3.16.60-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Takashi Iwai <tiwai@xxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Takashi Iwai <tiwai@xxxxxxx> [bwh: Backported to 3.16: adjust context] Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> --- 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--;
