[PATCH 3.16 255/305] ALSA: usb-audio: Fix UAF decrement if card has no live interfaces in card.c

[Ben Hutchings]

3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Hui Peng <benqu...@gmail.com>

commit 5f8cf712582617d523120df67d392059eaf2fc4b upstream.

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 <benqu...@gmail.com>
Reported-by: Mathias Payer <mathias.pa...@nebelwelt.net>
Signed-off-by: Hui Peng <benqu...@gmail.com>
Signed-off-by: Mathias Payer <mathias.pa...@nebelwelt.net>
Signed-off-by: Takashi Iwai <ti...@suse.de>
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
 sound/usb/card.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -594,9 +594,12 @@ snd_usb_audio_probe(struct usb_device *d
 
  __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);
  __err_val: