Re: [PATCH] media: stk1160: fix some bounds checking in stk1160_copy_video()

[Ricardo Ribalda]

Hi Dan

On Wed, 17 Apr 2024 at 19:51, Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
>
> The subtract in this condition is reversed.  The ->length is the length
> of the buffer.  The ->bytesused is how many bytes we have copied thus
> far.  When the condition is reversed that means the result of the
> subtraction is always negative but since it's unsigned then the result
> is a very high positive value.  That means the overflow check is never
> true.
>
> Fixes: 9cb2173e6ea8 ("[media] media: Add stk1160 new driver (easycap replacement)")
> Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> ---
> This patch is untested, I just spotted it in review.
>
> When this bug is fixed, the two checks for negative values of "lencopy"
> could be removed.  I wrote a version of this patch which removed the
> checks, but in the end I decided to leave the checks.  They're harmless.
>
>  drivers/media/usb/stk1160/stk1160-video.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/media/usb/stk1160/stk1160-video.c b/drivers/media/usb/stk1160/stk1160-video.c
> index 366f0e4a5dc0..bfb97ea352e7 100644
> --- a/drivers/media/usb/stk1160/stk1160-video.c
> +++ b/drivers/media/usb/stk1160/stk1160-video.c
> @@ -139,8 +139,8 @@ void stk1160_copy_video(struct stk1160 *dev, u8 *src, int len)
>          * Check if we have enough space left in the buffer.
>          * In that case, we force loop exit after copy.
>          */
> -       if (lencopy > buf->bytesused - buf->length) {
> -               lencopy = buf->bytesused - buf->length;
> +       if (lencopy > buf->length - buf->bytesused) {
> +               lencopy = buf->length - buf->bytesused;
>                 remain = lencopy;
>         }

I think it is a bit more complicated than bytesused.

bytesused does not take into consideration the actual position where
it is going to write.

What we really want to check is if

offset = dst - buf->mem;
if (offset + lencopy > buf->length) {
  lencopy = buf->length - offset;
  remain = lencopy;
}

>
> @@ -182,8 +182,8 @@ void stk1160_copy_video(struct stk1160 *dev, u8 *src, int len)
>                  * Check if we have enough space left in the buffer.
>                  * In that case, we force loop exit after copy.
>                  */
> -               if (lencopy > buf->bytesused - buf->length) {
> -                       lencopy = buf->bytesused - buf->length;
> +               if (lencopy > buf->length - buf->bytesused) {
> +                       lencopy = buf->length - buf->bytesused;
>                         remain = lencopy;
>                 }
>
> --
> 2.43.0
>


-- 
Ricardo Ribalda