ALSA: usb-audio: fix oops due to cleanup race when disconnecting

When a USB audio device is disconnected, snd_usb_audio_disconnect()
kills all audio URBs.  At the same time, the application, after being
notified of the disconnection, might close the device, in which case
ALSA calls the .hw_free callback, which should free the URBs too.

Commit de1b8b9 "[ALSA] Fix hang-up at disconnection of usb-audio"
prevented snd_usb_hw_free() from freeing the URBs to avoid a hang that
resulted from this race, but this introduced another race because the
URB callbacks could now be executed after snd_usb_hw_free() has
returned, and try to access already freed data.

Fix the first race by introducing a mutex to serialize the disconnect
callback and all PCM callbacks that manage URBs (hw_free and hw_params).

Reported-and-tested-by: Pierre-Louis Bossart <pierre-louis.bossart@intel.com>
Cc: <stable@kernel.org>
[CL: also serialize hw_params callback]
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/card.c

@@ -323,6 +323,7 @@ static int snd_usb_audio_create(struct usb_device *dev, int idx,
 		return -ENOMEM;
 	}
 
+	mutex_init(&chip->shutdown_mutex);
 	chip->index = idx;
 	chip->dev = dev;
 	chip->card = card;
@@ -531,6 +532,7 @@ static void snd_usb_audio_disconnect(struct usb_device *dev, void *ptr)
 	chip = ptr;
 	card = chip->card;
 	mutex_lock(&register_mutex);
+	mutex_lock(&chip->shutdown_mutex);
 	chip->shutdown = 1;
 	chip->num_interfaces--;
 	if (chip->num_interfaces <= 0) {
@@ -548,9 +550,11 @@ static void snd_usb_audio_disconnect(struct usb_device *dev, void *ptr)
 			snd_usb_mixer_disconnect(p);
 		}
 		usb_chip[chip->index] = NULL;
+		mutex_unlock(&chip->shutdown_mutex);
 		mutex_unlock(&register_mutex);
 		snd_card_free_when_closed(card);
 	} else {
+		mutex_unlock(&chip->shutdown_mutex);
 		mutex_unlock(&register_mutex);
 	}
 }

sound/usb/pcm.c

@@ -361,13 +361,15 @@ static int snd_usb_hw_params(struct snd_pcm_substream *substream,
 	}
 
 	if (changed) {
+		mutex_lock(&subs->stream->chip->shutdown_mutex);
 		/* format changed */
 		snd_usb_release_substream_urbs(subs, 0);
 		/* influenced: period_bytes, channels, rate, format, */
 		ret = snd_usb_init_substream_urbs(subs, params_period_bytes(hw_params),
 						  params_rate(hw_params),
 						  snd_pcm_format_physical_width(params_format(hw_params)) *
 							params_channels(hw_params));
+		mutex_unlock(&subs->stream->chip->shutdown_mutex);
 	}
 
 	return ret;
@@ -385,8 +387,9 @@ static int snd_usb_hw_free(struct snd_pcm_substream *substream)
 	subs->cur_audiofmt = NULL;
 	subs->cur_rate = 0;
 	subs->period_bytes = 0;
-	if (!subs->stream->chip->shutdown)
-		snd_usb_release_substream_urbs(subs, 0);
+	mutex_lock(&subs->stream->chip->shutdown_mutex);
+	snd_usb_release_substream_urbs(subs, 0);
+	mutex_unlock(&subs->stream->chip->shutdown_mutex);
 	return snd_pcm_lib_free_vmalloc_buffer(substream);
 }
 

sound/usb/usbaudio.h

@@ -36,6 +36,7 @@ struct snd_usb_audio {
 	struct snd_card *card;
 	u32 usb_id;
 	int shutdown;
+	struct mutex shutdown_mutex;
 	unsigned int txfr_quirk:1; /* Subframe boundaries on transfers */
 	int num_interfaces;
 	int num_suspended_intf;