[PATCH 4.4 049/117] ALSA: timer: Fix link corruption due to double start or stop

Greg Kroah-Hartman
2016-02-14T22:33:46Z

From: Takashi Iwai <tiwai@suse.de>

commit f784beb75ce82f4136f8a0960d3ee872f7109e09 upstream.

Although ALSA timer code got hardening for races, it still causes
use-after-free error. This is however rather a corrupted linked list,
not actually the concurrent accesses. Namely, when timer start is
triggered twice, list_add_tail() is called twice, too. This ends
up with the link corruption and triggers KASAN error.

The simplest fix would be replacing list_add_tail() with
list_move_tail(), but fundamentally it's the problem that we don't
check the double start/stop correctly. So, the right fix here is to
add the proper checks to snd_timer_start() and snd_timer_stop() (and
their variants).

BugLink: http://lkml.kernel.org/r/CACT4Y+ZyPRoMQjmawbvmCEDrkBD2BQuH7R09=eOkf5ESK8kJAw@mail.gmail.com
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
sound/core/timer.c | 30 ++++++++++++++++++++++++++++--
1 file changed, 28 insertions(+), 2 deletions(-)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -451,6 +451,10 @@ static int snd_timer_start_slave(struct
unsigned long flags;

spin_lock_irqsave(&slave_active_lock, flags);
+ if (timeri->flags & SNDRV_TIMER_IFLG_RUNNING) {
+ spin_unlock_irqrestore(&slave_active_lock, flags);
+ return -EBUSY;
+ }
timeri->flags |= SNDRV_TIMER_IFLG_RUNNING;
if (timeri->master && timeri->timer) {
spin_lock(&timeri->timer->lock);
@@ -475,7 +479,8 @@ int snd_timer_start(struct snd_timer_ins
return -EINVAL;
if (timeri->flags & SNDRV_TIMER_IFLG_SLAVE) {
result = snd_timer_start_slave(timeri);
- snd_timer_notify1(timeri, SNDRV_TIMER_EVENT_START);
+ if (result >= 0)
+ snd_timer_notify1(timeri, SNDRV_TIMER_EVENT_START);
return result;
}
timer = timeri->timer;
@@ -484,11 +489,18 @@ int snd_timer_start(struct snd_timer_ins
if (timer->card && timer->card->shutdown)
return -ENODEV;
spin_lock_irqsave(&timer->lock, flags);
+ if (timeri->flags & (SNDRV_TIMER_IFLG_RUNNING |
+ SNDRV_TIMER_IFLG_START)) {
+ result = -EBUSY;
+ goto unlock;
+ }
timeri->ticks = timeri->cticks = ticks;
timeri->pticks = 0;
result = snd_timer_start1(timer, timeri, ticks);
+ unlock:
spin_unlock_irqrestore(&timer->lock, flags);
- snd_timer_notify1(timeri, SNDRV_TIMER_EVENT_START);
+ if (result >= 0)
+ snd_timer_notify1(timeri, SNDRV_TIMER_EVENT_START);
return result;
}

@@ -502,6 +514,10 @@ static int _snd_timer_stop(struct snd_ti

if (timeri->flags & SNDRV_TIMER_IFLG_SLAVE) {
spin_lock_irqsave(&slave_active_lock, flags);
+ if (!(timeri->flags & SNDRV_TIMER_IFLG_RUNNING)) {
+ spin_unlock_irqrestore(&slave_active_lock, flags);
+ return -EBUSY;
+ }
timeri->flags &= ~SNDRV_TIMER_IFLG_RUNNING;
list_del_init(&timeri->ack_list);
list_del_init(&timeri->active_list);
@@ -512,6 +528,11 @@ static int _snd_timer_stop(struct snd_ti
if (!timer)
return -EINVAL;
spin_lock_irqsave(&timer->lock, flags);
+ if (!(timeri->flags & (SNDRV_TIMER_IFLG_RUNNING |
+ SNDRV_TIMER_IFLG_START))) {
+ spin_unlock_irqrestore(&timer->lock, flags);
+ return -EBUSY;
+ }
list_del_init(&timeri->ack_list);
list_del_init(&timeri->active_list);
if (timer->card && timer->card->shutdown) {
@@ -581,10 +602,15 @@ int snd_timer_continue(struct snd_timer_
if (timer->card && timer->card->shutdown)
return -ENODEV;
spin_lock_irqsave(&timer->lock, flags);
+ if (timeri->flags & SNDRV_TIMER_IFLG_RUNNING) {
+ result = -EBUSY;
+ goto unlock;
+ }
if (!timeri->cticks)
timeri->cticks = 1;
timeri->pticks = 0;
result = snd_timer_start1(timer, timeri, timer->sticks);
+ unlock:
spin_unlock_irqrestore(&timer->lock, flags);
snd_timer_notify1(timeri, SNDRV_TIMER_EVENT_CONTINUE);
return result;