[PATCH 4.3 055/157] ALSA: timer: Harden slave timer list handling

From: Greg Kroah-Hartman
Date: Wed Jan 27 2016 - 13:57:40 EST

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


From: Takashi Iwai <tiwai@xxxxxxx>

commit b5a663aa426f4884c71cd8580adae73f33570f0d upstream.

A slave timer instance might be still accessible in a racy way while
operating the master instance as it lacks of locking. Since the
master operation is mostly protected with timer->lock, we should cope
with it while changing the slave instance, too. Also, some linked
lists (active_list and ack_list) of slave instances aren't unlinked
immediately at stopping or closing, and this may lead to unexpected

This patch tries to address these issues. It adds spin lock of
timer->lock (either from master or slave, which is equivalent) in a
few places. For avoiding a deadlock, we ensure that the global
slave_active_lock is always locked at first before each timer lock.

Also, ack and active_list of slave instances are properly unlinked at
snd_timer_stop() and snd_timer_close().

Last but not least, remove the superfluous call of _snd_timer_stop()
at removing slave links. This is a noop, and calling it may confuse
readers wrt locking. Further cleanup will follow in a later patch.

Actually we've got reports of use-after-free by syzkaller fuzzer, and
this hopefully fixes these issues.

Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

sound/core/timer.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -215,11 +215,13 @@ static void snd_timer_check_master(struc
slave->slave_id == master->slave_id) {
list_move_tail(&slave->open_list, &master->slave_list_head);
+ spin_lock(&master->timer->lock);
slave->master = master;
slave->timer = master->timer;
if (slave->flags & SNDRV_TIMER_IFLG_RUNNING)
+ spin_unlock(&master->timer->lock);
@@ -346,15 +348,18 @@ int snd_timer_close(struct snd_timer_ins
/* remove slave links */
+ spin_lock_irq(&slave_active_lock);
+ spin_lock(&timer->lock);
list_for_each_entry_safe(slave, tmp, &timeri->slave_list_head,
open_list) {
- spin_lock_irq(&slave_active_lock);
- _snd_timer_stop(slave, 1, SNDRV_TIMER_EVENT_RESOLUTION);
list_move_tail(&slave->open_list, &snd_timer_slave_list);
slave->master = NULL;
slave->timer = NULL;
- spin_unlock_irq(&slave_active_lock);
+ list_del_init(&slave->ack_list);
+ list_del_init(&slave->active_list);
+ spin_unlock(&timer->lock);
+ spin_unlock_irq(&slave_active_lock);
@@ -441,9 +446,12 @@ static int snd_timer_start_slave(struct

spin_lock_irqsave(&slave_active_lock, flags);
timeri->flags |= SNDRV_TIMER_IFLG_RUNNING;
- if (timeri->master)
+ if (timeri->master && timeri->timer) {
+ spin_lock(&timeri->timer->lock);
+ spin_unlock(&timeri->timer->lock);
+ }
spin_unlock_irqrestore(&slave_active_lock, flags);
return 1; /* delayed start */
@@ -489,6 +497,8 @@ static int _snd_timer_stop(struct snd_ti
if (!keep_flag) {
spin_lock_irqsave(&slave_active_lock, flags);
timeri->flags &= ~SNDRV_TIMER_IFLG_RUNNING;
+ list_del_init(&timeri->ack_list);
+ list_del_init(&timeri->active_list);
spin_unlock_irqrestore(&slave_active_lock, flags);
goto __end;