Merge tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Don't destroy user event fields when removal fails

   User event fields are destroyed before the event is removed from
   visibility. But that can fail leaving the still visible event with no
   fields. Move the destroying of the fields to after the event is
   successfully removed from visibility.

 - Initialize function graph state is fork before calling
   copy_exec_state()

   For non-CLONE_VM forks, copy_exec_state() allocates a new
   task_exec_state. If that allocation fails, ftrace_graph_exit_task()
   will free the tasks ret_stack pointer. Since that pointer is still
   using the parent's ret_stack, it mistakenly frees the parent's
   pointer too.

   Call ftrace_graph_init() on the task first which will NULL out the
   new tasks's ret_stack and if the copy fails, it will not free
   anything.

 - Remove FGRAPH_MAX_INDEX

   The macro FGRAPH_MAX_INDEX was added but never used. Remove it.

 - Save ent_size in function graph printing of nested functions

   The function graph tracer needs to look at the next event to see if
   the next event is the return of the current function entry. If it is,
   it prints a single line:

	ktime_get();

   Otherwise it prints it like a nested function:

	tick_nohz_irq_exit() {
	    ktime_get();
	    kcpustat_irq_exit();
	}

   In order to look at the next event, it must save the current event so
   that it has the information to print from it. It saves the event in
   the iterator descriptor called "ent". What it doesn't save is the
   ent_size of the event which is now used to know if the function graph
   arguments are to be printed. The peek doesn't save the size so the
   size used happens to be that of the size of the last event that was
   seen.

   Save the entry event size in the iterator descriptor so that the
   correct size is used.

 - Fix several errors with freeing data in the histogram code

   The histogram code had a lot of leaked or or incorrect accounting
   when failures happen. Correct them.

 - Fix histogram regression of .percent and .graph modifiers

   Up until 6.3 histogram values could have "percent" or "graph"
   modifiers that changed how they were printed. But a change that added
   restricting histograms values from being strings, stack traces and
   other modifiers inadvertently prevented them from using the percent
   and graph modifiers, which were legal use cases for values.

   Put back the percent and graph modifiers.

 - Fix various typos in the comments

 - Set the trace_clock before initializing a histogram with clock
   argument

   The histogram API allows the user to specific which trace clock to
   use via a "clock=" string. The histogram is set up first before the
   clock is checked. If the passed in clock is not valid, it exits
   without fully fixing up the histogram leaving it on the list and a
   use-after-free can trigger.

   Update the clock argument first and if it fails then exit gracefully
   before the histogram trigger is placed on any lists.

 - Restore :mod: trailer after parsing in ftrace_set_clr_event

   The function ftrace_set_clr_event() modifies the parse string and
   needs to put it back to what was passed in. It searches for ":mod:"
   via a strsep() but fails to put back the first ':' in the string.

   Add back the ':' in the passed in string.

 - Take trace_array reference when opening a tracer options file

   The options files are dynamically created and some tracers add their
   own options. When a tracer adds their own list of options, the
   trace_array holding them has an array to hold the list of options for
   each tracer. This array increases in size via a krealloc(), and the
   new entry gets a newly allocated array to hold the options of the new
   tracer being added.

   The element in each entry of the tracer's option array holds a
   pointer back to the trace_array, a pointer to the tracer it is
   associated to, a pointer to the flags of the option.

   The issue is that these arrays are freed when the trace_array is
   freed when its instance it represents is removed from the instances
   directory. There's a race that an open of one of these options files
   can happen when the instance is being removed.

   Add a new helper function to be called by the open function of the
   options file to iterate all existing trace_arrays under a lock and
   find the one that has the given option element in one of it's tracer
   arrays. If found, then update the associated trace_array's reference
   counter to keep it from being freed. If not found, have the open call
   return -ENODEV.

 - Disable interrupts when acquiring the lock in rb_wake_up_waiters()

   The function rb_wake_up_waiters() assumes it will be called in
   interrupt context and does not disable irqs when taking
   cpu_buffer->reader_lock, which can be called in hard interrupt
   context. The issue is in PREEMPT_RT, this function is called in
   thread context leaving this lock open to a deadlock.

   Take the lock with interrupts disabled.

