[PATCH] drm: fix division-by-zero on dumb_create()

[Rob Clark]

On Sun, Aug 24, 2014 at 1:23 PM, David Herrmann <dh.herrmann at gmail.com> wrote:
> Kinda unexpected, but DIV_ROUND_UP() can overflow if passed an argument
> bigger than UINT_MAX - DIVISOR. Fix this by testing for "!cpp" before
> using it in the following division.
>
> Note that DIV_ROUND_UP() is defined as:
>         #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
>
> ..this will obviously overflow if (n + d - 1) is bigger than UINT_MAX.
>
> Reported-by: Tommi Rantala <tt.rantala at gmail.com>
> Signed-off-by: David Herrmann <dh.herrmann at gmail.com>
> ---
>  drivers/gpu/drm/drm_crtc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index fe94cc1..55057f7 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -4397,7 +4397,7 @@ int drm_mode_create_dumb_ioctl(struct drm_device *dev,
>
>         /* overflow checks for 32bit size calculations */
>         cpp = DIV_ROUND_UP(args->bpp, 8);
> -       if (cpp > 0xffffffffU / args->width)

might be an idea to add:

+   /* NOTE: DIV_ROUND_UP() can overflow */

or something along those lines..  I also assumed that DIV_ROUND_UP()
would not overflow, and would otherwise find that code funny looking.

(yeah, technically someone could figure that out via git-blame.. but still..)

other than that,

Reviewed-by: Rob Clark <robdclark at gmail.com>

> +       if (!cpp || cpp > 0xffffffffU / args->width)
>                 return -EINVAL;
>         stride = cpp * args->width;
>         if (args->height > 0xffffffffU / stride)
> --
> 2.1.0
>