linux-hardening - Re: [PATCH v2] drm/dp: Fix off-by-one in register cache size

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Date:   Fri, 4 Feb 2022 17:38:50 -0800
From:   Kees Cook <keescook@...omium.org>
To:     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, stable@...r.kernel.org,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Uma Shankar <uma.shankar@...el.com>,
        Jani Nikula <jani.nikula@...el.com>,
        Ankit Nautiyal <ankit.k.nautiyal@...el.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] drm/dp: Fix off-by-one in register cache size

Ping. This is a OOB read fix. Can something send this to Linus please?

-Kees

On Wed, Jan 05, 2022 at 09:33:10AM -0800, Kees Cook wrote:
> The pcon_dsc_dpcd array holds 13 registers (0x92 through 0x9E). Fix the
> math to calculate the max size. Found from a -Warray-bounds build:
> 
> drivers/gpu/drm/drm_dp_helper.c: In function 'drm_dp_pcon_dsc_bpp_incr':
> drivers/gpu/drm/drm_dp_helper.c:3130:28: error: array subscript 12 is outside array bounds of 'const u8[12]' {aka 'const unsigned char[12]'} [-Werror=array-bounds]
>  3130 |         buf = pcon_dsc_dpcd[DP_PCON_DSC_BPP_INCR - DP_PCON_DSC_ENCODER];
>       |               ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/gpu/drm/drm_dp_helper.c:3126:39: note: while referencing 'pcon_dsc_dpcd'
>  3126 | int drm_dp_pcon_dsc_bpp_incr(const u8 pcon_dsc_dpcd[DP_PCON_DSC_ENCODER_CAP_SIZE])
>       |                              ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> 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
> Fixes: e2e16da398d9 ("drm/dp_helper: Add support for Configuring DSC for HDMI2.1 Pcon")
> Cc: stable@...r.kernel.org
> Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> Link: https://lore.kernel.org/lkml/20211214001849.GA62559@embeddedor/
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> v1: https://lore.kernel.org/lkml/20211203084333.3105038-1-keescook@chromium.org/
> v2:
>  - add reviewed-by
>  - add cc:stable
> ---
>  include/drm/drm_dp_helper.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
> index 30359e434c3f..472dac376284 100644
> --- a/include/drm/drm_dp_helper.h
> +++ b/include/drm/drm_dp_helper.h
> @@ -456,7 +456,7 @@ struct drm_panel;
>  #define DP_FEC_CAPABILITY_1			0x091   /* 2.0 */
>  
>  /* DP-HDMI2.1 PCON DSC ENCODER SUPPORT */
> -#define DP_PCON_DSC_ENCODER_CAP_SIZE        0xC	/* 0x9E - 0x92 */
> +#define DP_PCON_DSC_ENCODER_CAP_SIZE        0xD	/* 0x92 through 0x9E */
>  #define DP_PCON_DSC_ENCODER                 0x092
>  # define DP_PCON_DSC_ENCODER_SUPPORTED      (1 << 0)
>  # define DP_PCON_DSC_PPS_ENC_OVERRIDE       (1 << 1)
> -- 
> 2.30.2
> 

-- 
Kees Cook