 - Use rcu_assign_pointer() for tmp_ops filter hash

   The tmp_ops used in update_ftrace_direct_mod() assigns its
   filter_hash field directly, but that field is annotated as __rcu and
   sparse complains. Assign it with rcu_assign_pointer()

 - Fix use-after-free in enable_trigger_private_data_free()

   The trace_event_call is accessed through the event_trigger_data's
   trace_event_file pointer to put the trace_event_call on freeing. The
   issue is that the trace_event_file data may have been freed already
   causing a use-after-free. Add a field to the event_trigger_data that
   points directly to the trace_event_call so that it can decrement its
   reference directly without needing to go through the
   trace_event_file.

 - Fix accounting of buffer data remote headers

   trace_buffer_desc_size() and trace_remote_alloc_buffer() undercount
   the number of pages is needed for the asked for size as it doesn't
   take into account the meta data on each page. Add a helper function
   to do the calculation properly and use that in these functions.

 - Catch nr_page_va overflow in ring_buffer_desc sizing

   The number of pages per remote ring buffer is capped by
   ring_buffer_desc::nr_page_va (32 bits). A buffer_size large enough to
   overflow that field would silently allocate a descriptor smaller than
   what was asked for.

 - Do not resize the subbuf order if any per_cpu buffer is disabled

   The mmapping of ring buffers disables resizing the subbuffers, but it
   is done per-cpu whereas the subbuf size change is done for all the
   per_cpu buffers under the buffer->mutex. It could change the size of
   some while the mapping is happening on others. Have the resize of the
   subbuf order check all the per_cpu buffers under the lock to see if
   any of them is disabled before starting and causing an inconsistency
   between buffers that are being mapped.

* tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: (25 commits)
  ring-buffer: Check resize_disabled before publishing the new subbuf order
  tracing/remotes: Catch nr_page_va overflow in ring_buffer_desc sizing
  tracing/remotes: Account for ring buffer page header in size calculation
  tracing: Don't dereference trace_event_file in deferred trigger free
  ftrace: Use rcu_assign_pointer() for tmp_ops filter hash
  ring-buffer: Acquire the lock with irqsave in rb_wake_up_waiters()
  tracing: Take trace_array reference when opening a tracer options file
  tracing: Fix ring_buffer_read_page_size() kernel-doc
  tracing: Restore :mod: trailer after parsing in ftrace_set_clr_event()
  tracing: Fix memory corruption from a "STACKTRACE" histogram key
  tracing: Fix memory corruption from the histogram stacktrace modifier
  tracing: Undo the registration when enabling the histogram trigger fails
  tracing: Take the reference before publishing the named histogram trigger
  tracing: Set the trace clock before registering the histogram trigger
  tracing: Fix typo "preceeded" in comment
  tracing: Fix typo "availabe" in comment
  tracing: Let histogram values keep the percent and graph modifiers
  tracing: Keep the entry count when the histogram stats allocation fails
  tracing: Free histogram the field rejected for a bad modifier
  tracing: Free histogram the var ref when its initialization fails
  ...

