CVE-2026-90179: apparmor: fix deadlock in complain-mode change_hat

CVE-2026-90179: apparmor: fix deadlock in complain-mode change_hat

[Greg Kroah-Hartman]

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

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

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

apparmor: fix deadlock in complain-mode change_hat

The use of change_hat when in complain mode can cause a deadlock
when the hat doesn't exist and a new learning profile is created
for the missing profile. This is because change_hat() has taken
the lock to search the hat list and creating the new learning
profile needs to take the lock to add it to the list.

>From the bug report:

Originally found in 7.0.0 in LTS ubuntu 26.04 with pam_apparmor + su
in complain mode set to change hats.  Then verified in newest
available vanilla kernel I've compiled to see if still present:

7.2-rc7 vanilla -> affected

checked also some other kernels:
6.18.44 vanilla -> affected
6.12.95 with debian patches -> unaffected

On systems without bug (for example 6.12.95 debian) it just prints:

aa_change_hat rc=0

On systems with bug, the executable always hangs, prints nothing and
becomes unkillable.  (And once stuck this way, it will cause any
further hat changes to also cause the changing process to get stuck)

Then in syslog you can find hint about cause:

kernel: INFO: task hat:3409 blocked for more than 483 seconds.
kernel:       Not tainted 7.2.0-rc7 #1
kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kernel: task:hat             state:D stack:0     pid:3409  tgid:3409  ppid:2605   task_flags:0x400000 flags:0x00080800
kernel: Call Trace:
kernel:  <TASK>
kernel:  __schedule+0x48f/0xfe0
kernel:  schedule+0x27/0xa0
kernel:  schedule_preempt_disabled+0x15/0x30
kernel:  __mutex_lock.constprop.0+0x569/0xa10
kernel:  aa_new_learning_profile+0x15f/0x210
kernel:  build_change_hat+0x19f/0x3b0
kernel:  change_hat.isra.0+0x5dd/0xd60
kernel:  aa_change_hat+0x2f3/0x710
kernel:  aa_setprocattr_changehat+0x121/0x1f0
kernel:  do_setattr+0x28c/0x340
kernel:  apparmor_setselfattr+0x20/0x50
kernel:  security_setselfattr+0xf6/0x110
kernel:  __x64_sys_lsm_set_self_attr+0x53/0x90
kernel:  do_syscall_64+0xdd/0x5e0
kernel:  ? __mod_memcg_lruvec_state+0xfd/0x260
kernel:  ? lruvec_stat_mod_folio+0x8d/0xd0
kernel:  ? __folio_mod_stat+0x2d/0x90
kernel:  ? map_anon_folio_pte_nopf+0xd1/0x1f0
kernel:  ? do_anonymous_page+0x184/0xa10
kernel:  ? __handle_mm_fault+0x805/0x870
kernel:  ? count_memcg_events+0xef/0x230
kernel:  ? handle_mm_fault+0x1f0/0x2f0
kernel:  ? do_user_addr_fault+0x2bb/0x7b0
kernel:  ? do_syscall_64+0x94/0x5e0
kernel:  ? exc_page_fault+0x75/0x160
kernel:  entry_SYSCALL_64_after_hwframe+0x76/0x7e
kernel: RIP: 0033:0x7f815e134c8d
kernel: RSP: 002b:00007fff6df94ea8 EFLAGS: 00000246 ORIG_RAX: 00000000000001cc
kernel: RAX: ffffffffffffffda RBX: 0000556d8c81d040 RCX: 00007f815e134c8d
kernel: RDX: 0000000000000046 RSI: 0000556d8c81d040 RDI: 0000000000000064
kernel: RBP: 00007fff6df94ef0 R08: 00007f815e212ac8 R09: 000000000000000c
kernel: R10: 0000000000000000 R11: 0000000000000246 R12: 0000556d8c81d010
kernel: R13: 0000000000000026 R14: 0000000000000046 R15: 0000000000000064
kernel:  </TASK>
kernel: INFO: task hat:3409 is blocked on a mutex likely owned by task hat:3409.

To fix the issue, lift the locking out of the core of
aa_new_learning_profile(), introduce a wrapper function that takes the
lock where needed, and have build_change_hat() call the core function
that no longer takes the lock.

In addition fix 4 other issues introduced by commit
32e92764d6f8d ("apparmor: grab ns lock and refresh when looking up changehat child profiles")
- aa_get_profile_rcu() was replaced-by: aa_get_profile without the
  accompanying rcu_dereference_protected()
- an extra aa_get_label(label) was introduced at the start of
  change_hat() without an accompanying aa_put_label() causing a
  reference count leak.
- a reference count leak was introduced in the label_is_stale(label)
  case, where the newest profile would be leaked instead of the
  label passed to the function.
- a potential UAF when the lookup walks up the tree with new_ns != ns
  the new label reference is put, and then used for the next lookup.
  The mutex_lock, will block replacement, and removal in the locked
  ns. However there are two cases where putting the reference can
  result in the label being freed even with the lock held.

  1. the label does not have a list reference (possible for temporary
     or special profiles) in which case the put can trigger the
     cleanup.
  2. the new label reference is in a different namespace, which does
     not have a lock held on it. This extends case 1 to also include
     replacement, and removal that could be occurring in the namespace
     new is in.

The Linux kernel CVE team has assigned CVE-2026-90179 to this issue.


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

	Issue introduced in 6.18.40 with commit b427061ca498309bf581bb6f8df4ffcfe4c772cd and fixed in 6.18.52 with commit 00cce5e457cb690a3b3b3d334e76e71049ec8ae5
	Issue introduced in 7.2 with commit 32e92764d6f8d251c1bca62be33793287b453a81 and fixed in 7.2.6 with commit 072fb5aeec2eac7d445bf28c7119a1d6f6a2e333
	Issue introduced in 7.2 with commit 32e92764d6f8d251c1bca62be33793287b453a81 and fixed in 7.3-rc1 with commit 4ec11f14d1d6fdda787d991b142537be7841d395
	Issue introduced in 5.10.261 with commit 862ae1fcc78aea9acab2ac87cccf6431f4682b8e
	Issue introduced in 5.15.212 with commit 65f95c1e7ce70bf339504e22473879e02026cfc4
	Issue introduced in 6.1.178 with commit 9735e300a1c9259bfe4dc5ecdba472bb25574e8a
	Issue introduced in 6.6.145 with commit 5acb878db41328dae4d3249bc87b9ee4f290a663
	Issue introduced in 6.12.97 with commit 6446dbe02f4a3592bf062b4d12105927993086af
	Issue introduced in 7.1.5 with commit 67ee65ec1a3eebdde76ccaa664d20dcf5b125a2f

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-2026-90179
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/domain.c
	security/apparmor/include/policy.h
	security/apparmor/policy.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/00cce5e457cb690a3b3b3d334e76e71049ec8ae5
	https://git.kernel.org/stable/c/072fb5aeec2eac7d445bf28c7119a1d6f6a2e333
	https://git.kernel.org/stable/c/4ec11f14d1d6fdda787d991b142537be7841d395