Re: [PATCH v2] drm/edid: In connector_bad_edid() cap num_of_ext by num_blocks read

[Ville Syrjälä]

On Wed, Oct 06, 2021 at 03:45:07PM -0700, Doug Anderson wrote:
> Hi,
> 
> On Tue, Oct 5, 2021 at 7:29 PM Douglas Anderson <dianders@xxxxxxxxxxxx> 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>
> > Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> > ---
> > 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.
> >
> > Changes in v2:
> > - Added a comment/changed math to help make it easier to grok.
> >
> >  drivers/gpu/drm/drm_edid.c | 15 ++++++++++++---
> >  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> Pushed this to drm-misc-fixes since the commit it fixes is fairly old.
> 
> fdc21c35aaa1 drm/edid: In connector_bad_edid() cap num_of_ext by num_blocks read

BTW seems kasan caught this for us [1]. I didn't notice we had a bug
open about it until now. Just Chris Wilson mentioned it to me in passing
quite a while ago, and I totally forgot about it until I saw your other
patch poking around the same code.

[1] https://gitlab.freedesktop.org/drm/intel/-/issues/4106

-- 
Ville Syrjälä
Intel