FAILED: patch "[PATCH] ALSA: aloop: Fix peer runtime UAF during format-change stop" failed to apply to 5.10-stable tree

PATCH New [email protected] 2 participants 2 messages May 4 – May 10

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[email protected]

author maintainer

2026-05-04 12:46 UTC #0 lore raw

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x e5c33cdc6f402eab8abd36ecf436b22c9d3a8aff
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026050447-aerospace-stingy-18bd@gregkh' --subject-prefix 'PATCH 5.10.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From e5c33cdc6f402eab8abd36ecf436b22c9d3a8aff Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?C=C3=A1ssio=20Gabriel?= <[email protected]>
Date: Fri, 24 Apr 2026 09:48:41 -0300
Subject: [PATCH] ALSA: aloop: Fix peer runtime UAF during format-change stop
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

loopback_check_format() may stop the capture side when playback starts with parameters that no longer match a running capture stream. Commit 826af7fa62e3 ("ALSA: aloop: Fix racy access at PCM trigger") moved the peer lookup under cable->lock, but the actual snd_pcm_stop() still runs after dropping that lock.

A concurrent close can clear the capture entry from cable->streams[] and detach or free its runtime while the playback trigger path still holds a stale peer substream pointer.

Keep a per-cable count of in-flight peer stops before dropping cable->lock, and make free_cable() wait for those stops before detaching the runtime. This preserves the existing behavior while making the peer runtime lifetime explicit.

Reported-by [email protected]

Closes https://syzkaller.appspot.com/bug?extid=8fa95c41eafbc9d2ff6f

Fixes 597603d615d2 ("ALSA: introduce the snd-aloop module for the PCM loopback")

Cc [email protected]

Signed-off-by Cássio Gabriel <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Takashi Iwai <[email protected]>

sound/drivers/aloop.c +30 ~~-13~~

index aa0d2fcb1a18..a37a1695f51c 100644
@@ -99,6 +99,9 @@ struct loopback_ops {
 struct loopback_cable {
 	spinlock_t lock;
 	struct loopback_pcm *streams[2];
+	/* in-flight peer stops running outside cable->lock */
+	atomic_t stop_count;
+	wait_queue_head_t stop_wait;
 	struct snd_pcm_hardware hw;
 	/* flags */
 	unsigned int valid;

@@ -366,8 +369,11 @@ static int loopback_check_format(struct loopback_cable *cable, int stream)
 				return 0;
 			if (stream == SNDRV_PCM_STREAM_CAPTURE)
 				return -EIO;
-			else if (cruntime->state == SNDRV_PCM_STATE_RUNNING)
+			else if (cruntime->state == SNDRV_PCM_STATE_RUNNING) {
+				/* close must not free the peer runtime below */
+				atomic_inc(&cable->stop_count);
 				stop_capture = true;
+			}
 		}
 
 		setup = get_setup(dpcm_play);

@@ -396,8 +402,11 @@ static int loopback_check_format(struct loopback_cable *cable, int stream)
 		}
 	}
 
-	if (stop_capture)
+	if (stop_capture) {
 		snd_pcm_stop(dpcm_capt->substream, SNDRV_PCM_STATE_DRAINING);
+		if (atomic_dec_and_test(&cable->stop_count))
+			wake_up(&cable->stop_wait);
+	}
 
 	return 0;
 }

@@ -1049,23 +1058,29 @@ static void free_cable(struct snd_pcm_substream *substream)
 	struct loopback *loopback = substream->private_data;
 	int dev = get_cable_index(substream);
 	struct loopback_cable *cable;
+	struct loopback_pcm *dpcm;
+	bool other_alive;
 
 	cable = loopback->cables[substream->number][dev];
 	if (!cable)
 		return;
-	if (cable->streams[!substream->stream]) {
-		/* other stream is still alive */
-		guard(spinlock_irq)(&cable->lock);
-		cable->streams[substream->stream] = NULL;
-	} else {
-		struct loopback_pcm *dpcm = substream->runtime->private_data;
 
-		if (cable->ops && cable->ops->close_cable && dpcm)
-			cable->ops->close_cable(dpcm);
-		/* free the cable */
-		loopback->cables[substream->number][dev] = NULL;
-		kfree(cable);
+	scoped_guard(spinlock_irq, &cable->lock) {
+		cable->streams[substream->stream] = NULL;
+		other_alive = cable->streams[!substream->stream];
 	}
+
+	/* Pair with the stop_count increment in loopback_check_format(). */
+	wait_event(cable->stop_wait, !atomic_read(&cable->stop_count));
+	if (other_alive)
+		return;
+
+	dpcm = substream->runtime->private_data;
+	if (cable->ops && cable->ops->close_cable && dpcm)
+		cable->ops->close_cable(dpcm);
+	/* free the cable */
+	loopback->cables[substream->number][dev] = NULL;
+	kfree(cable);
 }
 
 static int loopback_jiffies_timer_open(struct loopback_pcm *dpcm)

