x86/kprobes: Fix crash when probing CS CALL instructionsgrafted

When using eBPF to probe CS CALL instructions within a function,
a crash can be triggered.

The eBPF tool probes offset 257 of the __hrtimer_run_queues()
function:

<__hrtimer_run_queues+249>:  nopl   0x0(%rax,%rax,1)
<__hrtimer_run_queues+254>:  mov    %r14,%rdi
<__hrtimer_run_queues+257>:  cs call <__x86_indirect_thunk_r12>
<__hrtimer_run_queues+263>:  mov    %eax,%r12d
<__hrtimer_run_queues+266>:  xchg   %ax,%ax
<__hrtimer_run_queues+268>:  mov    %r13,%rdi

Which triggers this crash:

  BUG: unable to handle page fault for address: 00000000000f41c9
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP NOPTI
  CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P
  RIP: 0010:__hrtimer_run_queues+0x106/0x230

Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is
at the 6th byte of the above CS CALL instruction. Since the CS CALL
instruction occupies 6 bytes, the exception occurred in the middle of that
call instruction.

The root cause is that when using eBPF tools to probe in the middle of a
function, a kprobe with INT3 is used as the underlying implementation.

During single-step emulation of the original CALL instruction,
int3_emulate_call() assumes that the probed CALL instruction is 5 bytes
long. However, the actual CS-prefixed CALL instruction occupies 6 bytes,
so it constructs an incorrect exception return address. When the CPU
returns from the kprobe handler, the next instruction to be executed is at
the address of the last byte of that CS CALL instruction. Coincidentally,
starting from that address, the CPU fetches and decodes a completely
different instruction, which ultimately triggers a kernel crash.

Fix the issue by using the actual instruction length obtained from
the instruction decoder when constructing the exception return
address, rather than relying on the hardcoded CALL_INSN_SIZE macro.

[ mingo: Refined the changelog ]

Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Jinke Han <jinkehan@didiglobal.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000

