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

[Arthur Grillo]

On 08/11/23 13:36, Harry Wentland wrote:
> 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>

LGTM
Reviewed-by: Arthur Grillo <arthurgrillo at riseup.net>

Best Regards,
~Arthur Grillo

> ---
>  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);