[PATCH] drm/modes: Fix division by zero error

[Ziqi Zhao]

In the bug reported by Syzbot, the variable `den == (1 << 22)` and
`mode->vscan == (1 << 10)`, causing the multiplication to overflow and
accidentally make `den == 0`. To prevent any chance of overflow, we
replace `num` and `den` with 64-bit unsigned integers, and explicitly
check if the divisor `den` will overflow. If so, we employ full 64-bit
division with rounding; otherwise we keep the 64-bit to 32-bit division
that could potentially be better optimized.

In order to minimize the performance overhead, the overflow check for
`den` is wrapped with an `unlikely` condition. Please let me know if
this usage is appropriate.

#syz test:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

Signed-off-by: Ziqi Zhao <astrajoan at yahoo.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 ac9a406250c5..aa98bd7b8bc9 100644
--- a/drivers/gpu/drm/drm_modes.c
+++ b/drivers/gpu/drm/drm_modes.c
@@ -1285,13 +1285,13 @@ EXPORT_SYMBOL(drm_mode_set_name);
  */
 int drm_mode_vrefresh(const struct drm_display_mode *mode)
 {
-	unsigned int num, den;
+	unsigned long long num, den;
 
 	if (mode->htotal == 0 || mode->vtotal == 0)
 		return 0;
 
-	num = mode->clock;
-	den = mode->htotal * mode->vtotal;
+	num = mul_u32_u32(mode->clock, 1000);
+	den = mul_u32_u32(mode->htotal, mode->vtotal);
 
 	if (mode->flags & DRM_MODE_FLAG_INTERLACE)
 		num *= 2;
@@ -1300,7 +1300,10 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode)
 	if (mode->vscan > 1)
 		den *= mode->vscan;
 
-	return DIV_ROUND_CLOSEST_ULL(mul_u32_u32(num, 1000), den);
+	if (unlikely(den >> 32))
+		return div64_u64(num + (den >> 1), den);
+	else
+		return DIV_ROUND_CLOSEST_ULL(num, (unsigned int) den);
 }
 EXPORT_SYMBOL(drm_mode_vrefresh);
 
-- 
2.34.1