1 files changed, 144 insertions, 0 deletions

diff --git a/Documentation/userspace-api/check_exec.rst b/Documentation/userspace-api/check_exec.rst
new file mode 100644
index 000000000..05dfe3b56
--- /dev/null
+++ b/Documentation/userspace-api/check_exec.rst
@@ -0,0 +1,144 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. Copyright © 2024 Microsoft Corporation
+
+===================
+Executability check
+===================
+
+The ``AT_EXECVE_CHECK`` :manpage:`execveat(2)` flag, and the
+``SECBIT_EXEC_RESTRICT_FILE`` and ``SECBIT_EXEC_DENY_INTERACTIVE`` securebits
+are intended for script interpreters and dynamic linkers to enforce a
+consistent execution security policy handled by the kernel.  See the
+`samples/check-exec/inc.c`_ example.
+
+Whether an interpreter should check these securebits or not depends on the
+security risk of running malicious scripts with respect to the execution
+environment, and whether the kernel can check if a script is trustworthy or
+not.  For instance, Python scripts running on a server can use arbitrary
+syscalls and access arbitrary files.  Such interpreters should then be
+enlighten to use these securebits and let users define their security policy.
+However, a JavaScript engine running in a web browser should already be
+sandboxed and then should not be able to harm the user's environment.
+
+Script interpreters or dynamic linkers built for tailored execution environments
+(e.g. hardened Linux distributions or hermetic container images) could use
+``AT_EXECVE_CHECK`` without checking the related securebits if backward
+compatibility is handled by something else (e.g. atomic update ensuring that
+all legitimate libraries are allowed to be executed).  It is then recommended
+for script interpreters and dynamic linkers to check the securebits at run time
+by default, but also to provide the ability for custom builds to behave like if
+``SECBIT_EXEC_RESTRICT_FILE`` or ``SECBIT_EXEC_DENY_INTERACTIVE`` were always
+set to 1 (i.e. always enforce restrictions).
+
+AT_EXECVE_CHECK
+===============
+
+Passing the ``AT_EXECVE_CHECK`` flag to :manpage:`execveat(2)` only performs a
+check on a regular file and returns 0 if execution of this file would be
+allowed, ignoring the file format and then the related interpreter dependencies
+(e.g. ELF libraries, script's shebang).
+
+Programs should always perform this check to apply kernel-level checks against
+files that are not directly executed by the kernel but passed to a user space
+interpreter instead.  All files that contain executable code, from the point of
+view of the interpreter, should be checked.  However the result of this check
+should only be enforced according to ``SECBIT_EXEC_RESTRICT_FILE`` or
+``SECBIT_EXEC_DENY_INTERACTIVE.``.
+
+The main purpose of this flag is to improve the security and consistency of an
+execution environment to ensure that direct file execution (e.g.
+``./script.sh``) and indirect file execution (e.g. ``sh script.sh``) lead to
+the same result.  For instance, this can be used to check if a file is
+trustworthy according to the caller's environment.
+
+In a secure environment, libraries and any executable dependencies should also
+be checked.  For instance, dynamic linking should make sure that all libraries
+are allowed for execution to avoid trivial bypass (e.g. using ``LD_PRELOAD``).
+For such secure execution environment to make sense, only trusted code should
+be executable, which also requires integrity guarantees.
+
+To avoid race conditions leading to time-of-check to time-of-use issues,
+``AT_EXECVE_CHECK`` should be used with ``AT_EMPTY_PATH`` to check against a
+file descriptor instead of a path.
+
+SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE
+==========================================================
+
+When ``SECBIT_EXEC_RESTRICT_FILE`` is set, a process should only interpret or
+execute a file if a call to :manpage:`execveat(2)` with the related file
+descriptor and the ``AT_EXECVE_CHECK`` flag succeed.
+
+This secure bit may be set by user session managers, service managers,
+container runtimes, sandboxer tools...  Except for test environments, the
+related ``SECBIT_EXEC_RESTRICT_FILE_LOCKED`` bit should also be set.
+
+Programs should only enforce consistent restrictions according to the
+securebits but without relying on any other user-controlled configuration.
+Indeed, the use case for these securebits is to only trust executable code
+vetted by the system configuration (through the kernel), so we should be
+careful to not let untrusted users control this configuration.
+
+However, script interpreters may still use user configuration such as
+environment variables as long as it is not a way to disable the securebits
+checks.  For instance, the ``PATH`` and ``LD_PRELOAD`` variables can be set by
+a script's caller.  Changing these variables may lead to unintended code
+executions, but only from vetted executable programs, which is OK.  For this to
+make sense, the system should provide a consistent security policy to avoid
+arbitrary code execution e.g., by enforcing a write xor execute policy.
+
+When ``SECBIT_EXEC_DENY_INTERACTIVE`` is set, a process should never interpret
+interactive user commands (e.g. scripts).  However, if such commands are passed
+through a file descriptor (e.g. stdin), its content should be interpreted if a
+call to :manpage:`execveat(2)` with the related file descriptor and the
+``AT_EXECVE_CHECK`` flag succeed.
+
+For instance, script interpreters called with a script snippet as argument
+should always deny such execution if ``SECBIT_EXEC_DENY_INTERACTIVE`` is set.
+
+This secure bit may be set by user session managers, service managers,
+container runtimes, sandboxer tools...  Except for test environments, the
+related ``SECBIT_EXEC_DENY_INTERACTIVE_LOCKED`` bit should also be set.
+
+Here is the expected behavior for a script interpreter according to combination
+of any exec securebits:
+
+1. ``SECBIT_EXEC_RESTRICT_FILE=0`` and ``SECBIT_EXEC_DENY_INTERACTIVE=0``
+
+   Always interpret scripts, and allow arbitrary user commands (default).
+
+   No threat, everyone and everything is trusted, but we can get ahead of
+   potential issues thanks to the call to :manpage:`execveat(2)` with
+   ``AT_EXECVE_CHECK`` which should always be performed but ignored by the
+   script interpreter.  Indeed, this check is still important to enable systems
+   administrators to verify requests (e.g. with audit) and prepare for
+   migration to a secure mode.
+
+2. ``SECBIT_EXEC_RESTRICT_FILE=1`` and ``SECBIT_EXEC_DENY_INTERACTIVE=0``
+
+   Deny script interpretation if they are not executable, but allow
+   arbitrary user commands.
+
+   The threat is (potential) malicious scripts run by trusted (and not fooled)
+   users.  That can protect against unintended script executions (e.g. ``sh
+   /tmp/*.sh``).  This makes sense for (semi-restricted) user sessions.
+
+3. ``SECBIT_EXEC_RESTRICT_FILE=0`` and ``SECBIT_EXEC_DENY_INTERACTIVE=1``
+
+   Always interpret scripts, but deny arbitrary user commands.
+
+   This use case may be useful for secure services (i.e. without interactive
+   user session) where scripts' integrity is verified (e.g.  with IMA/EVM or
+   dm-verity/IPE) but where access rights might not be ready yet.  Indeed,
+   arbitrary interactive commands would be much more difficult to check.
+
+4. ``SECBIT_EXEC_RESTRICT_FILE=1`` and ``SECBIT_EXEC_DENY_INTERACTIVE=1``
+
+   Deny script interpretation if they are not executable, and also deny
+   any arbitrary user commands.
+
+   The threat is malicious scripts run by untrusted users (but trusted code).
+   This makes sense for system services that may only execute trusted scripts.
+
+.. Links
+.. _samples/check-exec/inc.c:
+   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/samples/check-exec/inc.c