Re: usb/core: slab-out-of-bounds read in cdc_parse_cdc_header

[Andrey Konovalov]

On Thu, Sep 21, 2017 at 4:07 PM, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Sep 21, 2017 at 03:51:44PM +0200, Andrey Konovalov wrote:
>> On Thu, Sep 21, 2017 at 10:04 AM, Greg Kroah-Hartman
>> <gregkh@xxxxxxxxxxxxxxxxxxx> 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.
>
> {sigh}  yes, you are right, counted this one wrong.
>
> With this patch, updated one below, does it fix your crash?

It does, thanks!

Tested-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>

>
> 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 < 3)) {
> +                       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;