1 files changed, 557 insertions, 0 deletions

diff --git a/Documentation/security/credentials.rst b/Documentation/security/credentials.rst
new file mode 100644
index 000000000..a39a2a2f6
--- /dev/null
+++ b/Documentation/security/credentials.rst
@@ -0,0 +1,557 @@
+====================
+Credentials in Linux
+====================
+
+By: David Howells <dhowells@redhat.com>
+
+.. contents:: :local:
+
+Overview
+========
+
+There are several parts to the security check performed by Linux when one
+object acts upon another:
+
+ 1. Objects.
+
+     Objects are things in the system that may be acted upon directly by
+     userspace programs.  Linux has a variety of actionable objects, including:
+
+	- Tasks
+	- Files/inodes
+	- Sockets
+	- Message queues
+	- Shared memory segments
+	- Semaphores
+	- Keys
+
+     As a part of the description of all these objects there is a set of
+     credentials.  What's in the set depends on the type of object.
+
+ 2. Object ownership.
+
+     Amongst the credentials of most objects, there will be a subset that
+     indicates the ownership of that object.  This is used for resource
+     accounting and limitation (disk quotas and task rlimits for example).
+
+     In a standard UNIX filesystem, for instance, this will be defined by the
+     UID marked on the inode.
+
+ 3. The objective context.
+
+     Also amongst the credentials of those objects, there will be a subset that
+     indicates the 'objective context' of that object.  This may or may not be
+     the same set as in (2) - in standard UNIX files, for instance, this is the
+     defined by the UID and the GID marked on the inode.
+
+     The objective context is used as part of the security calculation that is
+     carried out when an object is acted upon.
+
+ 4. Subjects.
+
+     A subject is an object that is acting upon another object.
+
+     Most of the objects in the system are inactive: they don't act on other
+     objects within the system.  Processes/tasks are the obvious exception:
+     they do stuff; they access and manipulate things.
+
+     Objects other than tasks may under some circumstances also be subjects.
+     For instance an open file may send SIGIO to a task using the UID and EUID
+     given to it by a task that called ``fcntl(F_SETOWN)`` upon it.  In this case,
+     the file struct will have a subjective context too.
+
+ 5. The subjective context.
+
+     A subject has an additional interpretation of its credentials.  A subset
+     of its credentials forms the 'subjective context'.  The subjective context
+     is used as part of the security calculation that is carried out when a
+     subject acts.
+
+     A Linux task, for example, has the FSUID, FSGID and the supplementary
+     group list for when it is acting upon a file - which are quite separate
+     from the real UID and GID that normally form the objective context of the
+     task.
+
+ 6. Actions.
+
+     Linux has a number of actions available that a subject may perform upon an
+     object.  The set of actions available depends on the nature of the subject
+     and the object.
+
+     Actions include reading, writing, creating and deleting files; forking or
+     signalling and tracing tasks.
+
+ 7. Rules, access control lists and security calculations.
+
+     When a subject acts upon an object, a security calculation is made.  This
+     involves taking the subjective context, the objective context and the
+     action, and searching one or more sets of rules to see whether the subject
+     is granted or denied permission to act in the desired manner on the
+     object, given those contexts.
+
+     There are two main sources of rules:
+
+     a. Discretionary access control (DAC):
+
+	 Sometimes the object will include sets of rules as part of its
+	 description.  This is an 'Access Control List' or 'ACL'.  A Linux
+	 file may supply more than one ACL.
+
+	 A traditional UNIX file, for example, includes a permissions mask that
+	 is an abbreviated ACL with three fixed classes of subject ('user',
+	 'group' and 'other'), each of which may be granted certain privileges
+	 ('read', 'write' and 'execute' - whatever those map to for the object
+	 in question).  UNIX file permissions do not allow the arbitrary
+	 specification of subjects, however, and so are of limited use.
+
+	 A Linux file might also sport a POSIX ACL.  This is a list of rules
+	 that grants various permissions to arbitrary subjects.
+
+     b. Mandatory access control (MAC):
+
+	 The system as a whole may have one or more sets of rules that get
+	 applied to all subjects and objects, regardless of their source.
+	 SELinux and Smack are examples of this.
+
+	 In the case of SELinux and Smack, each object is given a label as part
+	 of its credentials.  When an action is requested, they take the
+	 subject label, the object label and the action and look for a rule
+	 that says that this action is either granted or denied.
+
+
+Types of Credentials
+====================
+
+The Linux kernel supports the following types of credentials:
+
+ 1. Traditional UNIX credentials.
+
+	- Real User ID
+	- Real Group ID
+
+     The UID and GID are carried by most, if not all, Linux objects, even if in
+     some cases it has to be invented (FAT or CIFS files for example, which are
+     derived from Windows).  These (mostly) define the objective context of
