Re: [PATCH] media: uvcvideo: fix division by zero at stream start

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Johan,

On Tue, Oct 26, 2021 at 02:06:55PM +0200, Johan Hovold wrote:
> On Tue, Oct 26, 2021 at 02:15:20PM +0300, Laurent Pinchart wrote:
> > On Tue, Oct 26, 2021 at 11:55:05AM +0100, Kieran Bingham wrote:
> > > Quoting Johan Hovold (2021-10-26 10:55:11)
> > > > Add the missing bulk-endpoint max-packet sanity check to probe() to
> > > > avoid division by zero in uvc_alloc_urb_buffers() in case a malicious
> > > > device has broken descriptors (or when doing descriptor fuzz testing).
> > > > 
> > > > Note that USB core will reject URBs submitted for endpoints with zero
> > > > wMaxPacketSize but that drivers doing packet-size calculations still
> > > > need to handle this (cf. commit 2548288b4fb0 ("USB: Fix: Don't skip
> > > > endpoint descriptors with maxpacket=0")).
> > > > 
> > > > Fixes: c0efd232929c ("V4L/DVB (8145a): USB Video Class driver")
> > > > Cc: stable@vger.kernel.org      # 2.6.26
> > > > Signed-off-by: Johan Hovold <johan@kernel.org>
> > > > ---
> > > >  drivers/media/usb/uvc/uvc_video.c | 4 ++++
> > > >  1 file changed, 4 insertions(+)
> > > > 
> > > > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> > > > index e16464606b14..85ac5c1081b6 100644
> > > > --- a/drivers/media/usb/uvc/uvc_video.c
> > > > +++ b/drivers/media/usb/uvc/uvc_video.c
> > > > @@ -1958,6 +1958,10 @@ static int uvc_video_start_transfer(struct uvc_streaming *stream,
> > > >                 if (ep == NULL)
> > > >                         return -EIO;
> > > >  
> > > > +               /* Reject broken descriptors. */
> > > > +               if (usb_endpoint_maxp(&ep->desc) == 0)
> > > > +                       return -EIO;
> > > 
> > > Is there any value in identifying this with a specific return code like
> > > -ENODATA?
> > 
> > Going one step further, wouldn't it be better to fail probe() for those
> > devices ?
> 
> This is not how the driver works today. Look at the "return -EIO" just
> above in case the expected endpoint is missing. And similarly for the
> isochronous case for which zero wMaxPacket isn't handled until
> uvc_video_start_transfer() either (a few lines further up still).

That's a good point, it wouldn't be reasonable to ask you to fix all
that :-)

> Note however the copy-paste error in the commit message mentioning
> probe(), which is indeed where this would typically be handled.
> 
> Do you want me to resend or can you change
> 
> 	s/probe()/uvc_video_start_transfer()/
> 
> in the commit message when applying if you think this is acceptable as
> is?

I can fix this when applying.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

-- 
Regards,

Laurent Pinchart