linux-hardening - Re: [PATCH v2] [next] drm/radeon: Replace one-element array with flexible-array member

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Message-ID: <CADnq5_Mod90O=tN26+Yi74WPYxpVtss+LG_+_HZyFv2EtzR+MA@mail.gmail.com>
Date:   Tue, 1 Nov 2022 10:42:14 -0400
From:   Alex Deucher <alexdeucher@...il.com>
To:     Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
Cc:     Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] [next] drm/radeon: Replace one-element array with
 flexible-array member

On Fri, Oct 28, 2022 at 11:32 PM Paulo Miguel Almeida
<paulo.miguel.almeida.rodenas@...il.com> wrote:
>
> One-element arrays are deprecated, and we are replacing them with
> flexible array members instead. So, replace one-element array with
> flexible-array member in struct _ATOM_FAKE_EDID_PATCH_RECORD and
> refactor the rest of the code accordingly.
>
> It's worth mentioning that doing a build before/after this patch results
> in no binary output differences.
>
> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> routines on memcpy() and help us make progress towards globally
> enabling -fstrict-flex-arrays=3 [1].

This seems like a worthy goal, and I'm not opposed to the patch per
se, but it seems a bit at odds with what this header represents.
atombios.h represents the memory layout of the data stored in the ROM
on the GPU.  This changes the memory layout of that ROM.  We can work
around that in the driver code, but if someone were to take this
header to try and write some standalone tool or use it for something
else in the kernel, it would not reflect reality.

Alex


>
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://github.com/KSPP/linux/issues/239
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
>
> Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
> ---
> Changelog:
>
> v2: no binary output differences patch; report binary changes findings
>     on commit log. Res: Kees Cook.
>
>     This request was made in an identical, yet different, patch but the
>     same feedback applies.
>     https://lore.kernel.org/lkml/Y1x3MtRJ8ckXxlJn@mail.google.com/
>
> v1: https://lore.kernel.org/lkml/Y1trhRE3nK5iAY6q@mail.google.com/
> ---
>  drivers/gpu/drm/radeon/atombios.h        | 2 +-
>  drivers/gpu/drm/radeon/radeon_atombios.c | 7 +++++--
>  2 files changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/radeon/atombios.h b/drivers/gpu/drm/radeon/atombios.h
> index da35a970fcc0..235e59b547a1 100644
> --- a/drivers/gpu/drm/radeon/atombios.h
> +++ b/drivers/gpu/drm/radeon/atombios.h
> @@ -3615,7 +3615,7 @@ typedef struct _ATOM_FAKE_EDID_PATCH_RECORD
>  {
>    UCHAR ucRecordType;
>    UCHAR ucFakeEDIDLength;
> -  UCHAR ucFakeEDIDString[1];    // This actually has ucFakeEdidLength elements.
> +  UCHAR ucFakeEDIDString[];    // This actually has ucFakeEdidLength elements.
>  } ATOM_FAKE_EDID_PATCH_RECORD;
>
>  typedef struct  _ATOM_PANEL_RESOLUTION_PATCH_RECORD
> diff --git a/drivers/gpu/drm/radeon/radeon_atombios.c b/drivers/gpu/drm/radeon/radeon_atombios.c
> index 204127bad89c..4ad5a328d920 100644
> --- a/drivers/gpu/drm/radeon/radeon_atombios.c
> +++ b/drivers/gpu/drm/radeon/radeon_atombios.c
> @@ -1727,8 +1727,11 @@ struct radeon_encoder_atom_dig *radeon_atombios_get_lvds_info(struct
>                                                 }
>                                         }
>                                         record += fake_edid_record->ucFakeEDIDLength ?
> -                                               fake_edid_record->ucFakeEDIDLength + 2 :
> -                                               sizeof(ATOM_FAKE_EDID_PATCH_RECORD);
> +                                                 struct_size(fake_edid_record,
> +                                                             ucFakeEDIDString,
> +                                                             fake_edid_record->ucFakeEDIDLength) :
> +                                                 /* empty fake edid record must be 3 bytes long */
> +                                                 sizeof(ATOM_FAKE_EDID_PATCH_RECORD) + 1;
>                                         break;
>                                 case LCD_PANEL_RESOLUTION_RECORD_TYPE:
>                                         panel_res_record = (ATOM_PANEL_RESOLUTION_PATCH_RECORD *)record;
> --
> 2.37.3
>