linux-kernel - Re: [PATCH v2] drm/edid: In connector_bad_edid() cap num_of_ext by num

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Date:   Thu, 7 Oct 2021 02:00:09 +0300
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Doug Anderson <dianders@...omium.org>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        "Siqueira, Rodrigo" <Rodrigo.Siqueira@....com>,
        "Zuo, Jerry" <Jerry.Zuo@....com>, alexander.deucher@....com,
        "Wentland, Harry" <Harry.Wentland@....com>,
        Kuogee Hsieh <khsieh@...eaurora.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] drm/edid: In connector_bad_edid() cap num_of_ext by
 num_blocks read

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@...omium.org> 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@...ux.intel.com>
> > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > Reviewed-by: Ville Syrjälä <ville.syrjala@...ux.intel.com>
> > ---
> > 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