Code: static int _snd_timer_stop(struct snd_timer_instance * timeri,
			   int keep_flag, int event)
{
	struct snd_timer *timer;
	unsigned long flags;

	if (snd_BUG_ON(!timeri))
		return -ENXIO;

	if (timeri->flags & SNDRV_TIMER_IFLG_SLAVE) {
 		if (!keep_flag) {
 			spin_lock_irqsave(&slave_active_lock, flags);
 			timeri->flags &= ~SNDRV_TIMER_IFLG_RUNNING;
 			spin_unlock_irqrestore(&slave_active_lock, flags);
 		}
 		goto __end;
	}
	timer = timeri->timer;
	if (!timer)
		return -EINVAL;
	spin_lock_irqsave(&timer->lock, flags);
	list_del_init(&timeri->ack_list);
	list_del_init(&timeri->active_list);
	if ((timeri->flags & SNDRV_TIMER_IFLG_RUNNING) &&
	    !(--timer->running)) {
		timer->hw.stop(timer);
		if (timer->flags & SNDRV_TIMER_FLG_RESCHED) {
			timer->flags &= ~SNDRV_TIMER_FLG_RESCHED;
			snd_timer_reschedule(timer, 0);
			if (timer->flags & SNDRV_TIMER_FLG_CHANGE) {
				timer->flags &= ~SNDRV_TIMER_FLG_CHANGE;
				timer->hw.start(timer);
			}
		}
	}
	if (!keep_flag)
		timeri->flags &=
			~(SNDRV_TIMER_IFLG_RUNNING | SNDRV_TIMER_IFLG_START);
	spin_unlock_irqrestore(&timer->lock, flags);
      __end:
	if (event != SNDRV_TIMER_EVENT_RESOLUTION)
		snd_timer_notify1(timeri, event);
	return 0;
}

Vulnerability Type: DoS 
CWE ID: CWE-20
Summary: sound/core/timer.c in the Linux kernel before 4.4.1 retains certain linked lists after a close or stop action, which allows local users to cause a denial of service (system crash) via a crafted ioctl call, related to the (1) snd_timer_close and (2) _snd_timer_stop functions.
Commit Message: ALSA: timer: Harden slave timer list handling

A slave timer instance might be still accessible in a racy way while
operating the master instance as it lacks of locking.  Since the
master operation is mostly protected with timer->lock, we should cope
with it while changing the slave instance, too.  Also, some linked
lists (active_list and ack_list) of slave instances aren't unlinked
immediately at stopping or closing, and this may lead to unexpected
accesses.

This patch tries to address these issues.  It adds spin lock of
timer->lock (either from master or slave, which is equivalent) in a
few places.  For avoiding a deadlock, we ensure that the global
slave_active_lock is always locked at first before each timer lock.

Also, ack and active_list of slave instances are properly unlinked at
snd_timer_stop() and snd_timer_close().

Last but not least, remove the superfluous call of _snd_timer_stop()
at removing slave links.  This is a noop, and calling it may confuse
readers wrt locking.  Further cleanup will follow in a later patch.

Actually we've got reports of use-after-free by syzkaller fuzzer, and
this hopefully fixes these issues.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>