ALSA: timer: Fix race between stop and interrupt
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.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/core/timer.c b/sound/core/timer.c
index dea932a..a0405b0 100644
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -518,9 +518,13 @@
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;
}