[PATCH v6 8/8] doc: uefi: add anti-rollback documentation

[Takahiro Akashi]

Hi Kojima-san,

On Fri, May 19, 2023 at 07:32:14PM +0900, Masahisa Kojima wrote:
> This commit describe the procedure to configure lowest supported
> version in the device tree for anti-rollback protection.
> 
> Signed-off-by: Masahisa Kojima <masahisa.kojima at linaro.org>
> ---
> Newly created in v6
> 
>  doc/develop/uefi/uefi.rst | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/doc/develop/uefi/uefi.rst b/doc/develop/uefi/uefi.rst
> index efab0fc7b1..f1f13bb993 100644
> --- a/doc/develop/uefi/uefi.rst
> +++ b/doc/develop/uefi/uefi.rst
> @@ -539,6 +539,38 @@ since FMP protocol handles multiple image indexes.
>        [--fit | --raw | --guid <guid-string] \
>        <image_blob> <capsule_file_name>
>  
> +Anti-rollback Protection
> +~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +The anti-rollback protection is implemented differently from firmware versioning.
> +U-Boot implements the file-based EFI variable storage, it can be tampered
> +and not the right place to store the lowest supported version.
> +U-Boot uses device tree to store the lowest supported version, it is secured
> +as long as dtb is authenticated together with U-Boot image by the authenticated
> +capsule update, and the former stage boot loader verifies the image containing the dtb
> +when the system boots.

This is details of implementation.
You should rather mention the usage, i.e. how "anti-rollback" can be managed
and achieved using firmware-version and lowest-supported-version and that users
should always update their device tree to enforce the protection.
(If the lowest-supported-version is kept the same even after the firmware update,
anti-rollback won't work.)

-Takahiro Akashi

> +1. Insert the lowest supported version into a device tree
> +
> +.. code-block:: console
> +
> +    $ dtc -@ -I dts -O dtb -o version.dtbo version.dts
> +    $ fdtoverlay -i orig.dtb -o new.dtb -v version.dtbo
> +
> +where version.dts looks like::
> +
> +    /dts-v1/;
> +    /plugin/;
> +    &{/} {
> +            firmware-version {
> +                    image1 {
> +                            image-type-id = "09D7CF52-0720-4710-91D1-08469B7FE9C8";
> +                            image-index = <1>;
> +                            lowest-supported-version = <3>;
> +                    };
> +            };
> +    };
> +
>  Executing the boot manager
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~
>  
> -- 
> 2.17.1
>