[PATCH v2] drm/dp: Actually read Adjust Request Post Cursor2 register

[PATCH v2] drm/dp: Actually read Adjust Request Post Cursor2 register

[Kees Cook]

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@chromium.org>
---
v2: Fix missed array size change in intel_dp_check_mst_status()
---
 drivers/gpu/drm/i915/display/intel_dp.c |  8 ++++----
 include/drm/drm_dp_helper.h             | 10 +++++++++-
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 5a8206298691..97367afc7243 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -3647,17 +3647,17 @@ intel_dp_check_mst_status(struct intel_dp *intel_dp)
 
 	for (;;) {
 		/*
-		 * The +2 is because DP_DPRX_ESI_LEN is 14, but we then
+		 * The +10 is because while DP_DPRX_ESI_LEN is 14, we then
 		 * pass in "esi+10" to drm_dp_channel_eq_ok(), which
-		 * takes a 6-byte array. So we actually need 16 bytes
-		 * here.
+		 * takes a DP_LINK_STATUS_SIZE array. So we actually need
+		 * 10 bytes more than DP_LINK_STATUS_SIZE.
 		 *
 		 * Somebody who knows what the limits actually are
 		 * should check this, but for now this is at least
 		 * harmless and avoids a valid compiler warning about
 		 * using more of the array than we have allocated.
 		 */
-		u8 esi[DP_DPRX_ESI_LEN+2] = {};
+		u8 esi[DP_LINK_STATUS_SIZE + 10] = {};
 		bool handled;
 		int retry;
 
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 472dac376284..277643d2fe2c 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -1517,7 +1517,15 @@ enum drm_dp_phy {
 #define DP_MST_LOGICAL_PORT_0 8
 
 #define DP_LINK_CONSTANT_N_VALUE 0x8000
-#define DP_LINK_STATUS_SIZE	   6
+/*
+ * DPCD registers in link_status:
+ * Link Status:		0x202 through 0x204
+ * Sink Status:		0x205
+ * Adjust Request:	0x206 through 0x207
+ * Training Score:	0x208 through 0x20b
+ * AR Post Cursor2:	0x20c
+ */
+#define DP_LINK_STATUS_SIZE	   11
 bool drm_dp_channel_eq_ok(const u8 link_status[DP_LINK_STATUS_SIZE],
 			  int lane_count);
 bool drm_dp_clock_recovery_ok(const u8 link_status[DP_LINK_STATUS_SIZE],
-- 
2.30.2

Re: [PATCH v2] drm/dp: Actually read Adjust Request Post Cursor2 register

[Thierry Reding]

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On Fri, Dec 03, 2021 at 01:25:17AM -0800, Kees Cook 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@chromium.org>
> ---
> v2: Fix missed array size change in intel_dp_check_mst_status()
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c |  8 ++++----
>  include/drm/drm_dp_helper.h             | 10 +++++++++-
>  2 files changed, 13 insertions(+), 5 deletions(-)

This sounds very familiar and I vaguely recall typing up a patch like
that a long time ago. But I obviously failed because that never seems
to have made it upstream.

Or perhaps I'm misremembering and was thinking about this instead:

	https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/338590/

Bonus points for adding that comment with background information on why
we need this.

Reviewed-by: Thierry Reding <treding@nvidia.com>

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Re: [PATCH v2] drm/dp: Actually read Adjust Request Post Cursor2 register

[Kees Cook]

On Fri, Dec 03, 2021 at 04:28:56PM +0100, Thierry Reding wrote:
> On Fri, Dec 03, 2021 at 01:25:17AM -0800, Kees Cook 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@chromium.org>
> > ---
> > v2: Fix missed array size change in intel_dp_check_mst_status()
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp.c |  8 ++++----
> >  include/drm/drm_dp_helper.h             | 10 +++++++++-
> >  2 files changed, 13 insertions(+), 5 deletions(-)
> 
> This sounds very familiar and I vaguely recall typing up a patch like
> that a long time ago. But I obviously failed because that never seems
> to have made it upstream.
> 
> Or perhaps I'm misremembering and was thinking about this instead:
> 
> 	https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/338590/

Oh! Yeah, that's the same thing. Looks like that never made its way
upstream. :(

> 
> Bonus points for adding that comment with background information on why
> we need this.

Thanks! Yeah, I needed to really convince myself everything added up and
made sense, and figured I should try to capture that research. ;)

> Reviewed-by: Thierry Reding <treding@nvidia.com>

Thanks!

-Kees

-- 
Kees Cook