ALSA: aloop: Fix racy access at PCM trigger

The PCM trigger callback of aloop driver tries to check the PCM state
and stop the stream of the tied substream in the corresponding cable.
Since both check and stop operations are performed outside the cable
lock, this may result in UAF when a program attempts to trigger
frequently while opening/closing the tied stream, as spotted by
fuzzers.

For addressing the UAF, this patch changes two things:
- It covers the most of code in loopback_check_format() with
cable->lock spinlock, and add the proper NULL checks. This avoids
already some racy accesses.
- In addition, now we try to check the state of the capture PCM stream
that may be stopped in this function, which was the major pain point
leading to UAF.

Reported-by: [e-mail]
Closes: https://lore.kernel.org/[e-mail]
Cc: <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/drivers/aloop.c

···
 
 static int loopback_check_format(struct loopback_cable *cable, int stream)
 {
+	struct loopback_pcm *dpcm_play, *dpcm_capt;
 	struct snd_pcm_runtime *runtime, *cruntime;
 	struct loopback_setup *setup;
 	struct snd_card *card;
+	bool stop_capture = false;
 	int check;
 
-	if (cable->valid != CABLE_VALID_BOTH) {
-		if (stream == SNDRV_PCM_STREAM_PLAYBACK)
-			goto __notify;
-		return 0;
-	}
-	runtime = cable->streams[SNDRV_PCM_STREAM_PLAYBACK]->
-							substream->runtime;
-	cruntime = cable->streams[SNDRV_PCM_STREAM_CAPTURE]->
-							substream->runtime;
-	check = runtime->format != cruntime->format ||
-		runtime->rate != cruntime->rate ||
-		runtime->channels != cruntime->channels ||
-		is_access_interleaved(runtime->access) !=
-		is_access_interleaved(cruntime->access);
-	if (!check)
-		return 0;
-	if (stream == SNDRV_PCM_STREAM_CAPTURE) {
-		return -EIO;
-	} else {
-		snd_pcm_stop(cable->streams[SNDRV_PCM_STREAM_CAPTURE]->
-					substream, SNDRV_PCM_STATE_DRAINING);
-	      __notify:
-		runtime = cable->streams[SNDRV_PCM_STREAM_PLAYBACK]->
-							substream->runtime;
-		setup = get_setup(cable->streams[SNDRV_PCM_STREAM_PLAYBACK]);
-		card = cable->streams[SNDRV_PCM_STREAM_PLAYBACK]->loopback->card;
+	scoped_guard(spinlock_irqsave, &cable->lock) {
+		dpcm_play = cable->streams[SNDRV_PCM_STREAM_PLAYBACK];
+		dpcm_capt = cable->streams[SNDRV_PCM_STREAM_CAPTURE];
+
+		if (cable->valid != CABLE_VALID_BOTH) {
+			if (stream == SNDRV_PCM_STREAM_CAPTURE || !dpcm_play)
+				return 0;
+		} else {
+			if (!dpcm_play || !dpcm_capt)
+				return -EIO;
+			runtime = dpcm_play->substream->runtime;
+			cruntime = dpcm_capt->substream->runtime;
+			if (!runtime || !cruntime)
+				return -EIO;
+			check = runtime->format != cruntime->format ||
+			runtime->rate != cruntime->rate ||
+			runtime->channels != cruntime->channels ||
+			is_access_interleaved(runtime->access) !=
+			is_access_interleaved(cruntime->access);
+			if (!check)
+				return 0;
+			if (stream == SNDRV_PCM_STREAM_CAPTURE)
+				return -EIO;
+			else if (cruntime->state == SNDRV_PCM_STATE_RUNNING)
+				stop_capture = true;
+		}
+
+		setup = get_setup(dpcm_play);
+		card = dpcm_play->loopback->card;
+		runtime = dpcm_play->substream->runtime;
 		if (setup->format != runtime->format) {
 			snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE,
 							&setup->format_id);
···
 			setup->access = runtime->access;
 		}
 	}
+
+	if (stop_capture)
+		snd_pcm_stop(dpcm_capt->substream, SNDRV_PCM_STATE_DRAINING);
+
 	return 0;
 }