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Date: Tue, 5 Oct 2021 19:30:55 -0700 From: Doug Anderson <dianders@...omium.org> To: Ville Syrjälä <ville.syrjala@...ux.intel.com> Cc: dri-devel <dri-devel@...ts.freedesktop.org>, "Siqueira, Rodrigo" <Rodrigo.Siqueira@....com>, "Wentland, Harry" <Harry.Wentland@....com>, Kuogee Hsieh <khsieh@...eaurora.org>, "Zuo, Jerry" <Jerry.Zuo@....com>, alexander.deucher@....com, 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] drm/edid: In connector_bad_edid() cap num_of_ext by num_blocks read Hi, On Tue, Oct 5, 2021 at 8:46 AM Ville Syrjälä <ville.syrjala@...ux.intel.com> 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@...ux.intel.com> > > Signed-off-by: Douglas Anderson <dianders@...omium.org> > > --- > > 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@...ux.intel.com> > 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
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