drm/modes: Avoid divide by zero harder in drm_mode_vrefresh()

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: [email protected]
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=622bba18029bcde672e1
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Jani Nikula <[email protected]>

Author: vsyrjala

drivers/gpu/drm/drm_modes.c

@@ -1287,21 +1287,24 @@ 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)
 		den *= 2;
 	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))
+		return 0;
+
 	return DIV_ROUND_CLOSEST_ULL(mul_u32_u32(num, 1000), den);
 }
 EXPORT_SYMBOL(drm_mode_vrefresh);