tree e5b3fd50df32c9c9fa8b18f96079f5eb0a476888
parent 903d271a3f83826ef810a4b5dbbd9842cf0465d6
author Takashi Iwai <tiwai@suse.de> 1522701703 +0200
committer Takashi Iwai <tiwai@suse.de> 1522737400 +0200

ALSA: pcm: Fix UAF at PCM release via PCM timer access

The PCM runtime object is created and freed dynamically at PCM stream
open / close time.  This is tracked via substream->runtime, and it's
cleared at snd_pcm_detach_substream().

The runtime object assignment is protected by PCM open_mutex, so for
all PCM operations, it's safely handled.  However, each PCM substream
provides also an ALSA timer interface, and user-space can access to
this while closing a PCM substream.  This may eventually lead to a
UAF, as snd_pcm_timer_resolution() tries to access the runtime while
clearing it in other side.

Fortunately, it's the only concurrent access from the PCM timer, and
it merely reads runtime->timer_resolution field.  So, we can avoid the
race by reordering kfree() and wrapping the substream->runtime
clearance with the corresponding timer lock.

Reported-by: syzbot+8e62ff4e07aa2ce87826@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>