drm/vkms: Avoid reading beyond LUT array

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

Fixes: db1f254f2cfa ("drm/vkms: Add support to 1D gamma LUT")
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Cc: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231108163647.106853-6-harry.wentland@amd.com

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index 3c99fb8b54e2df54ab26f8b6b15b4fd30d41183f..e7441b227b3cea351291749002ae1296e77b62e5 100644 (file)
--- 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);