+     that object, with tasks being slightly different in some cases.
+
+	- Effective, Saved and FS User ID
+	- Effective, Saved and FS Group ID
+	- Supplementary groups
+
+     These are additional credentials used by tasks only.  Usually, an
+     EUID/EGID/GROUPS will be used as the subjective context, and real UID/GID
+     will be used as the objective.  For tasks, it should be noted that this is
+     not always true.
+
+ 2. Capabilities.
+
+	- Set of permitted capabilities
+	- Set of inheritable capabilities
+	- Set of effective capabilities
+	- Capability bounding set
+
+     These are only carried by tasks.  They indicate superior capabilities
+     granted piecemeal to a task that an ordinary task wouldn't otherwise have.
+     These are manipulated implicitly by changes to the traditional UNIX
+     credentials, but can also be manipulated directly by the ``capset()``
+     system call.
+
+     The permitted capabilities are those caps that the process might grant
+     itself to its effective or permitted sets through ``capset()``.  This
+     inheritable set might also be so constrained.
+
+     The effective capabilities are the ones that a task is actually allowed to
+     make use of itself.
+
+     The inheritable capabilities are the ones that may get passed across
+     ``execve()``.
+
+     The bounding set limits the capabilities that may be inherited across
+     ``execve()``, especially when a binary is executed that will execute as
+     UID 0.
+
+ 3. Secure management flags (securebits).
+
+     These are only carried by tasks.  These govern the way the above
+     credentials are manipulated and inherited over certain operations such as
+     execve().  They aren't used directly as objective or subjective
+     credentials.
+
+ 4. Keys and keyrings.
+
+     These are only carried by tasks.  They carry and cache security tokens
+     that don't fit into the other standard UNIX credentials.  They are for
+     making such things as network filesystem keys available to the file
+     accesses performed by processes, without the necessity of ordinary
+     programs having to know about security details involved.
+
+     Keyrings are a special type of key.  They carry sets of other keys and can
+     be searched for the desired key.  Each process may subscribe to a number
+     of keyrings:
+
+	- Per-thread keyring
+	- Per-process keyring
+	- Per-session keyring
+
+     When a process accesses a key, if not already present, it will normally be
+     cached on one of these keyrings for future accesses to find.
+
+     For more information on using keys, see ``Documentation/security/keys/*``.
+
+ 5. LSM
+
+     The Linux Security Module allows extra controls to be placed over the
+     operations that a task may do.  Currently Linux supports several LSM
+     options.
+
+     Some work by labelling the objects in a system and then applying sets of
+     rules (policies) that say what operations a task with one label may do to
+     an object with another label.
+
+ 6. AF_KEY
+
+     This is a socket-based approach to credential management for networking
+     stacks [RFC 2367].  It isn't discussed by this document as it doesn't
+     interact directly with task and file credentials; rather it keeps system
+     level credentials.
+
+
+When a file is opened, part of the opening task's subjective context is
+recorded in the file struct created.  This allows operations using that file
+struct to use those credentials instead of the subjective context of the task
+that issued the operation.  An example of this would be a file opened on a
+network filesystem where the credentials of the opened file should be presented
+to the server, regardless of who is actually doing a read or a write upon it.
+
+
+File Markings
+=============
+
+Files on disk or obtained over the network may have annotations that form the
+objective security context of that file.  Depending on the type of filesystem,
+this may include one or more of the following:
+
+ * UNIX UID, GID, mode;
+ * Windows user ID;
+ * Access control list;
+ * LSM security label;
+ * UNIX exec privilege escalation bits (SUID/SGID);
+ * File capabilities exec privilege escalation bits.
+
+These are compared to the task's subjective security context, and certain
+operations allowed or disallowed as a result.  In the case of execve(), the
+privilege escalation bits come into play, and may allow the resulting process
+extra privileges, based on the annotations on the executable file.
+
+
+Task Credentials
+================
+
+In Linux, all of a task's credentials are held in (uid, gid) or through
+(groups, keys, LSM security) a refcounted structure of type 'struct cred'.
+Each task points to its credentials by a pointer called 'cred' in its
+task_struct.
+
+Once a set of credentials has been prepared and committed, it may not be
+changed, barring the following exceptions:
+
+ 1. its reference count may be changed;
+
+ 2. the reference count on the group_info struct it points to may be changed;
+
+ 3. the reference count on the security data it points to may be changed;
+
+ 4. the reference count on any keyrings it points to may be changed;
+
+ 5. any keyrings it points to may be revoked, expired or have their security
+    attributes changed; and
+
+ 6. the contents of any keyrings to which it points may be changed (the whole
+    point of keyrings being a shared set of credentials, modifiable by anyone
