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Date:   Wed, 5 Jan 2022 14:11:50 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Lyude Paul <lyude@...hat.com>, linux-kernel@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] drm/dp: Remove common Post Cursor2 register handling

On Wed, Jan 05, 2022 at 08:00:50PM +0200, Jani Nikula wrote:
> On Wed, 05 Jan 2022, Kees Cook <keescook@...omium.org> wrote:
> > The link_status array was not large enough to read the Adjust Request
> > Post Cursor2 register, so remove the common helper function to avoid
> > an OOB read, 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],
> >       |                                          ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > Replace the only user of the helper with an open-coded fetch and decode,
> > similar to drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c.
> >
> > Fixes: 79465e0ffeb9 ("drm/dp: Add helper to get post-cursor adjustments")
> > Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
> > Cc: Maxime Ripard <mripard@...nel.org>
> > Cc: Thomas Zimmermann <tzimmermann@...e.de>
> > Cc: David Airlie <airlied@...ux.ie>
> > Cc: Daniel Vetter <daniel@...ll.ch>
> > Cc: dri-devel@...ts.freedesktop.org
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> > This is the alternative to:
> > https://lore.kernel.org/lkml/20211203084354.3105253-1-keescook@chromium.org/
> > ---
> >  drivers/gpu/drm/drm_dp_helper.c | 10 ----------
> >  drivers/gpu/drm/tegra/dp.c      | 11 ++++++++++-
> >  include/drm/drm_dp_helper.h     |  2 --
> >  3 files changed, 10 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> > index 23f9073bc473..c9528aa62c9c 100644
> > --- a/drivers/gpu/drm/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/drm_dp_helper.c
> > @@ -144,16 +144,6 @@ u8 drm_dp_get_adjust_tx_ffe_preset(const u8 link_status[DP_LINK_STATUS_SIZE],
> >  }
> >  EXPORT_SYMBOL(drm_dp_get_adjust_tx_ffe_preset);
> >  
> > -u8 drm_dp_get_adjust_request_post_cursor(const u8 link_status[DP_LINK_STATUS_SIZE],
> > -					 unsigned int lane)
> > -{
> > -	unsigned int offset = DP_ADJUST_REQUEST_POST_CURSOR2;
> > -	u8 value = dp_link_status(link_status, offset);
> > -
> > -	return (value >> (lane << 1)) & 0x3;
> > -}
> > -EXPORT_SYMBOL(drm_dp_get_adjust_request_post_cursor);
> > -
> >  static int __8b10b_clock_recovery_delay_us(const struct drm_dp_aux *aux, u8 rd_interval)
> >  {
> >  	if (rd_interval > 4)
> > diff --git a/drivers/gpu/drm/tegra/dp.c b/drivers/gpu/drm/tegra/dp.c
> > index 70dfb7d1dec5..f5535eb04c6b 100644
> > --- a/drivers/gpu/drm/tegra/dp.c
> > +++ b/drivers/gpu/drm/tegra/dp.c
> > @@ -549,6 +549,15 @@ static void drm_dp_link_get_adjustments(struct drm_dp_link *link,
> >  {
> >  	struct drm_dp_link_train_set *adjust = &link->train.adjust;
> >  	unsigned int i;
> > +	u8 post_cursor;
> > +	int err;
> > +
> > +	err = drm_dp_dpcd_read(link->aux, DP_ADJUST_REQUEST_POST_CURSOR2,
> > +			       &post_cursor, sizeof(post_cursor));
> 
> There's a drm_dp_dpcd_readb() for the common 1-byte reads. Other than
> that,
> 
> Reviewed-by: Jani Nikula <jani.nikula@...el.com>

Thanks!

> 
> Though obviously that's not enough to actually merge to tegra.

As in, "a review by Jani isn't sufficient to land via the tegra tree"?

What should next steps be?

-Kees

> 
> > +	if (err < 0) {
> > +		DRM_ERROR("failed to read post_cursor2: %d\n", err);
> > +		post_cursor = 0;
> > +	}
> >  
> >  	for (i = 0; i < link->lanes; i++) {
> >  		adjust->voltage_swing[i] =
> > @@ -560,7 +569,7 @@ static void drm_dp_link_get_adjustments(struct drm_dp_link *link,
> >  				DP_TRAIN_PRE_EMPHASIS_SHIFT;
> >  
> >  		adjust->post_cursor[i] =
> > -			drm_dp_get_adjust_request_post_cursor(status, i);
> > +			(post_cursor >> (i << 1)) & 0x3;
> >  	}
> >  }
> >  
> > diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
> > index 472dac376284..fdf3cf6ccc02 100644
> > --- a/include/drm/drm_dp_helper.h
> > +++ b/include/drm/drm_dp_helper.h
> > @@ -1528,8 +1528,6 @@ u8 drm_dp_get_adjust_request_pre_emphasis(const u8 link_status[DP_LINK_STATUS_SI
> >  					  int lane);
> >  u8 drm_dp_get_adjust_tx_ffe_preset(const u8 link_status[DP_LINK_STATUS_SIZE],
> >  				   int lane);
> > -u8 drm_dp_get_adjust_request_post_cursor(const u8 link_status[DP_LINK_STATUS_SIZE],
> > -					 unsigned int lane);
> >  
> >  #define DP_BRANCH_OUI_HEADER_SIZE	0xc
> >  #define DP_RECEIVER_CAP_SIZE		0xf
> 
> -- 
> Jani Nikula, Intel Open Source Graphics Center

-- 
Kees Cook

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