Subject: Re: [PATCH 07/18] [media] uvcvideo: prevent bounds-check bypass via speculative execution

From: Greg KH <[email-masked]>
Date: Sat, 6 Jan 2018 10:40:26 +0100

On Sat, Jan 06, 2018 at 10:09:07AM +0100, Greg KH wrote:
> On Fri, Jan 05, 2018 at 05:10:32PM -0800, Dan Williams wrote:
> > Static analysis reports that 'index' may be a user controlled value that
> > is used as a data dependency to read 'pin' from the
> > 'selector->baSourceID' array. In order to avoid potential leaks of
> > kernel memory values, block speculative execution of the instruction
> > stream that could issue reads based on an invalid value of 'pin'.
> > 
> > Based on an original patch by Elena Reshetova.
> > 
> > Cc: Laurent Pinchart <[email-masked]>
> > Cc: Mauro Carvalho Chehab <[email-masked]>
> > Cc: [email-masked]
> > Signed-off-by: Elena Reshetova <[email-masked]>
> > Signed-off-by: Dan Williams <[email-masked]>
> > ---
> >  drivers/media/usb/uvc/uvc_v4l2.c |    7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/media/usb/uvc/uvc_v4l2.c b/drivers/media/usb/uvc/uvc_v4l2.c
> > index 3e7e283a44a8..7442626dc20e 100644
> > --- a/drivers/media/usb/uvc/uvc_v4l2.c
> > +++ b/drivers/media/usb/uvc/uvc_v4l2.c
> > @@ -22,6 +22,7 @@
> >  #include <linux/mm.h>
> >  #include <linux/wait.h>
> >  #include <linux/atomic.h>
> > +#include <linux/compiler.h>
> >  
> >  #include <media/v4l2-common.h>
> >  #include <media/v4l2-ctrls.h>
> > @@ -810,6 +811,7 @@ static int uvc_ioctl_enum_input(struct file *file, void *fh,
> >  	struct uvc_entity *iterm = NULL;
> >  	u32 index = input->index;
> >  	int pin = 0;
> > +	__u8 *elem;
> >  
> >  	if (selector == NULL ||
> >  	    (chain->dev->quirks & UVC_QUIRK_IGNORE_SELECTOR_UNIT)) {
> > @@ -820,8 +822,9 @@ static int uvc_ioctl_enum_input(struct file *file, void *fh,
> >  				break;
> >  		}
> >  		pin = iterm->id;
> > -	} else if (index < selector->bNrInPins) {
> > -		pin = selector->baSourceID[index];
> > +	} else if ((elem = nospec_array_ptr(selector->baSourceID, index,
> > +					selector->bNrInPins))) {
> > +		pin = *elem;
> 
> I dug through this before, and I couldn't find where index came from
> userspace, I think seeing the coverity rule would be nice.

Ok, I take it back, this looks correct.  Ugh, the v4l ioctl api is
crazy complex (rightfully so), it's amazing that coverity could navigate
that whole thing :)

While I'm all for fixing this type of thing, I feel like we need to do
something "else" for this as playing whack-a-mole for this pattern is
going to be a never-ending battle for all drivers for forever.  Either
we need some way to mark this data path to make it easy for tools like
sparse to flag easily, or we need to catch the issue in the driver
subsystems, which unfortunatly, would harm the drivers that don't have
this type of issue (like here.)

I'm guessing that other operating systems, which don't have the luxury
of auditing all of their drivers are going for the "big hammer in the
subsystem" type of fix, right?

I don't have a good answer for this, but if there was some better way to
rewrite these types of patterns to just prevent the need for the
nospec_array_ptr() type thing, that might be the best overall for
everyone.  Much like ebpf did with their changes.  That way a simple
coccinelle rule would be able to catch the pattern and rewrite it.

Or am I just dreaming?

thanks,

greg k-h