ALSA: timer: Fix link corruption due to double start or stop

Although ALSA timer code got hardening for races, it still causes
use-after-free error.  This is however rather a corrupted linked list,
not actually the concurrent accesses.  Namely, when timer start is
triggered twice, list_add_tail() is called twice, too.  This ends
up with the link corruption and triggers KASAN error.

The simplest fix would be replacing list_add_tail() with
list_move_tail(), but fundamentally it's the problem that we don't
check the double start/stop correctly.  So, the right fix here is to
add the proper checks to snd_timer_start() and snd_timer_stop() (and
their variants).

BugLink: http://lkml.kernel.org/r/CACT4Y+ZyPRoMQjmawbvmCEDrkBD2BQuH7R09=eOkf5ESK8kJAw@mail.gmail.com
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/timer.c

@@ -451,6 +451,10 @@ static int snd_timer_start_slave(struct snd_timer_instance *timeri)
 	unsigned long flags;
 
 	spin_lock_irqsave(&slave_active_lock, flags);
+	if (timeri->flags & SNDRV_TIMER_IFLG_RUNNING) {
+		spin_unlock_irqrestore(&slave_active_lock, flags);
+		return -EBUSY;
+	}
 	timeri->flags |= SNDRV_TIMER_IFLG_RUNNING;
 	if (timeri->master && timeri->timer) {
 		spin_lock(&timeri->timer->lock);
@@ -475,7 +479,8 @@ int snd_timer_start(struct snd_timer_instance *timeri, unsigned int ticks)
 		return -EINVAL;
 	if (timeri->flags & SNDRV_TIMER_IFLG_SLAVE) {
 		result = snd_timer_start_slave(timeri);
-		snd_timer_notify1(timeri, SNDRV_TIMER_EVENT_START);
+		if (result >= 0)
+			snd_timer_notify1(timeri, SNDRV_TIMER_EVENT_START);
 		return result;
 	}
 	timer = timeri->timer;
@@ -484,11 +489,18 @@ int snd_timer_start(struct snd_timer_instance *timeri, unsigned int ticks)
 	if (timer->card && timer->card->shutdown)
 		return -ENODEV;
 	spin_lock_irqsave(&timer->lock, flags);
+	if (timeri->flags & (SNDRV_TIMER_IFLG_RUNNING |
+			     SNDRV_TIMER_IFLG_START)) {
+		result = -EBUSY;
+		goto unlock;
+	}
 	timeri->ticks = timeri->cticks = ticks;
 	timeri->pticks = 0;
 	result = snd_timer_start1(timer, timeri, ticks);
+ unlock:
 	spin_unlock_irqrestore(&timer->lock, flags);
-	snd_timer_notify1(timeri, SNDRV_TIMER_EVENT_START);
+	if (result >= 0)
+		snd_timer_notify1(timeri, SNDRV_TIMER_EVENT_START);
 	return result;
 }
 
@@ -502,6 +514,10 @@ static int _snd_timer_stop(struct snd_timer_instance *timeri, int event)
 
 	if (timeri->flags & SNDRV_TIMER_IFLG_SLAVE) {
 		spin_lock_irqsave(&slave_active_lock, flags);
+		if (!(timeri->flags & SNDRV_TIMER_IFLG_RUNNING)) {
+			spin_unlock_irqrestore(&slave_active_lock, flags);
+			return -EBUSY;
+		}
 		timeri->flags &= ~SNDRV_TIMER_IFLG_RUNNING;
 		list_del_init(&timeri->ack_list);
 		list_del_init(&timeri->active_list);
@@ -512,6 +528,11 @@ static int _snd_timer_stop(struct snd_timer_instance *timeri, int event)
 	if (!timer)
 		return -EINVAL;
 	spin_lock_irqsave(&timer->lock, flags);
+	if (!(timeri->flags & (SNDRV_TIMER_IFLG_RUNNING |
+			       SNDRV_TIMER_IFLG_START))) {
+		spin_unlock_irqrestore(&timer->lock, flags);
+		return -EBUSY;
+	}
 	list_del_init(&timeri->ack_list);
 	list_del_init(&timeri->active_list);
 	if (timer->card && timer->card->shutdown) {
@@ -581,10 +602,15 @@ int snd_timer_continue(struct snd_timer_instance *timeri)
 	if (timer->card && timer->card->shutdown)
 		return -ENODEV;
 	spin_lock_irqsave(&timer->lock, flags);
+	if (timeri->flags & SNDRV_TIMER_IFLG_RUNNING) {
+		result = -EBUSY;
+		goto unlock;
+	}
 	if (!timeri->cticks)
 		timeri->cticks = 1;
 	timeri->pticks = 0;
 	result = snd_timer_start1(timer, timeri, timer->sticks);
+ unlock:
 	spin_unlock_irqrestore(&timer->lock, flags);
 	snd_timer_notify1(timeri, SNDRV_TIMER_EVENT_CONTINUE);
 	return result;