Runtime Security Subsystem (RSS)
================================

Introduction
------------

Runtime Security Subsystem (RSS) is an Arm subsystem that provides a reference
implementation of the HES Host in the
`Arm Confidential Compute Architecture (CCA) <https://www.arm.com/architecture/security-features/arm-confidential-compute-architecture>`_.
It is designed to be integrated into A-profile compute subsystems that implement
Arm CCA, where it serves as the Root of Trust.

RSS initially boots from immutable code (BL1_1) in its internal ROM, before
jumping to BL1_2, which is provisioned and hash-locked in RSS OTP. The updatable
MCUBoot BL2 boot stage is loaded from host system flash into RSS SRAM, where it
is authenticated. BL2 loads and authenticates the TF-M runtime into RSS SRAM
from host flash. BL2 is also responsible for loading initial boot code into
other subsystems within the host.

The RSS platform port supports the TF-M Crypto, TF-M Initial Attestation,
Measured Boot and TF-M Platform services along with the corresponding
regression tests. It supports the IPC model in multi-core topology with
Isolation Level 1 and 2.

Building TF-M
-------------

Follow the instructions in :doc:`Build instructions </building/tfm_build_instruction>`.
Build TF-M with platform name: `arm/rss/<rss platform name>`

For example for building RSS for Total Compute platforms:
``-DTFM_PLATFORM=arm/rss/tc``

Signing host images
-------------------

RSS BL2 can load boot images into other subsystems within the host system. It
expects images to be signed, with the signatures attached to the images in the
MCUBoot metadata format.

The `imgtool Python package <https://pypi.org/project/imgtool/>`_ can be used to
sign images in the required format. To sign a host image using the development
key distributed with TF-M, use the following command::

    imgtool sign \
        -k <TF-M base directory>/bl2/ext/mcuboot/root-RSA-3072.pem \
        --public-key-format full \
        --max-align 8 \
        --align 1 \
        -v "0.0.1" \
        -s 1 \
        -H 0x1000 \
        --pad-header \
        -S 0x80000 \
        --pad \
        --boot-record "HOST" \
        -L <load address> \
        <binary infile> \
        <signed binary outfile>

The ``load address`` is the logical address in the RSS memory map to which BL2
will load the image. RSS FW expects the first host image to be loaded to address
``0x70000000`` (the beginning of the RSS ATU host access region), and each
subsequent host image to be loaded at an offset of ``0x100000`` from the
previous image. The RSS ATU should be configured to map these logical addresses
to the physical addresses in the host system that the images need to be loaded
to.

For more information on the ``imgtool`` parameters, see the MCUBoot
`imgtool documentation <https://docs.mcuboot.com/imgtool.html>`_.

.. warning::

    The TF-M development key must never be used in production. To generate a
    production key, follow the imgtool documentation.

Running the code
----------------

To run the built images, they need to be concatenated into binaries that can be
placed in ROM and flash. To create the ROM image, navigate to the TF-M build
directory and run the following ``srec_cat`` command::

    srec_cat \
        bl1_1.bin -Binary -offset 0x0 \
        bl1_provisioning_bundle.bin -Binary -offset 0xE000 \
        -o rom.bin -Binary

For development purposes, the OTP image is included as a provisioning bundle in
the ROM image and provisioned into OTP by BL1_1.

To create the flash image, the following ``fiptool`` command should be run.
``fiptool`` documentation can be found `here <https://trustedfirmware-a.readthedocs.io/en/latest/getting_started/tools-build.html?highlight=fiptool#building-and-using-the-fip-tool>`_.
Note that an up-to-date fiptool that supports the RSS UUIDs must be used.::

    fiptool create \
        --align 8192 --rss-bl2     bl2_signed.bin \
        --align 8192 --rss-ns      tfm_ns_signed.bin \
        --align 8192 --rss-s       tfm_s_signed.bin \
        --align 8192 --rss-scp-bl1 <signed Host SCP BL1 image> \
        --align 8192 --rss-ap-bl1  <signed Host AP BL1 image> \
        fip.bin

