ALSA: timer: Fix race between stop and interrupt

commit ed8b1d6d 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.

BugLink: http://lkml.kernel.org/r/CACT4Y+Y_Bm+7epAb=8Wi=AaWd+DYS7qawX52qxdCfOfY49vozQ@mail.gmail.comSigned-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.2: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

sound/core/timer.c

@@ -510,9 +510,13 @@ static int _snd_timer_stop(struct snd_timer_instance * timeri,
 				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;