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.

Fixes: c73be61cede5 ("pipe: Add general notification queue support")
Reported-by: Dae R. Jeong <threeearcat@gmail.com>
Closes: https://lore.kernel.org/all/ZT-S8Q7tyutcvu_q@dragonet/
Reported-by: AutonomousCodeSecurity@microsoft.com
Suggested-by: Dae R. Jeong <threeearcat@gmail.com>
Signed-off-by: Cen Zhang (Microsoft) <cenzhang@linux.microsoft.com>
Link: https://patch.msgid.link/20261009224715.46121-1-cenzhang@linux.microsoft.com
Reviewed-by: Bradley Morgan <brads@mainlining.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions