[PATCH] drm/i810: Prevent underflow in ioctl

[Chris Wilson]

Quoting Dan Carpenter (2019-10-04 11:22:51)
> The "used" variables here come from the user in the ioctl and it can be
> negative.  It could result in an out of bounds write.
> 
> Signed-off-by: Dan Carpenter <dan.carpenter at oracle.com>
> ---
>  drivers/gpu/drm/i810/i810_dma.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i810/i810_dma.c b/drivers/gpu/drm/i810/i810_dma.c
> index 2a77823b8e9a..e66c38332df4 100644
> --- a/drivers/gpu/drm/i810/i810_dma.c
> +++ b/drivers/gpu/drm/i810/i810_dma.c
> @@ -728,7 +728,7 @@ static void i810_dma_dispatch_vertex(struct drm_device *dev,
>         if (nbox > I810_NR_SAREA_CLIPRECTS)
>                 nbox = I810_NR_SAREA_CLIPRECTS;
>  
> -       if (used > 4 * 1024)
> +       if (used < 0 || used > 4 * 1024)
>                 used = 0;

Yes, as passed to the GPU instruction, negative used is invalid.

Then it is used as an offset into a memblock, where a negative offset
would be very bad.

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris