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

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Message-ID: <202112100853.FDB294B86@keescook>
Date:   Fri, 10 Dec 2021 08:53:45 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Harry Wentland <harry.wentland@....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 Fri, Dec 10, 2021 at 12:06:20PM +0200, Jani Nikula wrote:
> 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.

Okay, it sounds like you'd prefer the "make it tegra-specific" patch I
proposed. I will work that up as a proper patch and send it.

Thanks!

-- 
Kees Cook