ALSA: pcm: prevent UAF in snd_pcm_info

When the device descriptor is closed, the `substream->runtime` pointer
is freed. But another thread may be in the ioctl handler, case
SNDRV_CTL_IOCTL_PCM_INFO. This case calls snd_pcm_info_user() which
calls snd_pcm_info() which accesses the now freed `substream->runtime`.

Note: this fixes CVE-2017-0861

Signed-off-by: Robb Glasser <rglasser@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 2 insertions, 0 deletions

diff --git a/sound/core/pcm.c b/sound/core/pcm.c
index 9070f277f8db..09ee8c6b9f75 100644
--- a/sound/core/pcm.c
+++ b/sound/core/pcm.c
@@ -153,7 +153,9 @@ static int snd_pcm_control_ioctl(struct snd_card *card,
                                 err = -ENXIO;
                                 goto _error;
                         }
+                        mutex_lock(&pcm->open_mutex);
                         err = snd_pcm_info_user(substream, info);
+                        mutex_unlock(&pcm->open_mutex);
                 _error:
                         mutex_unlock(&register_mutex);
                         return err;