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

Kees Cook
2022-01-18T21:38:01Z

On Tue, Jan 18, 2022 at 01:11:48PM -0600, Gustavo A. R. Silva wrote:
>
>
> On 1/5/22 11:35, Kees Cook 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")
>
> This should be tagged for -stable:
>
> Cc: stable@vger.kernel.org

Ah yeah, good point. Added.

>
> > 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>
>
> Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks!

-Kees

>
> Thanks
> --
> Gustavo
>
> > ---
> > 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
> >

--
Kees Cook