From 5f8cf712582617d523120df67d392059eaf2fc4b Mon Sep 17 00:00:00 2001
From: Hui Peng <benquike@gmail.com>
Date: Mon, 3 Dec 2018 16:09:34 +0100
Subject: [PATCH] ALSA: usb-audio: Fix UAF decrement if card has no live
 interfaces in card.c
References: CVE-2018-19824,bsc#1118152
Git-commit: 5f8cf712582617d523120df67d392059eaf2fc4b
Patch-mainline: v4.20-rc6

If a USB sound card reports 0 interfaces, an error condition is triggered
and the function usb_audio_probe errors out. In the error path, there was a
use-after-free vulnerability where the memory object of the card was first
freed, followed by a decrement of the number of active chips. Moving the
decrement above the atomic_dec fixes the UAF.

[ The original problem was introduced in 3.1 kernel, while it was
  developed in a different form.  The Fixes tag below indicates the
  original commit but it doesn't mean that the patch is applicable
  cleanly. -- tiwai ]

Fixes: 362e4e49abe5 ("ALSA: usb-audio - clear chip->probing on error exit")
Reported-by: Hui Peng <benquike@gmail.com>
Reported-by: Mathias Payer <mathias.payer@nebelwelt.net>
Signed-off-by: Hui Peng <benquike@gmail.com>
Signed-off-by: Mathias Payer <mathias.payer@nebelwelt.net>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

---
 sound/usb/card.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/sound/usb/card.c b/sound/usb/card.c
index 2bfe4e80a6b9..a105947eaf55 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -682,9 +682,12 @@ static int usb_audio_probe(struct usb_interface *intf,
 
  __error:
 	if (chip) {
+		/* chip->active is inside the chip->card object,
+		 * decrement before memory is possibly returned.
+		 */
+		atomic_dec(&chip->active);
 		if (!chip->num_interfaces)
 			snd_card_free(chip->card);
-		atomic_dec(&chip->active);
 	}
 	mutex_unlock(&register_mutex);
 	return err;
-- 
2.19.2