ALSA: hda: hdac_stream: fix potential locking issue in snd_hdac_stream_assign()

The fields 'opened', 'running', 'assigned_key' are all protected by a
spinlock, but the spinlock is not taken when looking for a
stream. This can result in a possible race between assign() and
release().

Fix by taking the spinlock before walking through the bus stream list.
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210924192417.169243-2-pierre-louis.bossart@linux.intel.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/hda/hdac_stream.c

@@ -296,6 +296,7 @@ struct hdac_stream *snd_hdac_stream_assign(struct hdac_bus *bus,
 	int key = (substream->pcm->device << 16) | (substream->number << 2) |
 		(substream->stream + 1);
 
+	spin_lock_irq(&bus->reg_lock);
 	list_for_each_entry(azx_dev, &bus->stream_list, list) {
 		if (azx_dev->direction != substream->stream)
 			continue;
@@ -309,13 +310,12 @@ struct hdac_stream *snd_hdac_stream_assign(struct hdac_bus *bus,
 			res = azx_dev;
 	}
 	if (res) {
-		spin_lock_irq(&bus->reg_lock);
 		res->opened = 1;
 		res->running = 0;
 		res->assigned_key = key;
 		res->substream = substream;
-		spin_unlock_irq(&bus->reg_lock);
 	}
+	spin_unlock_irq(&bus->reg_lock);
 	return res;
 }
 EXPORT_SYMBOL_GPL(snd_hdac_stream_assign);