Trusted Firmware-M Profile Large Design

Introduction

As one of TF-M Profiles, Profile Large protects less resource-constrained Arm Cortex-M devices.

Compared to Profile Small [1] and Profile Medium [2], Profile Large aims to enable more secure features to support higher level of security required in more complex usage scenarios.

• Isolation level 3 enables additional isolation between Application RoT (App RoT) services.

• More crypto algorithms and cipher suites are selected to securely connect devices to remote services offered by various major Cloud Service Providers (CSP)

• Basic software countermeasures against physical attacks can be enabled.

Profile Large can be aligned as a reference implementation with the requirements defined in PSA Certified Level 3 Lightweight Protection Profile [3].

Overall design

TF-M Profile Large defines the following feature set:

• Firmware Framework

  • Inter-Process Communication (IPC) model [4]

  • Isolation level 3 [4]

• Internal Trusted Storage (ITS)

• Crypto

  • Support both symmetric ciphers and asymmetric ciphers

  • Asymmetric key based cipher suites defined in TLS 1.2 [5] to support direct secure connection to major CSPs, 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)

  • Asymmetric algorithms for firmware image signature verification

  • Key derivation

• Initial Attestation

  • Asymmetric key algorithm based Initial Attestation

• Secure boot

  • Anti-rollback protection

  • Multiple image boot

• Protected Storage (PS) if off-chip storage device is integrated

  • Data confidentiality

  • Data integrity

  • Rollback protection

• Software countermeasures against physical attacks

Design details

More details of TF-M Profile Large design are described in following sections.

Firmware framework

Profile Large selects IPC model and isolation level 3 by default.

Isolation level 3 supports additional isolation between App RoT services, compared to isolation level 2. It can protect RoT services from each other when their vendors don’t trust each other.

Crypto service

Profile Large supports direct connection to Cloud services via common protocols, such as TLS 1.2.

In some usage scenarios, PSA RoT can be managed by device manufacturer or other vendors and is out of control of application developers. Profile Large selects alternative crypto algorithms for each crypto function to support multiple common cipher suites required by various major CSPs. Therefore, application developers can support services for diverse CSPs on same devices with Profile Large, without relying on PSA RoT upgrades of crypto.

Devices meeting Profile Large should be in a position to offer at least two alternatives to every cryptographic primitive for symmetric, asymmetric and hash, and be able to use them for encryption, AEAD, signature and verification.

It will cost more resource in Profile Large to support more crypto algorithms and cipher suites, compared to Profile Medium [2].

Boot loader

BL2 implementation can be device specific. Devices may implement diverse boot processes with different features and configurations. However, the boot loader must support anti-rollback protection. Boot loader must be able to prevent unauthorized rollback, to protect devices from being downgraded to earlier versions with known vulnerabilities.

MCUBoot in TF-M is configured as multiple image boot by default in Profile Large. In multiple image boot, secure and non-secure images can be signed independently with different keys and they can be updated separately. It can support multiple vendors scenarios, in which non-secure and secure images are generated and updated by different vendors. Multiple image boot may cost larger memory footprint compared with single image boot.

Boot loader can implement software countermeasures to mitigate physical attacks.

Protected Storage

PS service is required if an off-chip storage device is integrated and used on the platform.

Anti-rollback protection in PS relies on non-volatile counter(s) provided by TF-M Platform Secure Partition (SP).

Software countermeasures against physical attacks

TF-M Profile Large enables TF-M Fault Injection Hardening (FIH) library Profile Medium by default. It enables the following countermeasure techniques:

• Control flow monitor

• Failure loop hardening

• Complex constants

• Redundant variables and condition checks

Refer to TF-M physical attack mitigation design document [6] for FIH library details.

Note

TF-M FIH library is still under development.

TF-M FIH library hardens TF-M critical execution steps to make physical attacks more difficult, together with device hardware countermeasures. It is not guaranteed that TF-M FIH library is able to mitigate all kinds of physical attacks.

Note

Implementation note

TF-M FIH library doesn’t cover platform specific critical configurations. Platforms shall implement software countermeasures against physical attacks to protect platform specific implementation.

