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

[Takashi Iwai]

On Tue, 12 Jan 2016 15:05:24 +0100,
Takashi Iwai wrote:
> 
> 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?

Gah, scratch this.  The second hunk causes a deadlock.
The revised patch is below (just containing the first hunk).


Takashi

---
diff --git a/sound/core/seq/seq_queue.c b/sound/core/seq/seq_queue.c
index 7dfd0f429410..0bec02e89d51 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... */