ALSA: seq: Fix potential concurrent access to the deleted pool

The input pool of a client might be deleted via the resize ioctl, the
the access to it should be covered by the proper locks.  Currently the
only missing place is the call in snd_seq_ioctl_get_client_pool(), and
this patch papers over it.

Reported-by: syzbot+4a75454b9ca2777f35c7@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/seq_clientmgr.c

@@ -1835,8 +1835,7 @@ static int snd_seq_ioctl_get_client_pool(struct snd_seq_client *client,
 	if (cptr->type == USER_CLIENT) {
 		info->input_pool = cptr->data.user.fifo_pool_size;
 		info->input_free = info->input_pool;
-		if (cptr->data.user.fifo)
-			info->input_free = snd_seq_unused_cells(cptr->data.user.fifo->pool);
+		info->input_free = snd_seq_fifo_unused_cells(cptr->data.user.fifo);
 	} else {
 		info->input_pool = 0;
 		info->input_free = 0;

sound/core/seq/seq_fifo.c

@@ -263,3 +263,20 @@ int snd_seq_fifo_resize(struct snd_seq_fifo *f, int poolsize)
 
 	return 0;
 }
+
+/* get the number of unused cells safely */
+int snd_seq_fifo_unused_cells(struct snd_seq_fifo *f)
+{
+	unsigned long flags;
+	int cells;
+
+	if (!f)
+		return 0;
+
+	snd_use_lock_use(&f->use_lock);
+	spin_lock_irqsave(&f->lock, flags);
+	cells = snd_seq_unused_cells(f->pool);
+	spin_unlock_irqrestore(&f->lock, flags);
+	snd_use_lock_free(&f->use_lock);
+	return cells;
+}

sound/core/seq/seq_fifo.h

@@ -53,5 +53,7 @@ int snd_seq_fifo_poll_wait(struct snd_seq_fifo *f, struct file *file, poll_table
 /* resize pool in fifo */
 int snd_seq_fifo_resize(struct snd_seq_fifo *f, int poolsize);
 
+/* get the number of unused cells safely */
+int snd_seq_fifo_unused_cells(struct snd_seq_fifo *f);
 
 #endif