linux-kernel - Re: usb/sounds: slab-out-of-bounds read in snd_usb_create

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Date:   Fri, 22 Sep 2017 15:55:17 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     alsa-devel@...a-project.org,
        Arvind Yadav <arvind.yadav.cs@...il.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: usb/sounds: slab-out-of-bounds read in snd_usb_create_streams

On Fri, Sep 22, 2017 at 3:19 PM, Takashi Iwai <tiwai@...e.de> wrote:
> On Fri, 22 Sep 2017 14:56:06 +0200,
> Andrey Konovalov wrote:
>>
>> Hi!
>>
>> I've got the following report while fuzzing the kernel with syzkaller.
>>
>> On commit ebb2c2437d8008d46796902ff390653822af6cc4 (Sep 18).
>>
>> It seems that there's no check that control_header has enough space
>> for uac1_ac_header_descriptor in snd_usb_create_streams().
>
> Indeed, this might be an issue.
> Does the patch below fix the problem?

I believe it does. It does fix the crash with my reproducer.

I'm not sure if the (rest_bytes <= 0) check is actually needed though.
It seems that if snd_usb_find_csint_desc() returns a non-null value,
it'll always be within the buffer.

Tested-by: Andrey Konovalov <andreyknvl@...gle.com>

>
>
> Takashi
>
> ---
> diff --git a/sound/usb/card.c b/sound/usb/card.c
> index 3dc36d913550..fc969efbb37c 100644
> --- a/sound/usb/card.c
> +++ b/sound/usb/card.c
> @@ -221,6 +221,7 @@ static int snd_usb_create_streams(struct snd_usb_audio *chip, int ctrlif)
>         struct usb_interface_descriptor *altsd;
>         void *control_header;
>         int i, protocol;
> +       int rest_bytes;
>
>         /* find audiocontrol interface */
>         host_iface = &usb_ifnum_to_if(dev, ctrlif)->altsetting[0];
> @@ -235,6 +236,13 @@ static int snd_usb_create_streams(struct snd_usb_audio *chip, int ctrlif)
>                 return -EINVAL;
>         }
>
> +       rest_bytes = (void *)(host_iface->extra + host_iface->extralen) -
> +               control_header;
> +       if (rest_bytes <= 0) {
> +               dev_err(&dev->dev, "invalid control header\n");
> +               return -EINVAL;
> +       }
> +
>         switch (protocol) {
>         default:
>                 dev_warn(&dev->dev,
> @@ -245,11 +253,21 @@ static int snd_usb_create_streams(struct snd_usb_audio *chip, int ctrlif)
>         case UAC_VERSION_1: {
>                 struct uac1_ac_header_descriptor *h1 = control_header;
>
> +               if (rest_bytes < sizeof(*h1)) {
> +                       dev_err(&dev->dev, "too short v1 buffer descriptor\n");
> +                       return -EINVAL;
> +               }
> +
>                 if (!h1->bInCollection) {
>                         dev_info(&dev->dev, "skipping empty audio interface (v1)\n");
>                         return -EINVAL;
>                 }
>
> +               if (rest_bytes < h1->bLength) {
> +                       dev_err(&dev->dev, "invalid buffer length (v1)\n");
> +                       return -EINVAL;
> +               }
> +
>                 if (h1->bLength < sizeof(*h1) + h1->bInCollection) {
>                         dev_err(&dev->dev, "invalid UAC_HEADER (v1)\n");
>                         return -EINVAL;
>
> --
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