Re: [PATCH v2] media: uvcvideo: Fix 'type' check leading to overflow

[Alistair Strachan]

On Wed, Dec 19, 2018 at 12:16 AM Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
>
> Hi Alistair,
>
> Thank you for the patch.
>
> On Wednesday, 19 December 2018 03:32:48 EET Alistair Strachan wrote:
> > From: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
>
> Are you sure you don't want to keep authorship ? I've merely reviewed v1 and
> proposed an alternative implementation :-) Let me know what you would prefer
> and I'll apply this to my tree.

Whatever attribution you think is best works for me! Thank you for
picking up this patch.

> > When initially testing the Camera Terminal Descriptor wTerminalType
> > field (buffer[4]), no mask is used. Later in the function, the MSB is
> > overloaded to store the descriptor subtype, and so a mask of 0x7fff
> > is used to check the type.
> >
> > If a descriptor is specially crafted to set this overloaded bit in the
> > original wTerminalType field, the initial type check will fail (falling
> > through, without adjusting the buffer size), but the later type checks
> > will pass, assuming the buffer has been made suitably large, causing an
> > overflow.
> >
> > Avoid this problem by checking for the MSB in the wTerminalType field.
> > If the bit is set, assume the descriptor is bad, and abort parsing it.
> >
> > Originally reported here:
> > https://groups.google.com/forum/#!topic/syzkaller/Ot1fOE6v1d8
> > A similar (non-compiling) patch was provided at that time.
> >
> > Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Alistair Strachan <astrachan@xxxxxxxxxx>
> > Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> > Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
> > Cc: linux-media@xxxxxxxxxxxxxxx
> > Cc: kernel-team@xxxxxxxxxxx
> > ---
> > v2: Use an alternative fix suggested by Laurent
> >  drivers/media/usb/uvc/uvc_driver.c | 14 +++++++++++---
> >  1 file changed, 11 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/media/usb/uvc/uvc_driver.c
> > b/drivers/media/usb/uvc/uvc_driver.c index bc369a0934a3..7fde3ce642c4
> > 100644
> > --- a/drivers/media/usb/uvc/uvc_driver.c
> > +++ b/drivers/media/usb/uvc/uvc_driver.c
> > @@ -1065,11 +1065,19 @@ static int uvc_parse_standard_control(struct
> > uvc_device *dev, return -EINVAL;
> >               }
> >
> > -             /* Make sure the terminal type MSB is not null, otherwise it
> > -              * could be confused with a unit.
> > +             /*
> > +              * Reject invalid terminal types that would cause issues:
> > +              *
> > +              * - The high byte must be non-zero, otherwise it would be
> > +              *   confused with a unit.
> > +              *
> > +              * - Bit 15 must be 0, as we use it internally as a terminal
> > +              *   direction flag.
> > +              *
> > +              * Other unknown types are accepted.
> >                */
> >               type = get_unaligned_le16(&buffer[4]);
> > -             if ((type & 0xff00) == 0) {
> > +             if ((type & 0x7f00) == 0 || (type & 0x8000) != 0) {
> >                       uvc_trace(UVC_TRACE_DESCR, "device %d videocontrol "
> >                               "interface %d INPUT_TERMINAL %d has invalid "
> >                               "type 0x%04x, skipping\n", udev->devnum,
>
> --
> Regards,
>
> Laurent Pinchart
>
>
>