+    with appropriate access).
+
+To alter anything in the cred struct, the copy-and-replace principle must be
+adhered to.  First take a copy, then alter the copy and then use RCU to change
+the task pointer to make it point to the new copy.  There are wrappers to aid
+with this (see below).
+
+A task may only alter its _own_ credentials; it is no longer permitted for a
+task to alter another's credentials.  This means the ``capset()`` system call
+is no longer permitted to take any PID other than the one of the current
+process. Also ``keyctl_instantiate()`` and ``keyctl_negate()`` functions no
+longer permit attachment to process-specific keyrings in the requesting
+process as the instantiating process may need to create them.
+
+
+Immutable Credentials
+---------------------
+
+Once a set of credentials has been made public (by calling ``commit_creds()``
+for example), it must be considered immutable, barring two exceptions:
+
+ 1. The reference count may be altered.
+
+ 2. While the keyring subscriptions of a set of credentials may not be
+    changed, the keyrings subscribed to may have their contents altered.
+
+To catch accidental credential alteration at compile time, struct task_struct
+has _const_ pointers to its credential sets, as does struct file.  Furthermore,
+certain functions such as ``get_cred()`` and ``put_cred()`` operate on const
+pointers, thus rendering casts unnecessary, but require to temporarily ditch
+the const qualification to be able to alter the reference count.
+
+
+Accessing Task Credentials
+--------------------------
+
+A task being able to alter only its own credentials permits the current process
+to read or replace its own credentials without the need for any form of locking
+-- which simplifies things greatly.  It can just call::
+
+	const struct cred *current_cred()
+
+to get a pointer to its credentials structure, and it doesn't have to release
+it afterwards.
+
+There are convenience wrappers for retrieving specific aspects of a task's
+credentials (the value is simply returned in each case)::
+
+	uid_t current_uid(void)		Current's real UID
+	gid_t current_gid(void)		Current's real GID
+	uid_t current_euid(void)	Current's effective UID
+	gid_t current_egid(void)	Current's effective GID
+	uid_t current_fsuid(void)	Current's file access UID
+	gid_t current_fsgid(void)	Current's file access GID
+	kernel_cap_t current_cap(void)	Current's effective capabilities
+	struct user_struct *current_user(void)  Current's user account
+
+There are also convenience wrappers for retrieving specific associated pairs of
+a task's credentials::
+
+	void current_uid_gid(uid_t *, gid_t *);
+	void current_euid_egid(uid_t *, gid_t *);
+	void current_fsuid_fsgid(uid_t *, gid_t *);
+
+which return these pairs of values through their arguments after retrieving
+them from the current task's credentials.
+
+
+In addition, there is a function for obtaining a reference on the current
+process's current set of credentials::
+
+	const struct cred *get_current_cred(void);
+
+and functions for getting references to one of the credentials that don't
+actually live in struct cred::
+
+	struct user_struct *get_current_user(void);
+	struct group_info *get_current_groups(void);
+
+which get references to the current process's user accounting structure and
+supplementary groups list respectively.
+
+Once a reference has been obtained, it must be released with ``put_cred()``,
+``free_uid()`` or ``put_group_info()`` as appropriate.
+
+
+Accessing Another Task's Credentials
+------------------------------------
+
+While a task may access its own credentials without the need for locking, the
+same is not true of a task wanting to access another task's credentials.  It
+must use the RCU read lock and ``rcu_dereference()``.
+
+The ``rcu_dereference()`` is wrapped by::
+
+	const struct cred *__task_cred(struct task_struct *task);
+
+This should be used inside the RCU read lock, as in the following example::
+
+	void foo(struct task_struct *t, struct foo_data *f)
+	{
+		const struct cred *tcred;
+		...
+		rcu_read_lock();
+		tcred = __task_cred(t);
+		f->uid = tcred->uid;
+		f->gid = tcred->gid;
+		f->groups = get_group_info(tcred->groups);
+		rcu_read_unlock();
+		...
+	}
+
+Should it be necessary to hold another task's credentials for a long period of
+time, and possibly to sleep while doing so, then the caller should get a
+reference on them using::
+
+	const struct cred *get_task_cred(struct task_struct *task);
+
+This does all the RCU magic inside of it.  The caller must call put_cred() on
+the credentials so obtained when they're finished with.
+
+.. note::
+   The result of ``__task_cred()`` should not be passed directly to
+   ``get_cred()`` as this may race with ``commit_cred()``.
+
+There is a convenience function to access bits of another task's credentials,
+hiding the RCU magic from the caller::
+
+	uid_t task_uid(task)		Task's real UID
+
+If the caller is holding the RCU read lock at the time anyway, then::
+
+	__task_cred(task)->uid
+
+should be used instead.  Similarly, if multiple aspects of a task's credentials
+need to be accessed, RCU read lock should be used, ``__task_cred()`` called,
+the result stored in a temporary pointer and then the credential aspects called
+from that before dropping the lock.  This prevents the potentially expensive
+RCU magic from being invoked multiple times.
+