Implementation

Overview

The basic idea is to add dedicated profile CMake configuration files under folder config/profile for TF-M Profile Large default configuration, the same as other TF-M Profiles do.

The top-level Profile Large config file collects all the necessary configuration flags and set them to default values, to explicitly enable the features required in Profile Large and disable the unnecessary ones, during TF-M build.

A platform/use case can provide a configuration extension file to overwrite Profile Large default setting and append other configurations. This configuration extension file can be added via parameter TFM_EXTRA_CONFIG_PATH in build command line.

The behaviour of the Profile Large build flow (particularly the order of configuration loading and overriding) can be found at Build configuration

The details of configurations will be covered in each module in Implementation details.

Implementation details

This section discusses the details of Profile Large implementation.

Top-level configuration files

The firmware framework configurations in config/profile/profile_large are shown below.

Table 33 Config flags in Profile Large top-level CMake config file

Configs	Descriptions	Default value
TFM_ISOLATION_LEVEL	Select level 3 isolation	3
TFM_PARTITION_INTERNAL_TRUSTED_STORAGE	Enable ITS SP	ON
TFM_PARTITION_CRYPTO	Enable Crypto service	ON
TFM_MBEDCRYPTO_CONFIG_PATH	MbedTLS config file path	${CMAKE_SOURCE_DIR}/lib/ext/mbedcrypto/mbedcrypto_config/tfm_mbedcrypto_config_profile_large.h
TFM_MBEDCRYPTO_PSA_CRYPTO_CONFIG_PATH	MbedTLS PSA config file path	${CMAKE_SOURCE_DIR}/lib/ext/mbedcrypto/mbedcrypto_config/crypto_config_profile_large.h
TFM_PARTITION_INITIAL_ATTESTATION	Enable Initial Attestation service	ON
TFM_PARTITION_PROTECTED_STORAGE [a]	Enable PS service	ON
TFM_PARTITION_PLATFORM	Enable TF-M Platform SP	ON

[a]

PS service is enabled by default. Platforms without off-chip storage devices can turn off TFM_PARTITION_PROTECTED_STORAGE to disable PS service. See Protected Storage Secure Partition for details.

Crypto service configurations

Crypto Secure Partition

TF-M Profile Large enables Crypto SP in top-level CMake config file and selects all the Crypto modules.

MbedTLS configurations

TF-M Profile Large adds a dedicated MbedTLS config file tfm_mbedcrypto_config_profile_large.h and MbedTLS PSA config file crypto_config_profile_large.h under /lib/ext/mbedcrypto/mbedcrypto_config folder, instead of the common one tfm_mbedcrypto_config_default.h and crypto_config_default.h [7].

Major MbedTLS configurations are set as listed below:

• Enable SHA256 and SHA512

• Enable generic message digest wrappers

• Enable AES

• Enable CCM mode, GCM mode and CBC mode for symmetric ciphers

• Disable other modes for symmetric ciphers

• Enable ECDH

• Enable ECDSA

• Enable RSA

• Select ECC curve secp256r1 and secp384r1

• Enable HMAC-based key derivation function

• Other configurations required by selected option above

A device/use case can append an extra config header to the Profile Large default MbedTLS config file to override the default settings. This can be done by setting the TFM_MBEDCRYPTO_PLATFORM_EXTRA_CONFIG_PATH cmake variable in the platform config file platform/ext<TFM_PLATFORM>/config.cmake. This cmake variable is a wrapper around the MBEDTLS_USER_CONFIG_FILE options, but is preferred as it keeps all configuration in cmake.

Internal Trusted Storage configurations

ITS service is enabled in top-level Profile Large CMake config file by default.

The internal transient buffer size ITS_BUF_SIZE [8] is set to 64 bytes by default. A platform/use case can overwrite the buffer size in its specific configuration extension according to its actual requirement of assets and Flash attributes.

Profile Large CMake config file won’t touch the configurations of device specific Flash hardware attributes.

Protected Storage Secure Partition

Data confidentiality, integrity and anti-rollback protection are enabled by default in PS.

If PS is selected, AES-CCM is used as AEAD algorithm by default. If platform hardware crypto accelerator supports the AEAD algorithm, the AEAD operations can be executed in hardware crypto accelerator.

