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Date:   Mon, 07 Feb 2022 13:04:55 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Kees Cook <keescook@...omium.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org,
        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: Re: [PATCH] drm/dp: Remove common Post Cursor2 register handling

On Fri, 04 Feb 2022, Kees Cook <keescook@...omium.org> wrote:
> Ping,
>
> This is a OOB read fix. Can someone please pick this up?

Daniel? Thierry?

As I said, I reviewed this but I'm not comfortable applying patches that
change the functionality of drivers I don't maintain.

BR,
Jani.


>
> -Kees
>
> On Wed, Jan 05, 2022 at 09:35:07AM -0800, 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")
>> 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
>> 

-- 
Jani Nikula, Intel Open Source Graphics Center

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