The aloop driver tries to stop the pending timer via timer_del() in the trigger callback and in the close callback. The former is correct, as it's an atomic operation, while the latter expects that the timer gets really removed and proceeds the resource releases after that. But timer_del() doesn't synchronize, hence the running timer may still access the released resources.

A similar situation can be also seen in the prepare callback after trigger(STOP) where the prepare tries to re-initialize the things while a timer is still running.

The problems like the above are seen indirectly in some syzkaller reports (although it's not 100% clear whether this is the only cause, as the race condition is quite narrow and not always easy to trigger).

For addressing these issues, this patch adds the explicit alls of timer_del_sync() in some places, so that the pending timer is properly killed / synced.

Cc: stable@vger.kernel.org Signed-off-by: Takashi Iwai tiwai@suse.de --- sound/drivers/aloop.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c index 0333143a1fa7..0a08e63e9c66 100644 --- a/sound/drivers/aloop.c +++ b/sound/drivers/aloop.c @@ -192,6 +192,11 @@ static inline void loopback_timer_stop(struct loopback_pcm *dpcm) dpcm->timer.expires = 0; }

+static inline void loopback_timer_stop_sync(struct loopback_pcm *dpcm) +{ + del_timer_sync(&dpcm->timer); +} + #define CABLE_VALID_PLAYBACK (1 << SNDRV_PCM_STREAM_PLAYBACK) #define CABLE_VALID_CAPTURE (1 << SNDRV_PCM_STREAM_CAPTURE) #define CABLE_VALID_BOTH (CABLE_VALID_PLAYBACK|CABLE_VALID_CAPTURE) @@ -326,6 +331,8 @@ static int loopback_prepare(struct snd_pcm_substream *substream) struct loopback_cable *cable = dpcm->cable; int bps, salign;

+ loopback_timer_stop_sync(dpcm); + salign = (snd_pcm_format_width(runtime->format) * runtime->channels) / 8; bps = salign * runtime->rate; @@ -744,7 +751,7 @@ static int loopback_close(struct snd_pcm_substream *substream) struct loopback *loopback = substream->private_data; struct loopback_pcm *dpcm = substream->runtime->private_data;

- loopback_timer_stop(dpcm); + loopback_timer_stop_sync(dpcm); mutex_lock(&loopback->cable_lock); free_cable(substream); mutex_unlock(&loopback->cable_lock);