If you already have a ``fip.bin`` containing host firmware images, RSS FIP
images can be patched in::

    fiptool update --align 8192 --rss-bl2 bl2_signed.bin fip.bin
    fiptool update --align 8192 --rss-ns  tfm_ns.bin fip.bin
    fiptool update --align 8192 --rss-s   tfm_s.bin fip.bin

If XIP mode is enabled, the following ``fiptool`` command should be run to
create the flash image::

    fiptool create \
        --align 8192 --rss-bl2           bl2_signed.bin \
        --align 8192 --rss-ns            tfm_ns.bin \
        --align 8192 --rss-s             tfm_s.bin \
        --align 8192 --rss-sic-tables-ns tfm_ns_sic_tables_signed.bin \
        --align 8192 --rss-sic-tables-s  tfm_s_sic_tables_signed.bin \
        --align 8192 --rss-scp-bl1       <signed Host SCP BL1 image> \
        --align 8192 --rss-ap-bl1        <signed Host AP BL1 image> \
        fip.bin

Once the FIP is prepared, a host flash image can be created using ``srec_cat``::

    srec_cat \
            fip.bin -Binary -offset 0x0 \
            -o host_flash.bin -Binary

If GPT support is enabled, and a host ``fip.bin`` and ``fip_gpt.bin`` has been
obtained, RSS images can be inserted by first patching the host FIP and then
inserting that patched FIP into the GPT image::

    sector_size=$(gdisk -l fip_gpt.bin | grep -i "sector size (logical):" | \
                sed 's/.*logical): \([0-9]*\) bytes/\1/')

    fip_label=" FIP_A$"
    fip_start_sector=$(gdisk -l fip_gpt.bin | grep "$fip_label" | awk '{print $2}')
    fip_sector_am=$(gdisk -l fip_gpt.bin | grep "$fip_label" | awk '{print $3 - $2}')

    dd if=fip.bin of=fip_gpt.bin bs=$sector_size seek=$fip_start_sector \
        count=$fip_sector_am conv=notrunc

    fip_label = " FIP_B$"
    fip_start_sector = $(gdisk -l fip_gpt.bin | grep "$fip_label" | awk '{print $2}')
    fip_sector_am = $(gdisk -l fip_gpt.bin | grep "$fip_label" | awk '{print $3 - $2}')

    dd if=fip.bin of=fip_gpt.bin bs=$sector_size seek=$fip_start_sector \
        count=$fip_sector_am conv=notrunc

To patch a ``fip_gpt.bin`` without having an initial ``fip.bin``, the FIP can be
extracted from the GPT image using the following commands (and can then be
patched and reinserted using the above commands)::

    sector_size=$(gdisk -l fip_gpt.bin | grep -i "sector size (logical):" | \
                sed 's/.*logical): \([0-9]*\) bytes/\1/')

    fip_label=" FIP_A$"
    fip_start_sector=$(gdisk -l fip_gpt.bin | grep "$fip_label" | awk '{print $2}')
    fip_sector_am=$(gdisk -l fip_gpt.bin | grep "$fip_label" | awk '{print $3 - $2}')

    dd if=fip_gpt.bin of=fip.bin bs=$sector_size skip=$fip_start_sector \
        count=$fip_sector_am conv=notrunc

Once the ``fip_gpt.bin`` is prepared, it is placed at the base of the host flash
image::

    srec_cat \
            fip_gpt.bin -Binary -offset 0x0 \
            -o host_flash.bin -Binary

The ROM binary should be placed in RSS ROM at ``0x11000000`` and the host flash
binary should be placed at the base of the host flash. For the TC platform,
this is at ``0x80000000``.

--------------

*Copyright (c) 2022-2023, Arm Limited. All rights reserved.*