[PATCH 3.16 091/366] ALSA: pcm: Fix UAF at PCM release via PCM timer access

Ben Hutchings
2018-10-14T15:25:41Z

From: Takashi Iwai <tiwai@suse.de>

commit a820ccbe21e8ce8e86c39cd1d3bc8c7d1cbb949b upstream.

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
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
sound/core/pcm.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

--- a/sound/core/pcm.c
+++ b/sound/core/pcm.c
@@ -28,6 +28,7 @@
#include <sound/core.h>
#include <sound/minors.h>
#include <sound/pcm.h>
+#include <sound/timer.h>
#include <sound/control.h>
#include <sound/info.h>

@@ -1002,8 +1003,13 @@ void snd_pcm_detach_substream(struct snd
#ifdef CONFIG_SND_PCM_XRUN_DEBUG
kfree(runtime->hwptr_log);
#endif
- kfree(runtime);
+ /* Avoid concurrent access to runtime via PCM timer interface */
+ if (substream->timer)
+ spin_lock_irq(&substream->timer->lock);
substream->runtime = NULL;
+ if (substream->timer)
+ spin_unlock_irq(&substream->timer->lock);
+ kfree(runtime);
put_pid(substream->pid);
substream->pid = NULL;
substream->pstr->substream_opened--;