Re: [PATCH 1/2] drm/modes: Avoid divide by zero harder in drm_mode_vrefresh()

["Nautiyal, Ankit K" <ankit.k.nautiyal@intel.com>]

On 11/29/2024 9:56 AM, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>
> drm_mode_vrefresh() is trying to avoid divide by zero
> by checking whether htotal or vtotal are zero. But we may
> still end up with a div-by-zero of vtotal*htotal*...
>
> Cc: stable@vger.kernel.org
> Reported-by: syzbot+622bba18029bcde672e1@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=622bba18029bcde672e1
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>   drivers/gpu/drm/drm_modes.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> index 6ba167a33461..71573b85d924 100644
> --- a/drivers/gpu/drm/drm_modes.c
> +++ b/drivers/gpu/drm/drm_modes.c
> @@ -1287,14 +1287,11 @@ EXPORT_SYMBOL(drm_mode_set_name);
>    */
>   int drm_mode_vrefresh(const struct drm_display_mode *mode)
>   {
> -	unsigned int num, den;
> +	unsigned int num = 1, den = 1;
>   
>   	if (mode->htotal == 0 || mode->vtotal == 0)
>   		return 0;
>   
> -	num = mode->clock;
> -	den = mode->htotal * mode->vtotal;
> -
>   	if (mode->flags & DRM_MODE_FLAG_INTERLACE)
>   		num *= 2;
>   	if (mode->flags & DRM_MODE_FLAG_DBLSCAN)
> @@ -1302,6 +1299,12 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode)
>   	if (mode->vscan > 1)
>   		den *= mode->vscan;
>   
> +	if (check_mul_overflow(mode->clock, num, &num))
> +		return 0;
> +
> +	if (check_mul_overflow(mode->htotal * mode->vtotal, den, &den))

Can mode->htotal * mode->vtotal result in overflow?

and we should add:

if (check_mul_overflow(mode->htotal, mode->vtotal, &prod))
	return 0;

Regards,

Ankit

> +		return 0;
> +
>   	return DIV_ROUND_CLOSEST_ULL(mul_u32_u32(num, 1000), den);
>   }
>   EXPORT_SYMBOL(drm_mode_vrefresh);