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 (file)
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -518,9 +518,13 @@ 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;
        }