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

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Message-ID: <202112082143.3BE8ABAE@keescook>
Date:   Wed, 8 Dec 2021 21:54:20 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Thierry Reding <treding@...dia.com>
Subject: Re: [PATCH] drm/dp: Actually read Adjust Request Post Cursor2
 register

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().
> 
> Instead of bumping the link status read size from 6 to 11 for all
> drivers I'd much rather see some other (maybe Tegra specific) solution
> to this.

Hmmm... Well given this is currently non-functional on Tegra (and is an
OOB memory read), how about just removing it entirely?

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..bb5bfa93950f 100644
--- a/drivers/gpu/drm/tegra/dp.c
+++ b/drivers/gpu/drm/tegra/dp.c
@@ -559,8 +559,7 @@ static void drm_dp_link_get_adjustments(struct drm_dp_link *link,
 			drm_dp_get_adjust_request_pre_emphasis(status, i) >>
 				DP_TRAIN_PRE_EMPHASIS_SHIFT;
 
-		adjust->post_cursor[i] =
-			drm_dp_get_adjust_request_post_cursor(status, i);
+		adjust->post_cursor[i] = 0;
 	}
 }
 
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 30359e434c3f..28378db676c8 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


Or maybe do a long link status read in Tegra only?

-- 
Kees Cook