Re: [PATCH] drm/edid: In connector_bad_edid() cap num_of_ext by num_blocks read

[Ville Syrjälä]

On Tue, Oct 05, 2021 at 08:10:28AM -0700, Douglas Anderson wrote:
> In commit e11f5bd8228f ("drm: Add support for DP 1.4 Compliance edid
> corruption test") the function connector_bad_edid() started assuming
> that the memory for the EDID passed to it was big enough to hold
> `edid[0x7e] + 1` blocks of data (1 extra for the base block). It
> completely ignored the fact that the function was passed `num_blocks`
> which indicated how much memory had been allocated for the EDID.
> 
> Let's fix this by adding a bounds check.
> 
> This is important for handling the case where there's an error in the
> first block of the EDID. In that case we will call
> connector_bad_edid() without having re-allocated memory based on
> `edid[0x7e]`.
> 
> Fixes: e11f5bd8228f ("drm: Add support for DP 1.4 Compliance edid corruption test")
> Reported-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
> This problem report came up in the context of a patch I sent out [1]
> and this is my attempt at a fix. The problem predates my patch,
> though. I don't personally know anything about DP compliance testing
> and what should be happening here, nor do I apparently have any
> hardware that actually reports a bad EDID. Thus this is just compile
> tested. I'm hoping that someone here can test this and make sure it
> seems OK to them.
> 
>  drivers/gpu/drm/drm_edid.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index 9b19eee0e1b4..ccfa08631c57 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -1843,8 +1843,9 @@ static void connector_bad_edid(struct drm_connector *connector,
>  	u8 num_of_ext = edid[0x7e];
>  
>  	/* Calculate real checksum for the last edid extension block data */
> -	connector->real_edid_checksum =
> -		drm_edid_block_checksum(edid + num_of_ext * EDID_LENGTH);
> +	if (num_of_ext <= num_blocks - 1)

Something about that doesn't really agree with my brain.
It's correct but when I read it I can't immediately see it.

I guess what I'd like to see is something like:
last_block = edid[0x7e];
if (last_block < num_blocks)
	connector->real_edid_checksum =
		drm_edid_block_checksum(edid + last_block * EDID_LENGTH);

Techically exactly the same thing, but I don't have to read
the comparison twice to convince myself that it's correct.

Anyways, this is
Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
either way.

> +		connector->real_edid_checksum =
> +			drm_edid_block_checksum(edid + num_of_ext * EDID_LENGTH);
>  
>  	if (connector->bad_edid_counter++ && !drm_debug_enabled(DRM_UT_KMS))
>  		return;
> -- 
> 2.33.0.800.g4c38ced690-goog

-- 
Ville Syrjälä
Intel