ALSA: timer: Fix use-after-free problem

When the timer instance was add into ack_list but was not currently in
process, the user could stop it via snd_timer_stop1() without delete it
from the ack_list. Then the user could free the timer instance and when
it was actually processed UAF occurred.

This issue could be reproduced via testcase snd_timer01 in ltp - running
several instances of that testcase at the same time.

What I actually met was that the ack_list of the timer broken and the
kernel went into deadloop with irqoff. That could be detected by
hardlockup detector on board or when we run it on qemu, we could use gdb
to dump the ack_list when the console has no response.

To fix this issue, we delete the timer instance from ack_list and
active_list unconditionally in snd_timer_stop1().

Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>
Suggested-by: Takashi Iwai <tiwai@suse.de>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20211103033517.80531-1-wangwensheng4@huawei.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/timer.c

@@ -624,13 +624,13 @@ static int snd_timer_stop1(struct snd_timer_instance *timeri, bool stop)
 	if (!timer)
 		return -EINVAL;
 	spin_lock_irqsave(&timer->lock, flags);
+	list_del_init(&timeri->ack_list);
+	list_del_init(&timeri->active_list);
 	if (!(timeri->flags & (SNDRV_TIMER_IFLG_RUNNING |
 			       SNDRV_TIMER_IFLG_START))) {
 		result = -EBUSY;
 		goto unlock;
 	}
-	list_del_init(&timeri->ack_list);
-	list_del_init(&timeri->active_list);
 	if (timer->card && timer->card->shutdown)
 		goto unlock;
 	if (stop) {