Re: [PATCH v2] drm/dp: Actually read Adjust Request Post Cursor2 register

[Kees Cook <keescook@chromium.org>]

On Fri, Dec 03, 2021 at 04:28:56PM +0100, Thierry Reding wrote:
> On Fri, Dec 03, 2021 at 01:25:17AM -0800, Kees Cook wrote:
> > The link_status array was not large enough to read the Adjust Request
> > Post Cursor2 register. Adjust the size to include it. Found with a
> > -Warray-bounds build:
> > 
> > drivers/gpu/drm/drm_dp_helper.c: In function 'drm_dp_get_adjust_request_post_cursor':
> > drivers/gpu/drm/drm_dp_helper.c:59:27: error: array subscript 10 is outside array bounds of 'const u8[6]' {aka 'const unsigned char[6]'} [-Werror=array-bounds]
> >    59 |         return link_status[r - DP_LANE0_1_STATUS];
> >       |                ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
> > drivers/gpu/drm/drm_dp_helper.c:147:51: note: while referencing 'link_status'
> >   147 | u8 drm_dp_get_adjust_request_post_cursor(const u8 link_status[DP_LINK_STATUS_SIZE],
> >       |                                          ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > 
> > Fixes: 79465e0ffeb9 ("drm/dp: Add helper to get post-cursor adjustments")
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> > v2: Fix missed array size change in intel_dp_check_mst_status()
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp.c |  8 ++++----
> >  include/drm/drm_dp_helper.h             | 10 +++++++++-
> >  2 files changed, 13 insertions(+), 5 deletions(-)
> 
> This sounds very familiar and I vaguely recall typing up a patch like
> that a long time ago. But I obviously failed because that never seems
> to have made it upstream.
> 
> Or perhaps I'm misremembering and was thinking about this instead:
> 
> 	https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/338590/

Oh! Yeah, that's the same thing. Looks like that never made its way
upstream. :(

> 
> Bonus points for adding that comment with background information on why
> we need this.

Thanks! Yeah, I needed to really convince myself everything added up and
made sense, and figured I should try to capture that research. ;)

> Reviewed-by: Thierry Reding <treding@nvidia.com>

Thanks!

-Kees

-- 
Kees Cook