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Date:   Thu, 21 Sep 2017 15:51:44 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jonathan Corbet <corbet@....net>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Jaejoong Kim <climbbb.kim@...il.com>,
        USB list <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: usb/core: slab-out-of-bounds read in cdc_parse_cdc_header

On Thu, Sep 21, 2017 at 10:04 AM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Thu, Sep 21, 2017 at 09:31:54AM +0200, Greg Kroah-Hartman wrote:
>> On Wed, Sep 20, 2017 at 04:45:08PM +0200, Andrey Konovalov wrote:
>> > Hi!
>> >
>> > I've got the following crash while fuzzing the kernel with syzkaller.
>> >
>> > On commit ebb2c2437d8008d46796902ff390653822af6cc4 (Sep 18).
>> >
>> > It looks like cdc_parse_cdc_header() doesn't validate buflen before
>> > accessing buffer[1], buffer[2] and so on. The only check present is
>> > while (buflen > 0).
>>
>> Ugh, you are right, let me go work on a patch, thanks for the report...
>
> Here's a first cut at a fix for this.  I think this should solve it, but
> it's early and my coffee has not fully kicked in...
>
> thanks,
>
> greg k-h
> -----------------
>
> diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
> index 4c38ea41ae96..028feaf01aa5 100644
> --- a/drivers/usb/core/message.c
> +++ b/drivers/usb/core/message.c
> @@ -2069,6 +2069,10 @@ int cdc_parse_cdc_header(struct usb_cdc_parsed_header *hdr,
>                         elength = 1;
>                         goto next_desc;
>                 }
> +               if ((buflen < elength) || (elength < 2)) {

Hi Greg,

I think this should check (elength < 3), since we access both
buffer[1] and buffer[2] after this check.

Thanks!

> +                       dev_err(&intf->dev, "invalid descriptor buffer length\n");
> +                       break;
> +               }
>                 if (buffer[1] != USB_DT_CS_INTERFACE) {
>                         dev_err(&intf->dev, "skipping garbage\n");
>                         goto next_desc;
>
> --
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