ALSA: seq: Fix racy pool initializations

ALSA sequencer core initializes the event pool on demand by invoking
snd_seq_pool_init() when the first write happens and the pool is
empty.  Meanwhile user can reset the pool size manually via ioctl
concurrently, and this may lead to UAF or out-of-bound accesses since
the function tries to vmalloc / vfree the buffer.

A simple fix is to just wrap the snd_seq_pool_init() call with the
recently introduced client->ioctl_mutex; as the calls for
snd_seq_pool_init() from other side are always protected with this
mutex, we can avoid the race.
Reported-by: 范龙飞 <long7573@126.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/seq_clientmgr.c

@@ -1003,7 +1003,7 @@ static ssize_t snd_seq_write(struct file *file, const char __user *buf,
 {
 	struct snd_seq_client *client = file->private_data;
 	int written = 0, len;
-	int err = -EINVAL;
+	int err;
 	struct snd_seq_event event;
 
 	if (!(snd_seq_file_flags(file) & SNDRV_SEQ_LFLG_OUTPUT))
@@ -1018,11 +1018,15 @@ static ssize_t snd_seq_write(struct file *file, const char __user *buf,
 
 	/* allocate the pool now if the pool is not allocated yet */ 
 	if (client->pool->size > 0 && !snd_seq_write_pool_allocated(client)) {
-		if (snd_seq_pool_init(client->pool) < 0)
+		mutex_lock(&client->ioctl_mutex);
+		err = snd_seq_pool_init(client->pool);
+		mutex_unlock(&client->ioctl_mutex);
+		if (err < 0)
 			return -ENOMEM;
 	}
 
 	/* only process whole events */
+	err = -EINVAL;
 	while (count >= sizeof(struct snd_seq_event)) {
 		/* Read in the event header from the user */
 		len = sizeof(event);