ALSA: pcm: Fix races among concurrent hw_params and hw_free calls

Currently we have neither proper check nor protection against the
concurrent calls of PCM hw_params and hw_free ioctls, which may result
in a UAF.  Since the existing PCM stream lock can't be used for
protecting the whole ioctl operations, we need a new mutex to protect
those racy calls.

This patch introduced a new mutex, runtime->buffer_mutex, and applies
it to both hw_params and hw_free ioctl code paths.  Along with it, the
both functions are slightly modified (the mmap_count check is moved
into the state-check block) for code simplicity.
Reported-by: Hu Jiahui <kirin.say@gmail.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20220322170720.3529-2-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/pcm.h

@@ -401,6 +401,7 @@ struct snd_pcm_runtime {
 	wait_queue_head_t tsleep;	/* transfer sleep */
 	struct fasync_struct *fasync;
 	bool stop_operating;		/* sync_stop will be called */
+	struct mutex buffer_mutex;	/* protect for buffer changes */
 
 	/* -- private section -- */
 	void *private_data;

sound/core/pcm.c

@@ -969,6 +969,7 @@ int snd_pcm_attach_substream(struct snd_pcm *pcm, int stream,
 	init_waitqueue_head(&runtime->tsleep);
 
 	runtime->status->state = SNDRV_PCM_STATE_OPEN;
+	mutex_init(&runtime->buffer_mutex);
 
 	substream->runtime = runtime;
 	substream->private_data = pcm->private_data;
@@ -1002,6 +1003,7 @@ void snd_pcm_detach_substream(struct snd_pcm_substream *substream)
 	} else {
 		substream->runtime = NULL;
 	}
+	mutex_destroy(&runtime->buffer_mutex);
 	kfree(runtime);
 	put_pid(substream->pid);
 	substream->pid = NULL;

sound/core/pcm_native.c

@@ -685,33 +685,40 @@ static int snd_pcm_hw_params_choose(struct snd_pcm_substream *pcm,
 	return 0;
 }
 
+#if IS_ENABLED(CONFIG_SND_PCM_OSS)
+#define is_oss_stream(substream)	((substream)->oss.oss)
+#else
+#define is_oss_stream(substream)	false
+#endif
+
 static int snd_pcm_hw_params(struct snd_pcm_substream *substream,
 			     struct snd_pcm_hw_params *params)
 {
 	struct snd_pcm_runtime *runtime;
-	int err, usecs;
+	int err = 0, usecs;
 	unsigned int bits;
 	snd_pcm_uframes_t frames;
 
 	if (PCM_RUNTIME_CHECK(substream))
 		return -ENXIO;
 	runtime = substream->runtime;
+	mutex_lock(&runtime->buffer_mutex);
 	snd_pcm_stream_lock_irq(substream);
 	switch (runtime->status->state) {
 	case SNDRV_PCM_STATE_OPEN:
 	case SNDRV_PCM_STATE_SETUP:
 	case SNDRV_PCM_STATE_PREPARED:
+		if (!is_oss_stream(substream) &&
+		    atomic_read(&substream->mmap_count))
+			err = -EBADFD;
 		break;
 	default:
-		snd_pcm_stream_unlock_irq(substream);
-		return -EBADFD;
+		err = -EBADFD;
+		break;
 	}
 	snd_pcm_stream_unlock_irq(substream);
-#if IS_ENABLED(CONFIG_SND_PCM_OSS)
-	if (!substream->oss.oss)
-#endif
-		if (atomic_read(&substream->mmap_count))
-			return -EBADFD;
+	if (err)
+		goto unlock;
 
 	snd_pcm_sync_stop(substream, true);
 
@@ -799,16 +806,21 @@ static int snd_pcm_hw_params(struct snd_pcm_substream *substream,
 	if (usecs >= 0)
 		cpu_latency_qos_add_request(&substream->latency_pm_qos_req,
 					    usecs);
-	return 0;
+	err = 0;
  _error:
+	if (err) {
 		/* hardware might be unusable from this time,
-	   so we force application to retry to set
-	   the correct hardware parameter settings */
+		 * so we force application to retry to set
+		 * the correct hardware parameter settings
+		 */
 		snd_pcm_set_state(substream, SNDRV_PCM_STATE_OPEN);
 		if (substream->ops->hw_free != NULL)
 			substream->ops->hw_free(substream);
 		if (substream->managed_buffer_alloc)
 			snd_pcm_lib_free_pages(substream);
+	}
+ unlock:
+	mutex_unlock(&runtime->buffer_mutex);
 	return err;
 }
 
@@ -848,26 +860,31 @@ static int do_hw_free(struct snd_pcm_substream *substream)
 static int snd_pcm_hw_free(struct snd_pcm_substream *substream)
 {
 	struct snd_pcm_runtime *runtime;
-	int result;
+	int result = 0;
 
 	if (PCM_RUNTIME_CHECK(substream))
 		return -ENXIO;
 	runtime = substream->runtime;
+	mutex_lock(&runtime->buffer_mutex);
 	snd_pcm_stream_lock_irq(substream);
 	switch (runtime->status->state) {
 	case SNDRV_PCM_STATE_SETUP:
 	case SNDRV_PCM_STATE_PREPARED:
+		if (atomic_read(&substream->mmap_count))
+			result = -EBADFD;
 		break;
 	default:
-		snd_pcm_stream_unlock_irq(substream);
-		return -EBADFD;
+		result = -EBADFD;
+		break;
 	}
 	snd_pcm_stream_unlock_irq(substream);
-	if (atomic_read(&substream->mmap_count))
-		return -EBADFD;
+	if (result)
+		goto unlock;
 	result = do_hw_free(substream);
 	snd_pcm_set_state(substream, SNDRV_PCM_STATE_OPEN);
 	cpu_latency_qos_remove_request(&substream->latency_pm_qos_req);
+ unlock:
+	mutex_unlock(&runtime->buffer_mutex);
 	return result;
 }