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

From: Laurent Pinchart <[email-masked]>
Date: Mon, 08 Jan 2018 13:23:08 +0200

Hi Dan,

Thank you for the patch.

On Saturday, 6 January 2018 03:10:32 EET 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'.

I won't repeat the arguments already made in the thread regarding having 
documented coverity rules for this, even if I agree with them.

> 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;
>  		list_for_each_entry(iterm, &chain->entities, chain) {
>  			if (!UVC_ENTITY_IS_ITERM(iterm))
>  				continue;

(adding a bit more context)

> 			if (iterm->id == pin)
> 				break;
> 		}
> 	}
> 
> 	if (iterm == NULL || iterm->id != pin)
> 		return -EINVAL;
> 
> 	memset(input, 0, sizeof(*input));
> 	input->index = index;
> 	strlcpy(input->name, iterm->name, sizeof(input->name));
> 	if (UVC_ENTITY_TYPE(iterm) == UVC_ITT_CAMERA)
> 		input->type = V4L2_INPUT_TYPE_CAMERA;

So pin is used to search for an entry in the chain->entities list. Entries in 
that list are allocated separately through kmalloc and can thus end up in 
different cache lines, so I agree we have an issue. However, this is mitigated 
by the fact that typical UVC devices have a handful (sometimes up to a dozen) 
entities, so an attacker would only be able to read memory values that are 
equal to the entity IDs used by the device. Entity IDs can be freely allocated 
but typically count continuously from 0. It would take a specially-crafted UVC 
device to be able to read all memory.

On the other hand, as this is nowhere close to being a fast path, I think we 
can close this potential hole as proposed in the patch. So,

Reviewed-by: Laurent Pinchart <[email-masked]>

Will you merge the whole series in one go, or would you like me to take the 
patch in my tree ? In the latter case I'll wait until the nospec_array_ptr() 
gets merged in mainline.

-- 
Regards,

Laurent Pinchart