From 69941c226dd8fe42cc330b227925a4f3d8ed7e37 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?C=C3=A1ssio=20Gabriel?= Date: Sun, 10 May 2026 21:38:00 +0800 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 ANBZ: #36180 commit 83bd62fa9620ac98d5d694bde14c50f98c8e7189 stable. commit e5c33cdc6f402eab8abd36ecf436b22c9d3a8aff upstream. 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: syzbot+8fa95c41eafbc9d2ff6f@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=8fa95c41eafbc9d2ff6f Fixes: 597603d615d2 ("ALSA: introduce the snd-aloop module for the PCM loopback") Cc: stable@vger.kernel.org Suggested-by: Takashi Iwai Signed-off-by: Cássio Gabriel Link: https://patch.msgid.link/20260424-alsa-aloop-peer-stop-uaf-v2-1-94e68101db8a@gmail.com Signed-off-by: Takashi Iwai [ 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 Signed-off-by: Greg Kroah-Hartman Fixes: CVE-2026-46090 Assisted-by: PatchPilot Signed-off-by: Dust Li --- sound/drivers/aloop.c | 40 +++++++++++++++++++++++++++------------- 1 file changed, 27 insertions(+), 13 deletions(-) diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c index 2c5f7e905ab8..3e9ee260adf2 100644 --- a/sound/drivers/aloop.c +++ b/sound/drivers/aloop.c @@ -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; @@ -994,24 +1001,29 @@ 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) @@ -1206,6 +1218,8 @@ 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; -- Gitee