+Should some other single aspect of another task's credentials need to be
+accessed, then this can be used::
+
+	task_cred_xxx(task, member)
+
+where 'member' is a non-pointer member of the cred struct.  For instance::
+
+	uid_t task_cred_xxx(task, suid);
+
+will retrieve 'struct cred::suid' from the task, doing the appropriate RCU
+magic.  This may not be used for pointer members as what they point to may
+disappear the moment the RCU read lock is dropped.
+
+
+Altering Credentials
+--------------------
+
+As previously mentioned, a task may only alter its own credentials, and may not
+alter those of another task.  This means that it doesn't need to use any
+locking to alter its own credentials.
+
+To alter the current process's credentials, a function should first prepare a
+new set of credentials by calling::
+
+	struct cred *prepare_creds(void);
+
+this locks current->cred_replace_mutex and then allocates and constructs a
+duplicate of the current process's credentials, returning with the mutex still
+held if successful.  It returns NULL if not successful (out of memory).
+
+The mutex prevents ``ptrace()`` from altering the ptrace state of a process
+while security checks on credentials construction and changing is taking place
+as the ptrace state may alter the outcome, particularly in the case of
+``execve()``.
+
+The new credentials set should be altered appropriately, and any security
+checks and hooks done.  Both the current and the proposed sets of credentials
+are available for this purpose as current_cred() will return the current set
+still at this point.
+
+When replacing the group list, the new list must be sorted before it
+is added to the credential, as a binary search is used to test for
+membership.  In practice, this means groups_sort() should be
+called before set_groups() or set_current_groups().
+groups_sort() must not be called on a ``struct group_list`` which
+is shared as it may permute elements as part of the sorting process
+even if the array is already sorted.
+
+When the credential set is ready, it should be committed to the current process
+by calling::
+
+	int commit_creds(struct cred *new);
+
+This will alter various aspects of the credentials and the process, giving the
+LSM a chance to do likewise, then it will use ``rcu_assign_pointer()`` to
+actually commit the new credentials to ``current->cred``, it will release
+``current->cred_replace_mutex`` to allow ``ptrace()`` to take place, and it
+will notify the scheduler and others of the changes.
+
+This function is guaranteed to return 0, so that it can be tail-called at the
+end of such functions as ``sys_setresuid()``.
+
+Note that this function consumes the caller's reference to the new credentials.
+The caller should _not_ call ``put_cred()`` on the new credentials afterwards.
+
+Furthermore, once this function has been called on a new set of credentials,
+those credentials may _not_ be changed further.
+
+
+Should the security checks fail or some other error occur after
+``prepare_creds()`` has been called, then the following function should be
+invoked::
+
+	void abort_creds(struct cred *new);
+
+This releases the lock on ``current->cred_replace_mutex`` that
+``prepare_creds()`` got and then releases the new credentials.
+
+
+A typical credentials alteration function would look something like this::
+
+	int alter_suid(uid_t suid)
+	{
+		struct cred *new;
+		int ret;
+
+		new = prepare_creds();
+		if (!new)
+			return -ENOMEM;
+
+		new->suid = suid;
+		ret = security_alter_suid(new);
+		if (ret < 0) {
+			abort_creds(new);
+			return ret;
+		}
+
+		return commit_creds(new);
+	}
+
+
+Managing Credentials
+--------------------
+
+There are some functions to help manage credentials:
+
+ - ``void put_cred(const struct cred *cred);``
+
+     This releases a reference to the given set of credentials.  If the
+     reference count reaches zero, the credentials will be scheduled for
+     destruction by the RCU system.
+
+ - ``const struct cred *get_cred(const struct cred *cred);``
+
+     This gets a reference on a live set of credentials, returning a pointer to
+     that set of credentials.
+
+
+Open File Credentials
+=====================
+
+When a new file is opened, a reference is obtained on the opening task's
+credentials and this is attached to the file struct as ``f_cred`` in place of
+``f_uid`` and ``f_gid``.  Code that used to access ``file->f_uid`` and
+``file->f_gid`` should now access ``file->f_cred->fsuid`` and
+``file->f_cred->fsgid``.
+
+It is safe to access ``f_cred`` without the use of RCU or locking because the
+pointer will not change over the lifetime of the file struct, and nor will the
+contents of the cred struct pointed to, barring the exceptions listed above
+(see the Task Credentials section).
+
+To avoid "confused deputy" privilege escalation attacks, access control checks
+during subsequent operations on an opened file should use these credentials
+instead of "current"'s credentials, as the file may have been passed to a more
+privileged process.
+
+Overriding the VFS's Use of Credentials
+=======================================
+
+Under some circumstances it is desirable to override the credentials used by
+the VFS, and that can be done by calling into such as ``vfs_mkdir()`` with a
+different set of credentials.  This is done in the following places:
+
+ * ``sys_faccessat()``.
+ * ``vfs_coredump()``.
+ * nfs4recover.c.