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

[Melissa Wen]

On 11/10, Arthur Grillo wrote:
> 
> 
> 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>

Nice catch. Thanks!

I'll already apply this one to drm-misc-next too.

CC'ing other VKMS maintainers/reviewers.

Melissa

> 
> 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);