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Cássio Gabriel 03f52a9c17 ALSA: aloop: Fix peer runtime UAF during format-change stop

[ Upstream commit e5c33cdc6f ]

loopback_check_format() may stop the capture side when playback starts
with parameters that no longer match a running capture stream. Commit
826af7fa62 ("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: 597603d615 ("ALSA: introduce the snd-aloop module for the PCM loopback")
Cc: stable@vger.kernel.org
Suggested-by: Takashi Iwai <tiwai@suse.com>
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260424-alsa-aloop-peer-stop-uaf-v2-1-94e68101db8a@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[ used scoped_guard(spinlock_irq) instead of guard(spinlock_irq) ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>