[Intel-gfx] [PATCH] drm/i915: Fix a potential integer overflow with framebuffers extending past 4 GiB

[Ville Syrjala]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

If we have framebuffers that are >= 4GiB in size we will overflow
the fb size check in intel_fill_fb_info().

Currently that is only possible with NV12 and CCS as offsets[1]
may be anything between 0 and 0xffffffff. ofsets[0] is currently
required to be 0 so we can't hit the overflow with any single
plane format (thanks to max fb size of 8kx8k and max stride of
32 KiB).

In the future we may allow almost any framebuffer to exceed 4GiB
in size so we really should fix the overflow. Not that the overflow
is particularly dangerous. It's mostly just a sanity check against
insane userspace. The display engine can't write to memory anyway
so I suppose in the worst case we might anger the hw by attempting
scanout past the end of the ggtt, or we might scan out some data
that we're not supposed to see from other parts of the ggtt.

Note that triggering this overflow depends on the driver
aligning the fb height to the next tile boundary to push the
calculated size above 4GiB. With linear buffers the effective
tile height is one so that never happens, and the core already
has a check for 32bit overflow of offsets[]+pitches[]*height.

Testcase: igt/kms_big_fb/x-tiled-addfb-size-offset-overflow
Testcase: igt/kms_big_fb/y-tiled-addfb-size-offset-overflow
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/i915/intel_display.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 2b77d9350a3a..2b474d049074 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -2636,9 +2636,10 @@ intel_fill_fb_info(struct drm_i915_private *dev_priv,
 		max_size = max(max_size, offset + size);
 	}
 
-	if (max_size * tile_size > obj->base.size) {
-		DRM_DEBUG_KMS("fb too big for bo (need %u bytes, have %zu bytes)\n",
-			      max_size * tile_size, obj->base.size);
+	if (mul_u32_u32(max_size, tile_size) > obj->base.size) {
+		DRM_DEBUG_KMS("fb too big for bo (need %llu bytes, have %zu bytes)\n",
+			      (unsigned long long) mul_u32_u32(max_size, tile_size),
+			      obj->base.size);
 		return -EINVAL;
 	}
 
-- 
2.16.4