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[2a02:3100:a16a:7801:fd8c:4b37:36b4:e53a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49b9266be71sm727447725e9.2.2026.09.01.01.47.00 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Tue, 01 Sep 2026 01:47:01 -0700 (PDT) From: Karl Mehltretter To: stable@vger.kernel.org Cc: Karl Mehltretter , Greg Kroah-Hartman , Jaroslav Kysela , Takashi Iwai , =?UTF-8?q?C=C3=A1ssio=20Gabriel?= , linux-sound@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+8fa95c41eafbc9d2ff6f@syzkaller.appspotmail.com, Takashi Iwai , Sasha Levin Subject: [PATCH 6.6.y 2/2] ALSA: aloop: Fix peer runtime UAF during format-change stop Date: Tue, 1 Sep 2026 10:46:37 +0200 Message-Id: <20260901084637.66106-3-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) In-Reply-To: <20260901084637.66106-1-kmehltretter@gmail.com> References: <2026050445-detention-cussed-a8c0@gregkh> <20260901084637.66106-1-kmehltretter@gmail.com> Precedence: bulk X-Mailing-List: linux-sound@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: Cássio Gabriel [ Upstream commit e5c33cdc6f402eab8abd36ecf436b22c9d3a8aff ] 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 [ used scoped_guard(spinlock_irq) instead of guard(spinlock_irq) ] Signed-off-by: Sasha Levin [ Karl Mehltretter: 6.12.y commit 03f52a9c1704 applies to 6.1.y/6.6.y unchanged; identical patch-id. ] Assisted-by: LLM Signed-off-by: Karl Mehltretter --- sound/drivers/aloop.c | 44 +++++++++++++++++++++++++++++-------------- 1 file changed, 30 insertions(+), 14 deletions(-) diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c index 67bbadcec02b0..64870381d66ec 100644 --- a/sound/drivers/aloop.c +++ b/sound/drivers/aloop.c @@ -98,6 +98,9 @@ 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; @@ -347,8 +350,11 @@ 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); @@ -371,8 +377,11 @@ 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; } @@ -1004,24 +1013,29 @@ 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); + 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) @@ -1216,6 +1230,8 @@ 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; -- 2.53.0