Re: [Intel-gfx] [PATCH 1/8] drm/fb: More paranoia in addfb checks

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Nov 15, 2019 at 10:21:13AM +0100, Daniel Vetter wrote:
> - Our limit is uint32_t, make that explicit.
> 
> - Untangle the one overflow check, I think (but not sure) that with
>   all three together you could overflow the uint64_t and it'd look
>   cool again.

It can't overflow. All theree inputs are u32 so the max value
you can get is 0xffffffff00000000.

> Hence two steps. Also go with the more common (and imo
>   safer approach) 

Also results in multiple divisions which is needlessly expensive.
The original is just mul+add.

> of reducing the range we accept, instead of trying
>   to compute the overflow in high enough precision.
> 
> - The above would blow up if we get a 0 pitches, so check for that
>   too, but only if block_size is a thing.
> 
> Cc: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  drivers/gpu/drm/drm_framebuffer.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
> index 57564318ceea..3141c6ed6dd2 100644
> --- a/drivers/gpu/drm/drm_framebuffer.c
> +++ b/drivers/gpu/drm/drm_framebuffer.c
> @@ -214,15 +214,20 @@ static int framebuffer_check(struct drm_device *dev,
>  			return -EINVAL;
>  		}
>  
> -		if (min_pitch > UINT_MAX)
> +		if (min_pitch > U8_MAX)

U8?

>  			return -ERANGE;
>  
> -		if ((uint64_t) height * r->pitches[i] + r->offsets[i] > UINT_MAX)
> -			return -ERANGE;
> +		if (block_size) {
> +			if (r->pitches[i] < min_pitch) {
> +				DRM_DEBUG_KMS("bad pitch %u for plane %d\n", r->pitches[i], i);
> +				return -EINVAL;
> +			}
>  
> -		if (block_size && r->pitches[i] < min_pitch) {
> -			DRM_DEBUG_KMS("bad pitch %u for plane %d\n", r->pitches[i], i);
> -			return -EINVAL;
> +			if (height > U8_MAX / r->pitches[i])
> +				return -ERANGE;
> +
> +			if (r->offsets[i] > U8_MAX / r->pitches[i] - height)
> +				return -ERANGE;
>  		}
>  
>  		if (r->modifier[i] && !(r->flags & DRM_MODE_FB_MODIFIERS)) {
> -- 
> 2.24.0
> 
> _______________________________________________
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> Intel-gfx@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Ville Syrjälä
Intel
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