@@ -1260,6 +1275,8 @@ static int loopback_open(struct snd_pcm_substream *substream)
 			goto unlock;
 		}
 		spin_lock_init(&cable->lock);
+		atomic_set(&cable->stop_count, 0);
+		init_waitqueue_head(&cable->stop_wait);
 		cable->hw = loopback_pcm_hardware;
 		if (loopback->timer_source)
 			cable->ops = &loopback_snd_timer_ops;

Sasha Levin

maintainer

2026-05-10 13:38 UTC #1 lore raw

PATCH 1/1 ALSA: aloop: Fix peer runtime UAF during format-change stop

From: Cássio Gabriel <[email protected]>

[ Upstream commit e5c33cdc6f402eab8abd36ecf436b22c9d3a8aff ]

A concurrent close can clear the capture entry from cable->streams[] and detach or free its runtime while the playback trigger path still holds a stale peer substream pointer.

Reported-by [email protected]

Closes https://syzkaller.appspot.com/bug?extid=8fa95c41eafbc9d2ff6f

Fixes 597603d615d2 ("ALSA: introduce the snd-aloop module for the PCM loopback")

Cc [email protected]

Signed-off-by Cássio Gabriel <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Takashi Iwai <[email protected]>

[ collapsed inc/snd_pcm_stop/dec into the existing inline call site and used spin_lock_irq/unlock_irq instead of scoped_guard ]

Signed-off-by Sasha Levin <[email protected]>

---
 sound/drivers/aloop.c | 40 +++++++++++++++++++++++++++-------------
 1 file changed, 27 insertions(+), 13 deletions(-)

sound/drivers/aloop.c +27 ~~-13~~

index fb45a32d99cd9..314ced32efbbc 100644
@@ -99,6 +99,9 @@ struct loopback_ops {
 struct loopback_cable {
 	spinlock_t lock;
 	struct loopback_pcm *streams[2];
+	/* in-flight peer stops running outside cable->lock */
+	atomic_t stop_count;
+	wait_queue_head_t stop_wait;
 	struct snd_pcm_hardware hw;
 	/* flags */
 	unsigned int valid;

@@ -342,8 +345,12 @@ static int loopback_check_format(struct loopback_cable *cable, int stream)
 	if (stream == SNDRV_PCM_STREAM_CAPTURE) {
 		return -EIO;
 	} else {
+		/* close must not free the peer runtime below */
+		atomic_inc(&cable->stop_count);
 		snd_pcm_stop(cable->streams[SNDRV_PCM_STREAM_CAPTURE]->
 					substream, SNDRV_PCM_STATE_DRAINING);
+		if (atomic_dec_and_test(&cable->stop_count))
+			wake_up(&cable->stop_wait);
 	      __notify:
 		runtime = cable->streams[SNDRV_PCM_STREAM_PLAYBACK]->
 							substream->runtime;

@@ -995,24 +1002,29 @@ static void free_cable(struct snd_pcm_substream *substream)
 	struct loopback *loopback = substream->private_data;
 	int dev = get_cable_index(substream);
 	struct loopback_cable *cable;
+	struct loopback_pcm *dpcm;
+	bool other_alive;
 
 	cable = loopback->cables[substream->number][dev];
 	if (!cable)
 		return;
-	if (cable->streams[!substream->stream]) {
-		/* other stream is still alive */
-		spin_lock_irq(&cable->lock);
-		cable->streams[substream->stream] = NULL;
-		spin_unlock_irq(&cable->lock);
-	} else {
-		struct loopback_pcm *dpcm = substream->runtime->private_data;
 
-		if (cable->ops && cable->ops->close_cable && dpcm)
-			cable->ops->close_cable(dpcm);
-		/* free the cable */
-		loopback->cables[substream->number][dev] = NULL;
-		kfree(cable);
-	}
+	spin_lock_irq(&cable->lock);
+	cable->streams[substream->stream] = NULL;
+	other_alive = cable->streams[!substream->stream] != NULL;
+	spin_unlock_irq(&cable->lock);
+
+	/* Pair with the stop_count increment in loopback_check_format(). */
+	wait_event(cable->stop_wait, !atomic_read(&cable->stop_count));
+	if (other_alive)
+		return;
+
+	dpcm = substream->runtime->private_data;
+	if (cable->ops && cable->ops->close_cable && dpcm)
+		cable->ops->close_cable(dpcm);
+	/* free the cable */
+	loopback->cables[substream->number][dev] = NULL;
+	kfree(cable);
 }
 
 static int loopback_jiffies_timer_open(struct loopback_pcm *dpcm)

@@ -1207,6 +1219,8 @@ static int loopback_open(struct snd_pcm_substream *substream)
 			goto unlock;
 		}
 		spin_lock_init(&cable->lock);
+		atomic_set(&cable->stop_count, 0);
+		init_waitqueue_head(&cable->stop_wait);
 		cable->hw = loopback_pcm_hardware;
 		if (loopback->timer_source)
 			cable->ops = &loopback_snd_timer_ops;

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