[RFC PATCH v3 05/23] drm/vkms: Avoid reading beyond LUT array

[Harry Wentland]

When the floor LUT index (drm_fixp2int(lut_index) is the last
index of the array the ceil LUT index will point to an entry
beyond the array. Make sure we guard against it and use the
value of the floor LUT index.

v3:
 - Drop bits from commit description that didn't contribute
   anything of value

Signed-off-by: Harry Wentland <harry.wentland at amd.com>
Cc: Arthur Grillo <arthurgrillo at riseup.net>
---
 drivers/gpu/drm/vkms/vkms_composer.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index 6f942896036e..25b6b73bece8 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -123,6 +123,8 @@ static u16 apply_lut_to_channel_value(const struct vkms_color_lut *lut, u16 chan
 				      enum lut_channel channel)
 {
 	s64 lut_index = get_lut_index(lut, channel_value);
+	u16 *floor_lut_value, *ceil_lut_value;
+	u16 floor_channel_value, ceil_channel_value;
 
 	/*
 	 * This checks if `struct drm_color_lut` has any gap added by the compiler
@@ -130,11 +132,15 @@ static u16 apply_lut_to_channel_value(const struct vkms_color_lut *lut, u16 chan
 	 */
 	static_assert(sizeof(struct drm_color_lut) == sizeof(__u16) * 4);
 
-	u16 *floor_lut_value = (__u16 *)&lut->base[drm_fixp2int(lut_index)];
-	u16 *ceil_lut_value = (__u16 *)&lut->base[drm_fixp2int_ceil(lut_index)];
+	floor_lut_value = (__u16 *)&lut->base[drm_fixp2int(lut_index)];
+	if (drm_fixp2int(lut_index) == (lut->lut_length - 1))
+		/* We're at the end of the LUT array, use same value for ceil and floor */
+		ceil_lut_value = floor_lut_value;
+	else
+		ceil_lut_value = (__u16 *)&lut->base[drm_fixp2int_ceil(lut_index)];
 
-	u16 floor_channel_value = floor_lut_value[channel];
-	u16 ceil_channel_value = ceil_lut_value[channel];
+	floor_channel_value = floor_lut_value[channel];
+	ceil_channel_value = ceil_lut_value[channel];
 
 	return lerp_u16(floor_channel_value, ceil_channel_value,
 			lut_index & DRM_FIXED_DECIMAL_MASK);
-- 
2.42.1