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Subject[PATCH 4.16 111/196] ALSA: pcm: Fix UAF at PCM release via PCM timer access
4.16-stable review patch.  If anyone has any objections, please let me know.


From: Takashi Iwai <>

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.

Cc: <>
Signed-off-by: Takashi Iwai <>
Signed-off-by: Greg Kroah-Hartman <>

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>

@@ -1054,8 +1055,13 @@ void snd_pcm_detach_substream(struct snd
PAGE_ALIGN(sizeof(struct snd_pcm_mmap_control)));
- 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);
substream->pid = NULL;

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