From 362bca57f5d78220f8b5907b875961af9436e229 Mon Sep 17 00:00:00 2001
From: Robb Glasser <rglasser@google.com>
Date: Tue, 5 Dec 2017 09:16:55 -0800
Subject: [PATCH] ALSA: pcm: prevent UAF in snd_pcm_info
Git-commit: 362bca57f5d78220f8b5907b875961af9436e229
Patch-mainline: v4.15-rc3
References: bsc#1051510

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>

---
 sound/core/pcm.c | 2 ++
 1 file changed, 2 insertions(+)

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;
-- 
2.15.1