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

From: Greg KH <[email-masked]>
Date: Sat, 6 Jan 2018 10:09:07 +0100

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.

And if this value really is user controlled, then why is this the only
v4l driver that is affected?  This is a common callback.

thanks,

greg k-h