Re: [PATCH] USB: s2255 & stkwebcam: fix oops with malicious USB descriptors

[Johan Hovold]

On Tue, Apr 16, 2019 at 01:26:45PM +0200, Johan Hovold wrote:
> On Thu, Apr 11, 2019 at 12:54:12PM +0800, Young Xiao wrote:
> > From: Young Xiao <YangX92@xxxxxxxxxxx>
> > 
> > The driver expects at least one valid endpoint. If given
> > malicious descriptors that specify 0 for the number of endpoints,
> > it will crash in the probe function.  Ensure there is at least
> > one endpoint on the interface before using it.
> 
> Why do claim it will crash?

Ok, I see now that Björn already pointed this out to you in your
updated version of this patch.

> > This vulnerability is same as CVE-2016-2188.
> 
> Note that the "fix" for this CVE that you're now copying was incomplete.
> Here's the proper fix:
> 
> 	b7321e81fc36 ("USB: iowarrior: fix NULL-deref at probe")
> 
> > Signed-off-by: Young Xiao <YangX92@xxxxxxxxxxx>
> > ---
> >  drivers/media/usb/s2255/s2255drv.c       | 7 +++++++
> >  drivers/media/usb/stkwebcam/stk-webcam.c | 6 ++++++
> >  2 files changed, 13 insertions(+)
> > 
> > diff --git a/drivers/media/usb/s2255/s2255drv.c b/drivers/media/usb/s2255/s2255drv.c
> > index 5b3e54b..7fdf159 100644
> > --- a/drivers/media/usb/s2255/s2255drv.c
> > +++ b/drivers/media/usb/s2255/s2255drv.c
> > @@ -2263,6 +2263,13 @@ static int s2255_probe(struct usb_interface *interface,
> >  	iface_desc = interface->cur_altsetting;
> >  	dev_dbg(&interface->dev, "num EP: %d\n",
> >  		iface_desc->desc.bNumEndpoints);
> > +
> > +	if (iface_desc->desc.bNumEndpoints < 1) {
> > +		dev_err(&interface->dev, "Invalid number of endpoints\n");
> > +		retval = -EINVAL;
> > +		goto error;
> > +	}
> > +
> >  	for (i = 0; i < iface_desc->desc.bNumEndpoints; ++i) {
> 
> Besides that you didn't even bother compile-testing this, there is no
> bug here to fix to begin with.
> 
> If bNumEndpoints is zero this loop will execute and the driver bails out
> just after since dev->read_endpoint is NULL.

That was meant to read "will never execute".

Johan