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Subject[PATCH v2] ALSA: seq: 2nd attempt at fixing race creating a q
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
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).

Reported-by: Dmitry Vyukov <>
Cc: <>
Cc: Takashi Iwai <>
Signed-off-by: Daniel Mentz <>
sound/core/seq/seq_clientmgr.c | 13 ++++---------
sound/core/seq/seq_queue.c | 14 +++++++++-----
sound/core/seq/seq_queue.h | 2 +-
3 files changed, 14 insertions(+), 15 deletions(-)

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index 272c55fe17c8..ea2d0ae85bd3 100644
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -1502,16 +1502,11 @@ static int snd_seq_ioctl_unsubscribe_port(struct snd_seq_client *client,
static int snd_seq_ioctl_create_queue(struct snd_seq_client *client, void *arg)
struct snd_seq_queue_info *info = arg;
- int result;
struct snd_seq_queue *q;

- result = snd_seq_queue_alloc(client->number, info->locked, info->flags);
- if (result < 0)
- return result;
- q = queueptr(result);
- if (q == NULL)
- return -EINVAL;
+ q = snd_seq_queue_alloc(client->number, info->locked, info->flags);
+ if (IS_ERR(q))
+ return PTR_ERR(q);

info->queue = q->queue;
info->locked = q->locked;
@@ -1521,7 +1516,7 @@ static int snd_seq_ioctl_create_queue(struct snd_seq_client *client, void *arg)
if (!info->name[0])
snprintf(info->name, sizeof(info->name), "Queue-%d", q->queue);
strlcpy(q->name, info->name, sizeof(q->name));
- queuefree(q);
+ snd_use_lock_free(&q->use_lock);

return 0;
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);
- return -ENOMEM;
+ return ERR_PTR(-ENOMEM);
- return q->queue;
+ return q;

/* delete a queue - queue must be owned by the client */
diff --git a/sound/core/seq/seq_queue.h b/sound/core/seq/seq_queue.h
index 30c8111477f6..719093489a2c 100644
--- a/sound/core/seq/seq_queue.h
+++ b/sound/core/seq/seq_queue.h
@@ -71,7 +71,7 @@ void snd_seq_queues_delete(void);

/* create new queue (constructor) */
-int snd_seq_queue_alloc(int client, int locked, unsigned int flags);
+struct snd_seq_queue *snd_seq_queue_alloc(int client, int locked, unsigned int flags);

/* delete queue (destructor) */
int snd_seq_queue_delete(int client, int queueid);
 \ /
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