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    Subject[PATCH 3.16 255/305] ALSA: usb-audio: Fix UAF decrement if card has no live interfaces in card.c
    3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

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

    From: Hui Peng <benquike@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 <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>
    Signed-off-by: Takashi Iwai <tiwai@suse.de>
    Signed-off-by: Ben Hutchings <ben@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:
    \
     
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