[alsa-devel] sound: use-after-free in snd_timer_stop

[Takashi Iwai]

On Tue, 12 Jan 2016 11:22:09 +0100,
Dmitry Vyukov wrote:
> 
> Hello,
> 
> I've hit the following use-after-free while running syzkaller fuzzer.
> It is followed by a splat of other reports and finally kernel death.
> I wasn't able to reproduce it with a standalone C program (there is
> probably some global state  involved). But it reproduces by replaying
> fuzzer logs in a loop (you will need Go toolchain):
> 
> $ go get github.com/google/syzkaller
> $ cd $GOPATH/src/github.com/google/syzkaller
> $ make executor execprog

Just a note: I had to run "mkdir bin" beforehand.
Also for my distro, I had to remove -static.

> $ scp bin/syz-executor bin/syz-execprog your at machine
> $ scp snd_timer_stop your at machine # the attached file
> on test machine:
> $ ./syz-execprog -executor ./syz-executor -cover=0 -repeat=0 -procs=16
> snd_timer_stop

I tried this but couldn't reproduce the issue on my machine,
unfortunately.  (Instead I hit another kernel panic regarding apparmor
:)

But looking through your log, it seems like a missing race
protection.  Does the patch below work for you?


thanks,

Takashi

---
diff --git a/sound/core/seq/seq_queue.c b/sound/core/seq/seq_queue.c
index 7dfd0f429410..8ea17d0a4299 100644
--- a/sound/core/seq/seq_queue.c
+++ b/sound/core/seq/seq_queue.c
@@ -142,8 +142,10 @@ static struct snd_seq_queue *queue_new(int owner, int locked)
 static void queue_delete(struct snd_seq_queue *q)
 {
 	/* stop and release the timer */
+	mutex_lock(&q->timer_mutex);
 	snd_seq_timer_stop(q->timer);
 	snd_seq_timer_close(q);
+	mutex_unlock(&q->timer_mutex);
 	/* wait until access free */
 	snd_use_lock_sync(&q->use_lock);
 	/* release resources... */
@@ -470,7 +472,9 @@ int snd_seq_queue_timer_close(int queueid)
 	queue = queueptr(queueid);
 	if (queue == NULL)
 		return -EINVAL;
+	mutex_lock(&queue->timer_mutex);
 	snd_seq_timer_close(queue);
+	mutex_unlock(&queue->timer_mutex);
 	queuefree(queue);
 	return result;
 }