CVE-2026-99179: watch_queue: Fix note pool publication race in watch_queue_set_size()

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

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

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

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

watch_queue: Fix note pool publication race in watch_queue_set_size()

There is a race between watch_queue_set_size() and post_one_notification().
watch_queue_set_size() publishes wqueue->notes, notes_bitmap, nr_pages and
nr_notes with plain stores, and post_one_notification() reads them with
plain loads under a different lock, so nothing orders the four stores
against the poster's loads: depending on the build, notes can be stored
last (gcc 14 on x86-64 does this).  Since keyctl_watch_key() lets a watch
attach to a queue before it is sized, post_one_notification() can already
be running on another CPU at that moment: it reads nr_notes > 0, passes
its bounds check, and then reads notes while it is still NULL,
dereferencing notes[0].

An unprivileged user triggers this reliably by watching one of its own
keys through a notification pipe and racing ioctl(IOC_WATCH_QUEUE_SET_SIZE)
on that pipe against keyctl(KEYCTL_SETPERM) on the key from another CPU,
triggering:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  RIP: 0010:post_one_notification.isra.0+0xa2/0x1e0
  Call Trace:
   __post_watch_notification+0x148/0x180
   keyctl_setperm_key+0x119/0x130
   do_syscall_64+0x108/0x4c0
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
  Kernel panic - not syncing: Fatal exception in interrupt

Fix this by publishing nr_notes last with smp_store_release(), and by
reading it once with smp_load_acquire() in post_one_notification() before
the bitmap and the page array are touched.  A poster that observes a
non-zero nr_notes then also observes the pool behind it; one that observes
zero drops the notification as before.

The Linux kernel CVE team has assigned CVE-2026-99179 to this issue.


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

	Issue introduced in 5.8 with commit c73be61cede5882f9605a852414db559c0ebedfd and fixed in 6.18.59 with commit 811fb6d0b07605e8e8d93bde8748be4b3028809a
	Issue introduced in 5.8 with commit c73be61cede5882f9605a852414db559c0ebedfd and fixed in 7.2.13 with commit 9e73a6754457ffed8399b835552013b9363e51d1
	Issue introduced in 5.8 with commit c73be61cede5882f9605a852414db559c0ebedfd and fixed in 7.3 with commit 5e531d62f1f8834faf7a4eed0d41105cf6570a8f

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-2026-99179
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:
	kernel/watch_queue.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/811fb6d0b07605e8e8d93bde8748be4b3028809a
	https://git.kernel.org/stable/c/9e73a6754457ffed8399b835552013b9363e51d1
	https://git.kernel.org/stable/c/5e531d62f1f8834faf7a4eed0d41105cf6570a8f