[PATCH 3.12 044/142] ALSA: timer: Fix race between stop and interrupt

From: Jiri Slaby
Date: Wed Feb 24 2016 - 06:04:58 EST

From: Takashi Iwai <tiwai@xxxxxxx>

3.12-stable review patch. If anyone has any objections, please let me know.


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.

BugLink: http://lkml.kernel.org/r/CACT4Y+Y_Bm+7epAb=8Wi=AaWd+DYS7qawX52qxdCfOfY49vozQ@xxxxxxxxxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
sound/core/timer.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/sound/core/timer.c b/sound/core/timer.c
index ef62ffbcfa8c..d90d8f4b85fe 100644
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -508,9 +508,13 @@ 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;
+ if (timeri->timer)
+ spin_unlock(&timeri->timer->lock);
spin_unlock_irqrestore(&slave_active_lock, flags);
goto __end;