ALSA: seq: 2nd attempt at fixing race creating a queue

commit 4842e98f26dd80be3623c4714a244ba52ea096a8 ("ALSA: seq: Fix race at
creating a queue") attempted to fix a race reported by syzkaller. That
fix has been described as follows:

"
When a sequencer queue is created in snd_seq_queue_alloc(),it adds the
new queue element to the public list before referencing it.  Thus the
queue might be deleted before the call of snd_seq_queue_use(), and it
results in the use-after-free error, as spotted by syzkaller.

The fix is to reference the queue object at the right time.
"

Even with that fix in place, syzkaller reported a use-after-free error.
It specifically pointed to the last instruction "return q->queue" in
snd_seq_queue_alloc(). The pointer q is being used after kfree() has
been called on it.

It turned out that there is still a small window where a race can
happen. The window opens at
snd_seq_ioctl_create_queue()->snd_seq_queue_alloc()->queue_list_add()
and closes at
snd_seq_ioctl_create_queue()->queueptr()->snd_use_lock_use(). Between
these two calls, a different thread could delete the queue and possibly
re-create a different queue in the same location in queue_list.

This change prevents this situation by calling snd_use_lock_use() from
snd_seq_queue_alloc() prior to calling queue_list_add(). It is then the
caller's responsibility to call snd_use_lock_free(&q->use_lock).

Fixes: 4842e98f26dd ("ALSA: seq: Fix race at creating a queue")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Daniel Mentz <danielmentz@google.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 9 insertions, 5 deletions

diff --git a/sound/core/seq/seq_queue.c b/sound/core/seq/seq_queue.c
index 450c5187eecb..79e0c5604ef8 100644
--- a/sound/core/seq/seq_queue.c
+++ b/sound/core/seq/seq_queue.c
@@ -184,22 +184,26 @@ void __exit snd_seq_queues_delete(void)
 static void queue_use(struct snd_seq_queue *queue, int client, int use);
 
 /* allocate a new queue -
- * return queue index value or negative value for error
+ * return pointer to new queue or ERR_PTR(-errno) for error
+ * The new queue's use_lock is set to 1. It is the caller's responsibility to
+ * call snd_use_lock_free(&q->use_lock).
  */
-int snd_seq_queue_alloc(int client, int locked, unsigned int info_flags)
+struct snd_seq_queue *snd_seq_queue_alloc(int client, int locked, unsigned int info_flags)
 {
 	struct snd_seq_queue *q;
 
 	q = queue_new(client, locked);
 	if (q == NULL)
-		return -ENOMEM;
+		return ERR_PTR(-ENOMEM);
 	q->info_flags = info_flags;
 	queue_use(q, client, 1);
+	snd_use_lock_use(&q->use_lock);
 	if (queue_list_add(q) < 0) {
+		snd_use_lock_free(&q->use_lock);
 		queue_delete(q);
-		return -ENOMEM;
+		return ERR_PTR(-ENOMEM);
 	}
-	return q->queue;
+	return q;
 }
 
 /* delete a queue - queue must be owned by the client */