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

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Date:   Fri, 10 Dec 2021 12:06:20 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Kees Cook <keescook@...omium.org>,
        Harry Wentland <harry.wentland@....com>
Cc:     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, 09 Dec 2021, Kees Cook <keescook@...omium.org> wrote:
> 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. ;)

Post Cursor2 was completely optional for the transmitter even before it
was deprecated.

And now we'd be adding 5 bytes extra to all link status reads. To fix
the only user of drm_dp_get_adjust_request_post_cursor() that apparently
has never worked as intended. I'm just not convinced.

I was trying to look through the implications of DP_LINK_STATUS_SIZE
increase, and at least drm_dp_dpcd_read_phy_link_status() comes across
as something probably needing attention.


BR,
Jani.

-- 
Jani Nikula, Intel Open Source Graphics Center