CVE-2026-89762: apparmor: fix cred UAF caused by begin_current_label_crit_section()

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

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

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

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

apparmor: fix cred UAF caused by begin_current_label_crit_section()

AppArmor's begin_current_label_crit_section() is a scary function called
from lots of LSM hooks (in particular VFS/socket-related ones) that checks
if the label referenced by the current creds is marked FLAG_STALE, and if
so, attempts to use aa_replace_current_label() to replace the creds with an
updated version that uses a new label.

The first problem with this is that it would directly lead to UAF of
`struct cred` if anything in the kernel takes a pointer to the current
creds and accesses these past a security hook invocation that replaces
creds, like so:
```
const struct cred *cred = current_cred();
alloc_file_pseudo(...);
uid_t uid = cred->euid;
```
I don't know if anything in the kernel actually does this, but I think it
is very surprising that this pattern could lead to UAF.

The second problem is that things go wrong when aa_replace_current_label()
runs with overridden credentials. aa_replace_current_label() bails out if
`current_cred() != current_real_cred()` (mirroring the check in
proc_pid_attr_write()), but this check can't actually reliably detect
overridden credentials because the overridden creds can be the same as the
objective creds.

So in approximately the following scenario, things go wrong:

1. task begins with <creds A> (as both objective and subjective creds),
   with refcount=2
2. task grabs an extra reference on <creds A> for overriding
3. task calls override_creds(<creds A>), which returns a pointer to the old
   subjective creds (<creds A>)
4. task enters AppArmor LSM hook
5. AppArmor checks that objective/subjective creds are equal
6. AppArmor replaces both cred pointers with <creds B> and drops 2 refs on
   <creds A>
7. task leaves AppArmor LSM hook
8. task calls revert_creds(<creds A>)
9. now task->cred is <creds A> while task->real_cred is <creds B>, but the
   task_struct logically holds two references to <creds B>
10. another task drops the extra reference on <creds A> that was used for
    overriding, refcount drops to 0
11. now task->real_cred points to freed creds

At this point, any access to current_cred() will be UAF.

I have a test case where I run aa-disable on a profile while a process
using that profile is blocked on splice() from a FUSE passthrough file into
a full pipe; after the profile update, the pipe becomes empty, splice()
resumes, the credentials go out of sync, and a subsequent getuid() syscall
results in a KASAN UAF splat.

To fix this, instead of directly replacing creds, do it via task_work that
will run at the end of the current syscall. (The point in time at which the
cred replacement happens should have no correctness impact; it is just a
performance optimization to avoid unnecessarily touching the refcount of
the new label.)

Note that AppArmor still performs direct cred replacements in the
sb_pivotroot LSM hook after this change, and that direct cred replacements
can still happen in VFS ->write() callbacks via proc_pid_attr_write().

There are two options for what to do with aa_dup_task_ctx(): Either
explicitly reset new->label_replacement_pending after the entire
aa_task_ctx has been copied, or switch to manually copying members over.
I am switching to manually copying members over because that should make
bugs more obvious.

The Linux kernel CVE team has assigned CVE-2026-89762 to this issue.


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

	Issue introduced in 2.6.36 with commit c75afcd153f6147d3b094f45a1d87e5df7f4f053 and fixed in 6.12.109 with commit 361488984d668235438faac28635f3632351407f
	Issue introduced in 2.6.36 with commit c75afcd153f6147d3b094f45a1d87e5df7f4f053 and fixed in 6.18.50 with commit 587a6a92b93ec314c583bbf747413af170d42540
	Issue introduced in 2.6.36 with commit c75afcd153f6147d3b094f45a1d87e5df7f4f053 and fixed in 7.2.4 with commit 580f777d6d9fd07fc034bd1aeba5c30fd48871a2
	Issue introduced in 2.6.36 with commit c75afcd153f6147d3b094f45a1d87e5df7f4f053 and fixed in 7.3-rc1 with commit 3f4ae5fab613dca01d6a2a8210dd832e009fcf47

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-2026-89762
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:
	security/apparmor/include/cred.h
	security/apparmor/include/task.h
	security/apparmor/task.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/361488984d668235438faac28635f3632351407f
	https://git.kernel.org/stable/c/587a6a92b93ec314c583bbf747413af170d42540
	https://git.kernel.org/stable/c/580f777d6d9fd07fc034bd1aeba5c30fd48871a2
	https://git.kernel.org/stable/c/3f4ae5fab613dca01d6a2a8210dd832e009fcf47