From: Norbert Szetei Date: Mon, 20 Jul 2026 07:09:55 +0000 (+0200) Subject: ALSA: timer: don't re-enter an instance callback that is still running X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=70d28bfcd6224eed75986b3b987b997e59643fa4;p=thirdparty%2Fkernel%2Flinux.git ALSA: timer: don't re-enter an instance callback that is still running The userspace-driven timer (utimer) TRIGGER ioctl calls snd_timer_interrupt() directly with no serialization, so two threads triggering the same utimer can run snd_timer_interrupt() on one snd_timer concurrently. snd_timer_process_callbacks() drops timer->lock around each instance callback and marks the in-flight callback with the single SNDRV_TIMER_IFLG_CALLBACK bit; snd_timer_close_locked() waits on that bit to drain an in-flight callback before freeing the instance. The bit cannot represent two concurrent callbacks: when a second interrupt re-queues an instance whose callback is still running, both run at once, the first to finish clears the bit, and the close-path drain then frees the instance (and its callback_data) while the other callback is still live - a use-after-free reachable by any user able to open /dev/snd/timer, both via a user timer instance and via a sequencer queue timer bound to the utimer. snd_timer_interrupt() sets IFLG_CALLBACK before dropping timer->lock, so a concurrent interrupt already observes it under the lock. Skip re-queuing an instance (and its slaves) to the ack/sack list while its callback is in flight; the accumulated pticks are delivered on the next tick, so no event is lost. Fixes: 37745918e0e7 ("ALSA: timer: Introduce virtual userspace-driven timers") Cc: stable@vger.kernel.org Suggested-by: Takashi Iwai Assisted-by: Claude:claude-opus-4-8 Signed-off-by: Norbert Szetei Signed-off-by: Takashi Iwai Link: https://patch.msgid.link/6F9B6501-8E65-4265-B02C-7EFB240D1664@doyensec.com --- diff --git a/sound/core/timer.c b/sound/core/timer.c index 51c6ac4df9f4..0cd01ee72226 100644 --- a/sound/core/timer.c +++ b/sound/core/timer.c @@ -927,12 +927,15 @@ void snd_timer_interrupt(struct snd_timer * timer, unsigned long ticks_left) ack_list_head = &timer->ack_list_head; else ack_list_head = &timer->sack_list_head; - if (list_empty(&ti->ack_list)) + /* don't requeue an instance whose callback is still running */ + if (list_empty(&ti->ack_list) && + !(ti->flags & SNDRV_TIMER_IFLG_CALLBACK)) list_add_tail(&ti->ack_list, ack_list_head); list_for_each_entry(ts, &ti->slave_active_head, active_list) { ts->pticks = ti->pticks; ts->resolution = resolution; - if (list_empty(&ts->ack_list)) + if (list_empty(&ts->ack_list) && + !(ts->flags & SNDRV_TIMER_IFLG_CALLBACK)) list_add_tail(&ts->ack_list, ack_list_head); } }