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). 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 Build instructions. 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 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.

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. 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.