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

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

Hi Greg,

On Saturday, 6 January 2018 11:40:26 EET Greg KH wrote:
> 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 <laurent.pinchart@ideasonboard.com>
> >> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> >> Cc: linux-media@vger.kernel.org
> >> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> >> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> >> ---
> >> 
> >>  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.

That's my concern too, as even if we managed to find and fix all the 
occurrences of the problematic patterns (and we won't), new ones will keep 
being merged all the time.

> 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.)

But how would you do so ?

> 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?

Other operating systems that ship closed-source drivers authored by hardware 
vendors and not reviewed by third parties will likely stay vulnerable forever. 
That's a small concern though as I expect those drivers to contain much large 
security holes anyway.

> 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?

Likely, but sometimes dreams come true :-) Do you have an idea how this could 
be done ?

-- 
Regards,

Laurent Pinchart