linux-kernel - Re: [PATCH] drm/dp: Actually read Adjust Request Post Cursor2 register

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Message-ID: <202112091539.6B349AC@keescook>
Date:   Thu, 9 Dec 2021 15:41:26 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Harry Wentland <harry.wentland@....com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-hardening@...r.kernel.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Thierry Reding <treding@...dia.com>
Subject: Re: [PATCH] drm/dp: Actually read Adjust Request Post Cursor2
 register

On Thu, Dec 09, 2021 at 05:20:45PM -0500, Harry Wentland wrote:
> 
> 
> On 2021-12-09 01:23, Kees Cook wrote:
> > On Wed, Dec 08, 2021 at 01:19:28PM +0200, Jani Nikula wrote:
> >> On Fri, 03 Dec 2021, Kees Cook <keescook@...omium.org> 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@...omium.org>
> >>
> >> Using DP_ADJUST_REQUEST_POST_CURSOR2 has been deprecated since DP 1.3
> >> published in 2014, and Tegra is the only user of
> >> drm_dp_get_adjust_request_post_cursor().
> > 
> > I see POST_CURSOR2 is used here too:
> > 
> > drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
> > 
> 
> Looks like we read and parse that in the admgpu driver without
> using drm_dp_get_adjust_request_post_cursor.

Right, and probably that could be switched to use it, but I'm not sure
what the impact of the larger link_status read is.

> 
> I don't have a strong feeling but I liked your original
> patch a bit better. I'm not sure what it means when part
> of a spec is deprecated. Once a spec is written display
> vendors might implement it. We should make sure that
> displays like that are always handled in a sane manner.

Jani, Dave, any guidance here? I'm fine with whatever, but the current
code is for sure broken. ;)

-Kees

-- 
Kees Cook