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. Reported-by: syzbot+622bba18029bcde67...@syzkaller.appspotmail.com Signed-off-by: Ziqi Zhao <astraj...@yahoo.com> --- V1 -> V2: address style comments suggested by Jani Nikula <jani.nik...@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 ac9a406250c5..137101960690 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; + u64 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 > UINT_MAX)) + return DIV64_U64_ROUND_CLOSEST(num, den); + + return DIV_ROUND_CLOSEST_ULL(num, (u32) den); } EXPORT_SYMBOL(drm_mode_vrefresh); -- 2.34.1