Re: [PATCH] drm/dp: Remove common Post Cursor2 register handling

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 05 Jan 2022, Kees Cook <keescook@chromium.org> wrote:
> On Wed, Jan 05, 2022 at 08:00:50PM +0200, Jani Nikula wrote:
>> On Wed, 05 Jan 2022, Kees Cook <keescook@chromium.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@linux.intel.com>
>> > Cc: Maxime Ripard <mripard@kernel.org>
>> > Cc: Thomas Zimmermann <tzimmermann@suse.de>
>> > Cc: David Airlie <airlied@linux.ie>
>> > Cc: Daniel Vetter <daniel@ffwll.ch>
>> > Cc: dri-devel@lists.freedesktop.org
>> > Signed-off-by: Kees Cook <keescook@chromium.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@intel.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"?

Yeah. Or, in this case, via any tree, really.

> What should next steps be?

Get an ack from tegra and/or drm-misc maintainers. All the relevant
folks and lists are in the recipients already.

BR,
Jani.


>
> -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

-- 
Jani Nikula, Intel Open Source Graphics Center