Re: [PATCH] ALSA: seq: close a re-opened queue timer in the destructor

[Takashi Iwai]

On Tue, 14 Jul 2026 10:29:23 +0200,
Norbert Szetei wrote:
> 
> queue_delete() closes the queue timer, then frees it. snd_seq_timer_close()
> clears q->timer->timeri. snd_use_lock_sync() then drains borrowers, and
> snd_seq_timer_delete() frees q->timer.
> 
> A borrower can re-open the timer inside that window. A SET_QUEUE_CLIENT
> that took a queueptr() use_lock reference before the queue was unlinked
> runs snd_seq_timer_open() after the close. Open refuses re-open only while
> timeri is set, and the close just cleared it, so it re-opens timeri.
> 
> snd_seq_timer_delete() does not close that instance. Its snd_seq_timer_stop()
> is a no-op, because running was cleared first. So it frees q->timer with the
> instance still live. The queue is freed next.
> 
> The instance stays on the global timer with callback_data pointing at the
> freed queue. A non-owner START on the unlocked queue arms it. The next tick
> derefs the freed queue in snd_seq_timer_interrupt().
> 
> Reachable by an unprivileged user with access to /dev/snd/seq. No CAP and
> no queue ownership required.
> 
> Close any lingering instance in the destructor. There, ->timeri can no
> longer change: the queue is unlinked and all use_lock borrowers have
> drained, so no snd_seq_queue_use() can re-open it. Close it before clearing
> q->timer. snd_timer_close() waits for any in-flight snd_seq_timer_interrupt()
> to finish, and that callback still reads q->timer (via snd_seq_check_queue()),
> so q->timer must stay valid until it drains.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Norbert Szetei <norbert@xxxxxxxxxxxx>

Applied now.  Thanks.


Takashi