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Date:   Wed,  5 Jan 2022 09:35:07 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Cc:     Kees Cook <keescook@...omium.org>,
        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,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        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: [PATCH] drm/dp: Remove common Post Cursor2 register handling

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));
+	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
-- 
2.30.2

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