[PATCH 3.2 35/67] ALSA: timer: Fix link corruption due to double start or stop

[Ben Hutchings]

3.2.78-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@xxxxxxx>

commit f784beb75ce82f4136f8a0960d3ee872f7109e09 upstream.

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@xxxxxxxxxxxxxx
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
[bwh: Backported to 3.2: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 sound/core/timer.c | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -443,6 +443,10 @@ static int snd_timer_start_slave(struct
 	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);
@@ -467,18 +471,26 @@ int snd_timer_start(struct snd_timer_ins
 		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;
 	if (timer == NULL)
 		return -EINVAL;
 	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;
 }
 
@@ -494,6 +506,10 @@ static int _snd_timer_stop(struct snd_ti
 	if (timeri->flags & SNDRV_TIMER_IFLG_SLAVE) {
 		if (!keep_flag) {
 			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);
@@ -505,6 +521,11 @@ static int _snd_timer_stop(struct snd_ti
 	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 ((timeri->flags & SNDRV_TIMER_IFLG_RUNNING) &&
@@ -570,10 +591,15 @@ int snd_timer_continue(struct snd_timer_
 	if (! timer)
 		return -EINVAL;
 	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;