UPSTREAM: ALSA: timer: Fix race between stop and interrupt

commit ed8b1d6d2c741ab26d60d499d7fbb7ac801f0f51 upstream. A slave timer element also unlinks at snd_timer_stop() but it takes only slave_active_lock. When a slave is assigned to a master, however, this may become a race against the master's interrupt handling, eventually resulting in a list corruption. The actual bug could be seen with a syzkaller fuzzer test case in BugLink below. As a fix, we need to take timeri->timer->lock when timer isn't NULL, i.e. assigned to a master, while the assignment to a master itself is protected by slave_active_lock. Bug: 37240993 Change-Id: Ib6eae144d5fdc92546d2210bcd6bc56454ad3e42 BugLink: http://lkml.kernel.org/r/CACT4Y+Y_Bm+7epAb=8Wi=AaWd+DYS7qawX52qxdCfOfY49vozQ@mail.gmail.com Signed-off-by: Takashi Iwai <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> Signed-off-by: Siqi Lin <[email protected]> Signed-off-by: Francisco Franco <[email protected]>
franciscofranco · May 11, 2018 · d51e81a · d51e81a
1 parent 31f4912
commit d51e81a
Showing 1 changed file with 4 additions and 0 deletions.
diff --git a/sound/core/timer.c b/sound/core/timer.c
@@ -517,9 +517,13 @@ static int _snd_timer_stop(struct snd_timer_instance *timeri, int event)
 			spin_unlock_irqrestore(&slave_active_lock, flags);
 			return -EBUSY;
 		}
+		if (timeri->timer)
+			spin_lock(&timeri->timer->lock);
 		timeri->flags &= ~SNDRV_TIMER_IFLG_RUNNING;
 		list_del_init(&timeri->ack_list);
 		list_del_init(&timeri->active_list);
+		if (timeri->timer)
+			spin_unlock(&timeri->timer->lock);
 		spin_unlock_irqrestore(&slave_active_lock, flags);
 		goto __end;
 	}