From 9661bf674d6a82b76e4ae424438a8ce1e3ed855d Mon Sep 17 00:00:00 2001 From: Takashi Iwai Date: Fri, 6 May 2022 12:10:13 +0300 Subject: ALSA: pcm: Fix potential AB/BA lock with buffer_mutex and mmap_lock commit bc55cfd5718c7c23e5524582e9fa70b4d10f2433 upstream. syzbot caught a potential deadlock between the PCM runtime->buffer_mutex and the mm->mmap_lock. It was brought by the recent fix to cover the racy read/write and other ioctls, and in that commit, I overlooked a (hopefully only) corner case that may take the revert lock, namely, the OSS mmap. The OSS mmap operation exceptionally allows to re-configure the parameters inside the OSS mmap syscall, where mm->mmap_mutex is already held. Meanwhile, the copy_from/to_user calls at read/write operations also take the mm->mmap_lock internally, hence it may lead to a AB/BA deadlock. A similar problem was already seen in the past and we fixed it with a refcount (in commit b248371628aa). The former fix covered only the call paths with OSS read/write and OSS ioctls, while we need to cover the concurrent access via both ALSA and OSS APIs now. This patch addresses the problem above by replacing the buffer_mutex lock in the read/write operations with a refcount similar as we've used for OSS. The new field, runtime->buffer_accessing, keeps the number of concurrent read/write operations. Unlike the former buffer_mutex protection, this protects only around the copy_from/to_user() calls; the other codes are basically protected by the PCM stream lock. The refcount can be a negative, meaning blocked by the ioctls. If a negative value is seen, the read/write aborts with -EBUSY. In the ioctl side, OTOH, they check this refcount, too, and set to a negative value for blocking unless it's already being accessed. Reported-by: syzbot+6e5c88838328e99c7e1c@syzkaller.appspotmail.com Fixes: dca947d4d26d ("ALSA: pcm: Fix races among concurrent read/write and buffer changes") Cc: Link: https://lore.kernel.org/r/000000000000381a0d05db622a81@google.com Link: https://lore.kernel.org/r/20220330120903.4738-1-tiwai@suse.de Signed-off-by: Takashi Iwai [OP: backport to 5.4: adjusted context] Signed-off-by: Ovidiu Panait Signed-off-by: Greg Kroah-Hartman --- include/sound/pcm.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include') diff --git a/include/sound/pcm.h b/include/sound/pcm.h index 24273d0f770b..f0045f842a60 100644 --- a/include/sound/pcm.h +++ b/include/sound/pcm.h @@ -396,6 +396,7 @@ struct snd_pcm_runtime { wait_queue_head_t tsleep; /* transfer sleep */ struct fasync_struct *fasync; struct mutex buffer_mutex; /* protect for buffer changes */ + atomic_t buffer_accessing; /* >0: in r/w operation, <0: blocked */ /* -- private section -- */ void *private_data; -- cgit v1.2.3