=no exit=-13 [...] ppid=272 pid=289 auid=0 uid=0 gid=0 [...] comm="sh" [...] + type=PROCTITLE msg=audit(1729738800.221:33): proctitle=7368002D63006563686F203E202F6574632F706173737764 + type=LANDLOCK_ACCESS msg=audit(1729738800.221:34): domain=1a6fdc679 blockers=fs.write_file path="/etc/passwd" dev="vda2" ino=143821 + type=SYSCALL msg=audit(1729738800.221:34): arch=c000003e syscall=257 success=no exit=-13 [...] ppid=272 pid=289 auid=0 uid=0 gid=0 [...] comm="sh" [...] + type=PROCTITLE msg=audit(1729738800.221:34): proctitle=7368002D63006563686F203E202F6574632F706173737764 + type=LANDLOCK_DOMAIN msg=audit(1729738800.261:35): domain=1a6fdc679 status=deallocated denials=2 + + +Event filtering +--------------- + +If you get spammed with audit logs related to Landlock, this is either an +attack attempt or a bug in the security policy. We can put in place some +filters to limit noise with two complementary ways: + +- with sys_landlock_restrict_self()'s flags, or + ``LANDLOCK_ADD_RULE_QUIET`` (ABI 10+) if we can fix the sandboxed + programs, +- or with audit rules (see :manpage:`auditctl(8)`). + +Additional documentation +======================== + +* `Linux Audit Documentation`_ +* Documentation/userspace-api/landlock.rst +* Documentation/security/landlock.rst +* https://landlock.io + +.. Links +.. _Linux Audit Documentation: + https://github.com/linux-audit/audit-documentation/wiki diff --git a/Documentation/admin-guide/LSM/tomoyo.rst b/Documentation/admin-guide/LSM/tomoyo.rst new file mode 100644 index 000000000..bdb2c2e2a --- /dev/null +++ b/Documentation/admin-guide/LSM/tomoyo.rst @@ -0,0 +1,52 @@ +====== +TOMOYO +====== + +What is TOMOYO? +=============== + +TOMOYO is a name-based MAC extension (LSM module) for the Linux kernel. + +LiveCD-based tutorials are available at + +https://tomoyo.sourceforge.net/1.8/ubuntu12.04-live.html +https://tomoyo.sourceforge.net/1.8/centos6-live.html + +Though these tutorials use non-LSM version of TOMOYO, they are useful for you +to know what TOMOYO is. + +How to enable TOMOYO? +===================== + +Build the kernel with ``CONFIG_SECURITY_TOMOYO=y`` and pass ``security=tomoyo`` on +kernel's command line. + +Please see https://tomoyo.sourceforge.net/2.6/ for details. + +Where is documentation? +======================= + +User <-> Kernel interface documentation is available at +https://tomoyo.sourceforge.net/2.6/policy-specification/index.html . + +Materials we prepared for seminars and symposiums are available at +https://sourceforge.net/projects/tomoyo/files/docs/ . +Below lists are chosen from three aspects. + +What is TOMOYO? + TOMOYO Linux Overview + https://sourceforge.net/projects/tomoyo/files/docs/lca2009-takeda.pdf + TOMOYO Linux: pragmatic and manageable security for Linux + https://sourceforge.net/projects/tomoyo/files/docs/freedomhectaipei-tomoyo.pdf + TOMOYO Linux: A Practical Method to Understand and Protect Your Own Linux Box + https://sourceforge.net/projects/tomoyo/files/docs/PacSec2007-en-no-demo.pdf + +What can TOMOYO do? + Deep inside TOMOYO Linux + https://sourceforge.net/projects/tomoyo/files/docs/lca2009-kumaneko.pdf + The role of "pathname based access control" in security. + https://sourceforge.net/projects/tomoyo/files/docs/lfj2008-bof.pdf + +History of TOMOYO? + Realities of Mainlining + https://sourceforge.net/projects/tomoyo/files/docs/lfj2008.pdf -- cgit v1.3.1