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

[Doug Anderson]

Hi,

On Tue, Oct 5, 2021 at 8:46 AM Ville Syrjälä
<ville.syrjala@xxxxxxxxxxxxxxx> wrote:
>
> 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.

Yeah, my brain had to work way too hard when I read over my patch too.
I've changed to your math _plus_ a big comment explaining it. I added
your review tag. I'll give this another day or so and then land.

https://lore.kernel.org/r/20211005192905.v2.1.Ib059f9c23c2611cb5a9d760e7d0a700c1295928d@changeid

-Doug