[SRU][T][PATCH] ALSA: usb-audio: Fix UAF decrement if card has no live interfaces in card.c

[Tyler Hicks]

On 2019-02-18 22:49:58, Hui Wang wrote:
> From: Hui Peng <benquike at gmail.com>
> 
> 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 at gmail.com>
> Reported-by: Mathias Payer <mathias.payer at nebelwelt.net>
> Signed-off-by: Hui Peng <benquike at gmail.com>
> Signed-off-by: Mathias Payer <mathias.payer at nebelwelt.net>
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Takashi Iwai <tiwai at suse.de>
> 
> CVE-2018-19824
> 
> (backported from commit 5f8cf712582617d523120df67d392059eaf2fc4b)
> Signed-off-by: Hui Wang <hui.wang at canonical.com>

This is a low risk change. It required a backport but you've got a good
understanding of the problem and what's required of the backport.

Acked-by: Tyler Hicks <tyhicks at canonical.com>

Thanks for working on this!

Tyler

> ---
>  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 75b1040ef3bb..dfbd152c0962 100644
> --- a/sound/usb/card.c
> +++ b/sound/usb/card.c
> @@ -588,9 +588,12 @@ snd_usb_audio_probe(struct usb_device *dev,
>  
>   __error:
>  	if (chip) {
> +		/* chip->probing is inside the chip->card object,
> +		 * set the value before memory is possibly returned.
> +		 */
> +		chip->probing = 0;
>  		if (!chip->num_interfaces)
>  			snd_card_free(chip->card);
> -		chip->probing = 0;
>  	}
>  	mutex_unlock(&register_mutex);
>   __err_val:
> -- 
> 2.17.1
> 
> 
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