If platforms don’t integrate any off-chip storage device, platforms can disable PS in platform specific configuration extension file via platform/ext<TFM_PLATFORM>/config.cmake.

BL2 setting

Profile Large enables MCUBoot provided by TF-M by default. A platform can overwrite this configuration by disabling MCUBoot in its configuration extension file platform/ext<TFM_PLATFORM>/config.cmake.

If MCUBoot provided by TF-M is enabled, multiple image boot is selected by default.

If a device implements its own boot loader, the configurations are implementation defined.

Software countermeasure against physical attacks

Profile Large selects TF-M FIH library Profile Medium by specifying -DTFM_FIH_PROFILE=MEDIUM in top-level CMake config file.

System integrators shall implement software countermeasures in platform specific implementations.

Device configuration extension

To change default configurations and add platform specific configurations, a platform can add a platform configuration file at platform/ext<TFM_PLATFORM>/config.cmake

Test configuration

Some cryptography tests are disabled due to the reduced MbedTLS config. Profile Large specific test configurations are also specified in Profile Large top-level CMake config file config/profile/profile_large_test.cmake.

Table 34 Profile Large crypto test configuration

Configs	Default value	Descriptions
TFM_CRYPTO_TEST_ALG_CBC	ON	Test CBC cryptography mode
TFM_CRYPTO_TEST_ALG_CCM	ON	Test CCM cryptography mode
TFM_CRYPTO_TEST_ALG_CFB	OFF	Test CFB cryptography mode
TFM_CRYPTO_TEST_ALG_ECB	OFF	Test ECB cryptography mode
TFM_CRYPTO_TEST_ALG_CTR	OFF	Test CTR cryptography mode
TFM_CRYPTO_TEST_ALG_OFB	OFF	Test OFB cryptography mode
TFM_CRYPTO_TEST_ALG_GCM	ON	Test GCM cryptography mode
TFM_CRYPTO_TEST_ALG_SHA_384	OFF	Test SHA-384 cryptography algorithm
TFM_CRYPTO_TEST_ALG_SHA_512	ON	Test SHA-512 cryptography algorithm
TFM_CRYPTO_TEST_HKDF	ON	Test HMAC-based key derivation function
TFM_CRYPTO_TEST_ECDH	ON	Test ECDH key agreement algorithm
TFM_CRYPTO_TEST_CHACHA20	OFF	Test ChaCha20 stream cipher
TFM_CRYPTO_TEST_CHACHA20_POLY1305	OFF	Test ChaCha20-Poly1305 AEAD algorithm
TFM_CRYPTO_TEST_SINGLE_PART_FUNCS	OFF	Test single-part operations in hash, MAC, AEAD and symmetric ciphers

Platform support

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

Building Profile Large

To build Profile Large, argument TFM_PROFILE in build command line should be set to profile_large.

Take AN521 as an example:

The following commands build Profile Large without test cases on AN521 with build type MinSizeRel, built by Armclang.

cd <TFM root dir>
mkdir build && cd build
cmake -DTFM_PLATFORM=arm/mps2/an521 \
      -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
      -DTFM_PROFILE=profile_large \
      -DCMAKE_BUILD_TYPE=MinSizeRel \
      ../
cmake --build ./ -- install

The following commands build Profile Large with regression test cases on AN521 with build type MinSizeRel, built by Armclang.

cd <TFM root dir>
mkdir build && cd build
cmake -DTFM_PLATFORM=arm/mps2/an521 \
      -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
      -DTFM_PROFILE=profile_large \
      -DCMAKE_BUILD_TYPE=MinSizeRel \
      -DTEST_S=ON -DTEST_NS=ON \
      ../
cmake --build ./ -- install

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

Reference

[1]

Trusted Firmware-M Profile Small Design

[2] (1,2)

Trusted Firmware-M Profile Medium Design

[3]

PSA Certified Level 3 Lightweight Protection Profile

[4] (1,2)

Arm Platform Security Architecture Firmware Framework 1.0

[5]

The Transport Layer Security (TLS) Protocol Version 1.2

[6]

Physical attack mitigation in Trusted Firmware-M

[7]

Crypto design

[8]

ITS integration guide

[9]

TF-M build instruction

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