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

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

[Norbert Szetei]

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@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Norbert Szetei <norbert@doyensec.com>
---
 sound/core/seq/seq_timer.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/sound/core/seq/seq_timer.c b/sound/core/seq/seq_timer.c
index 4cd7211ccf48..419288eec4bb 100644
--- a/sound/core/seq/seq_timer.c
+++ b/sound/core/seq/seq_timer.c
@@ -61,12 +61,23 @@ struct snd_seq_timer *snd_seq_timer_new(void)
 void snd_seq_timer_delete(struct snd_seq_timer **tmr)
 {
 	struct snd_seq_timer *t = *tmr;
-	*tmr = NULL;
+	struct snd_timer_instance *ti;
 
 	if (t == NULL) {
 		pr_debug("ALSA: seq: snd_seq_timer_delete() called with NULL timer\n");
 		return;
 	}
+
+	scoped_guard(spinlock_irq, &t->lock) {
+		ti = t->timeri;
+		t->timeri = NULL;
+	}
+	if (ti) {
+		snd_timer_close(ti);
+		snd_timer_instance_free(ti);
+	}
+
+	*tmr = NULL;
 	t->running = 0;
 
 	/* reset time */
-- 
2.55.0

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@vger.kernel.org
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Norbert Szetei <norbert@doyensec.com>

Applied now.  Thanks.


Takashi