CVE-2026-23159: perf: sched: Fix perf crash with new is_user_task() helper

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

perf: sched: Fix perf crash with new is_user_task() helper

In order to do a user space stacktrace the current task needs to be a user
task that has executed in user space. It use to be possible to test if a
task is a user task or not by simply checking the task_struct mm field. If
it was non NULL, it was a user task and if not it was a kernel task.

But things have changed over time, and some kernel tasks now have their
own mm field.

An idea was made to instead test PF_KTHREAD and two functions were used to
wrap this check in case it became more complex to test if a task was a
user task or not[1]. But this was rejected and the C code simply checked
the PF_KTHREAD directly.

It was later found that not all kernel threads set PF_KTHREAD. The io-uring
helpers instead set PF_USER_WORKER and this needed to be added as well.

But checking the flags is still not enough. There's a very small window
when a task exits that it frees its mm field and it is set back to NULL.
If perf were to trigger at this moment, the flags test would say its a
user space task but when perf would read the mm field it would crash with
at NULL pointer dereference.

Now there are flags that can be used to test if a task is exiting, but
they are set in areas that perf may still want to profile the user space
task (to see where it exited). The only real test is to check both the
flags and the mm field.

Instead of making this modification in every location, create a new
is_user_task() helper function that does all the tests needed to know if
it is safe to read the user space memory or not.

[1] https://lore.kernel.org/all/20250425204120.639530125@goodmis.org/

The Linux kernel CVE team has assigned CVE-2026-23159 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.6.116 with commit 34b5aba8511a12fb2e9bd3124835cb4087187dac and fixed in 6.6.123 with commit d84a4836dc246b7dc244e46a08ff992956b68db0
	Issue introduced in 6.12.57 with commit 8d79f96e477c4b8e3fca2efb36b269d8960a2285 and fixed in 6.12.69 with commit 5aac392fcd3d981d7997f1a0766829e1afdeac2e
	Issue introduced in 6.18 with commit 90942f9fac05702065ff82ed0bade0d08168d4ea and fixed in 6.18.9 with commit a28fce0365e1cb9cb8c04c893b9334e5ca9d9f1c
	Issue introduced in 6.18 with commit 90942f9fac05702065ff82ed0bade0d08168d4ea and fixed in 6.19 with commit 76ed27608f7dd235b727ebbb12163438c2fbb617
	Issue introduced in 6.17.7 with commit 5050083e1a2f3e5e29cee0205c40e5864b52601d

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-23159
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	include/linux/sched.h
	kernel/events/callchain.c
	kernel/events/core.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/d84a4836dc246b7dc244e46a08ff992956b68db0
	https://git.kernel.org/stable/c/5aac392fcd3d981d7997f1a0766829e1afdeac2e
	https://git.kernel.org/stable/c/a28fce0365e1cb9cb8c04c893b9334e5ca9d9f1c
	https://git.kernel.org/stable/c/76ed27608f7dd235b727ebbb12163438c2fbb617