linux-hardening - Re: [PATCH] media: uvcvideo: Silence memcpy() run-time false positive warnings

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Date:   Fri, 6 Jan 2023 12:43:44 +0100
From:   Ricardo Ribalda <ribalda@...omium.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        ionut_n2001@...oo.com, Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] media: uvcvideo: Silence memcpy() run-time false positive warnings

Hi Kees

Thanks for the patch

Would it make more sense to replace *mem with a structure/union. Something like:
https://patchwork.linuxtv.org/project/linux-media/patch/20221214-uvc-status-alloc-v4-0-f8e3e2994ebd@chromium.org/

Regards!

On Fri, 6 Jan 2023 at 07:19, Kees Cook <keescook@...omium.org> wrote:
>
> The memcpy() in uvc_video_decode_meta() intentionally copies across the
> length and flags members and into the trailing buf flexible array.
> Split the copy so that the compiler can better reason about (the lack
> of) buffer overflows here. Avoid the run-time false positive warning:
>
>   memcpy: detected field-spanning write (size 12) of single field "&meta->length" at drivers/media/usb/uvc/uvc_video.c:1355 (size 1)
>
> Additionally fix a typo in the documentation for struct uvc_meta_buf.
>
> Reported-by: ionut_n2001@...oo.com
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=216810
> Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
> Cc: linux-media@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 4 +++-
>  include/uapi/linux/uvcvideo.h     | 2 +-
>  2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index d2eb9066e4dc..b67347ab4181 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -1352,7 +1352,9 @@ static void uvc_video_decode_meta(struct uvc_streaming *stream,
>         if (has_scr)
>                 memcpy(stream->clock.last_scr, scr, 6);
>
> -       memcpy(&meta->length, mem, length);
> +       meta->length = mem[0];
> +       meta->flags  = mem[1];
> +       memcpy(meta->buf, &mem[2], length - 2);
>         meta_buf->bytesused += length + sizeof(meta->ns) + sizeof(meta->sof);
>
>         uvc_dbg(stream->dev, FRAME,
> diff --git a/include/uapi/linux/uvcvideo.h b/include/uapi/linux/uvcvideo.h
> index 8288137387c0..a9d0a64007ba 100644
> --- a/include/uapi/linux/uvcvideo.h
> +++ b/include/uapi/linux/uvcvideo.h
> @@ -86,7 +86,7 @@ struct uvc_xu_control_query {
>   * struct. The first two fields are added by the driver, they can be used for
>   * clock synchronisation. The rest is an exact copy of a UVC payload header.
>   * Only complete objects with complete buffers are included. Therefore it's
> - * always sizeof(meta->ts) + sizeof(meta->sof) + meta->length bytes large.
> + * always sizeof(meta->ns) + sizeof(meta->sof) + meta->length bytes large.
>   */
>  struct uvc_meta_buf {
>         __u64 ns;
> --
> 2.34.1
>


-- 
Ricardo Ribalda