[PATCH v9 08/10] doc: uefi: add anti-rollback documentation

[Takahiro Akashi]

On Wed, Jun 07, 2023 at 02:41:58PM +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>
> ---
> No update since v7
> 
> Changes in v7:
> - describe the usage
> 
> Newly created in v6
> 
>  doc/develop/uefi/uefi.rst | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/doc/develop/uefi/uefi.rst b/doc/develop/uefi/uefi.rst
> index 30b90a09d5..ffd13cebe9 100644
> --- a/doc/develop/uefi/uefi.rst
> +++ b/doc/develop/uefi/uefi.rst
> @@ -537,6 +537,45 @@ where signature.dts looks like::
>              };
>      };
>  
> +Anti-rollback Protection
> +************************
> +
> +Anti-rollback prevents unintentional installation of outdated firmware.
> +To enable anti-rollback, you must add the lowest-supported-version property
> +to dtb and specify --fw-version when creating a capsule file with the
> +mkeficapsule tool.
> +When executing capsule update, U-Boot checks if fw_version is greater than
> +or equal to lowest-supported-version. If fw_version is less than
> +lowest-supported-version, the update will fail.
> +For example, if lowest-supported-version is set to 7 and you run capsule
> +update using a capsule file with --fw-version of 5, the update will fail.
> +When the --fw-version in the capsule file is updated, lowest-supported-version
> +in the dtb might be updated accordingly.
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I think that you should explicitly mention that a user always needs to update dtb
*manually* in order to enforce anti-rollback to *any* older version.
Otherwise, people may recognize that dtb be updated by the system (U-Boot).

-Takahiro Akashi

> +
> +To insert the lowest supported version into a dtb
> +
> +.. 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>;
> +                    };
> +            };
> +    };
> +
> +The properties of image-type-id and image-index must match the value
> +defined in the efi_fw_image array as image_type_id and image_index.
> +
>  Executing the boot manager
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~
>  
> -- 
> 2.34.1
>