Trusted Firmware-M Profile Medium-ARoT-less

Introduction

TF-M Profile Medium-ARoT-less is a reference implementation to align with security requirements defined in PSA Certified ARoT-less Level 2 protection profile (PSA Certified ARoT-less) [1].

TF-M Profile Medium-ARoT-less is defined based on TF-M Profile Medium [2], which aligns with PSA Certified Level 2 Protection Profile [3].

Overall design

TF-M Profile Medium-ARoT-less defines the following feature set:

• Firmware Framework

  • Secure Function (SFN) model [4]

  • Isolation level 1 [5]

• Internal Trusted Storage (ITS)

• Crypto

  • Support both symmetric cryptography and asymmetric cryptography

  • Asymmetric key based cipher suite suggested in TLS/DTLS profiles for IoT [6] and CoAP [7], including

    • Authenticated Encryption with Associated Data (AEAD) algorithm

    • Asymmetric key algorithm based signature and verification

    • Public-key cryptography based key exchange

    • Hash function

    • HMAC for default Pseudorandom Function (PRF)

  • Asymmetric digital signature and verification for Initial Attestation Token (IAT)

• Initial Attestation

  • Asymmetric key algorithm based Initial Attestation

• Lightweight boot

  • Anti-rollback protection

  • Multiple image boot

• Firmware Update (FWU) RoT Service

Design details

Most of design in Profile Medium-ARoT-less is identical to that in Profile Medium. Refer to Profile Medium document [2] for details. Only the differences between Profile Medium-ARoT-less and Profile Medium are specified below.

Firmware framework

PSA Certified ARoT-less [1] is only applicable to devices that don’t support Application RoT (ARoT) services. The platform only consists of PSA RoT domain(s) in SPE making it unnecessary to implement the isolation between ARoT and PSA RoT. Therefore, this profile selects isolation level 1 to simplify implementation and optimize memory footprint and performance.

Since only isolation level 1 is required, this profile enables SFN model rather than IPC mode for further simplification.

Protected Storage

Protected Storage (PS) is implemented as an Application RoT service in TF-M by default. Therefore, PS is disabled by default in this profile.

FWU RoT Service

As PSA Certified ARoT-less requests, FWU RoT Service is enabled by default to support secure update of platform if the platform supports FWU.

Note

Implementation note

The entire secure update sequence involves multiple agents and components, including bootloader, TF-M FWU RoT Service, image update application(s), remote server(s), etc.

The secure update sequence is implementation-defined. Here is a reference of TF-M integration with FreeRTOS OTA [8].

Implementation

The basic idea is to add dedicated profile CMake configuration files config/profile/profile_medium_arotless for this profile default configuration.

This top-level config file collects all the fundamental configuration flags and set them to default values, to explicitly configured the features required in this profile, during TF-M build.

A platform/use case overwrite the default settings to configure this profile.

The behavior of the build flow (particularly the order of configuration loading and overriding) can be found at Build configuration.

The default configurations in config/profile/profile_medium_arotless are shown below.

Table 31 Config flags in Profile Medium-ARoT-less top-level CMake config file

Configs	Default value	Descriptions
TFM_ISOLATION_LEVEL	1	Select level 1 isolation
CONFIG_TFM_SPM_BACKEND	SFN	Select SFN model
TFM_PARTITION_INTERNAL_TRUSTED_STORAGE	ON	Enable ITS SP
ITS_BUF_SIZE	32	ITS internal transient buffer size
TFM_PARTITION_CRYPTO	ON	Enable Crypto service
CRYPTO_ASYM_ENCRYPT_MODULE_ENABLED	OFF	Enable Crypto asymmetric encryption operations
TFM_MBEDCRYPTO_CONFIG_PATH	${CMAKE_SOURCE_DIR}/lib/ext/mbedcrypto/mbedcrypto_config/tfm_mbedcrypto_config_profile_medium.h	Re-use Profile Medium configuration
TFM_MBEDCRYPTO_PSA_CRYPTO_CONFIG_PATH	${CMAKE_SOURCE_DIR}/lib/ext/mbedcrypto/mbedcrypto_config/crypto_config_profile_medium.h	Re-use Profile Medium configuration
TFM_PARTITION_INITIAL_ATTESTATION	ON	Enable Initial Attestation service
TFM_PARTITION_FIRMWARE_UPDATE	ON	Enable Firmware Update service
TFM_PARTITION_PROTECTED_STORAGE	OFF	Disable PS service
TFM_PARTITION_PLATFORM	ON	Enable TF-M Platform SP

Note

Where a configuration is the same as the default in config/config_default.cmake, it might be omitted from the profile configuration file.

Note

Implementation note

If the platform doesn’t support secure update functionalities, FWU service will be disabled automatically. A warning will be thrown out during build.

Regression test configuration

FWU regression tests and SFN regression tests are selected by default when regression tests are enabled. Other implementations are the same as those in Profile Medium.

Platform support

To enable Profile Medium-ARoT-less on a platform, the platform specific CMake file should be added into the platform support list in top-level Profile Medium-ARoT-less CMake config file.

Building Profile Medium-ARoT-less

To build Profile Medium-ARoT-less, argument TFM_PROFILE in build command line should be set to profile_medium_arotless.

Take Musca-B1 as an example. The following commands build Profile Medium-ARoT-less without test cases on Musca-B1 with build type MinSizeRel, built by Armclang.

cd <TFM root dir>
mkdir build && cd build
cmake -DTFM_PLATFORM=musca-b1 \
      -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
      -DTFM_PROFILE=profile_medium_arotless \
      -DCMAKE_BUILD_TYPE=MinSizeRel \
      ../
cmake --build ./ -- install

The following commands build Profile Medium-ARoT-less with regression test cases on Musca-B1 with build type MinSizeRel, built by Armclang.

cd <TFM root dir>
mkdir build && cd build
cmake -DTFM_PLATFORM=musca-b1 \
      -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
      -DTFM_PROFILE=profile_medium_arotless \
      -DCMAKE_BUILD_TYPE=MinSizeRel \
      -DTEST_S=ON -DTEST_NS=ON \
      ../
cmake --build ./ -- install

Note

• For devices with more constrained memory and flash requirements, it is possible to build with either only TEST_S enabled or only TEST_NS enabled. This will decrease the size of the test images. Note that both test suites must still be run to ensure correct operation.

More details of building instructions and parameters can be found TF-M build instruction guide [9].

Reference

[1] (1,2)

SESIP Profile for PSA Certified ARoT-less Level 2

[2] (1,2)

Trusted Firmware-M Profile Medium Design

[3]

SESIP Profile for PSA Certified Level 2

[4]

Arm Firmware Framework for M 1.1 Extensions

[5]

Arm Platform Security Architecture Firmware Framework 1.0

[6]

Transport Layer Security (TLS) / Datagram Transport Layer Security (DTLS) Profiles for the Internet of Things

[7]

The Constrained Application Protocol (CoAP)

[8]

Secure OTA Updates for Cortex-M Devices with FreeRTOS

[9]

TF-M build instruction

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