Running applications with Trusted Firmware-M

On nRF5340 and nRF9160, you can use Trusted Firmware-M (TF-M) as an alternative to Secure Partition Manager for running an application from the non-secure area of the memory.

Overview

TF-M is the reference implementation of Platform Security Architecture (PSA).

It provides a highly configurable set of software components to create a Trusted Execution Environment. This is achieved by a set of secure run time services such as Secure Storage, Cryptography, Audit Logs, and Attestation. Additionally, secure boot via MCUboot in TF-M ensures integrity of run time software and supports firmware upgrade.

Support for TF-M in nRF Connect SDK is currently experimental. TF-M is a framework which will be extended for new functions and use cases beyond the scope of SPM.

For official documentation, see the TF-M documentation.

The TF-M implementation in nRF Connect SDK is currently demonstrated in the following samples:

Building

TF-M is one of the images that are built as part of a multi-image application. If TF-M is used in the application, SPM will not be included in it. For more information about multi-image builds, see Multi-image builds.

To add TF-M to your build, enable the CONFIG_BUILD_WITH_TFM configuration option by adding it to your prj.conf file. To build a minimal version of TF-M, you must also enable the CONFIG_TFM_MINIMAL configuration.

Note

If you use menuconfig to enable CONFIG_BUILD_WITH_TFM, you must also enable its dependencies.

You must build TF-M using a non-secure build target. The following targets are currently supported:

  • nrf5340dk_nrf5340_cpuapp_ns

  • nrf9160dk_nrf9160_ns

When building for nrf9160dk_nrf9160_ns, UART1 must be disabled in the non-secure application, because it is used by the TF-M secure application. Otherwise, the non-secure application will fail to run. The recommended way to do this is to copy the .overlay file from the TF-M Hello World sample.

Enabling secure services

When using the Nordic Security Module, if CONFIG_BUILD_WITH_TFM is enabled together with CONFIG_NORDIC_SECURITY_BACKEND, the TF-M secure image will enable the use of the hardware acceleration of Arm CryptoCell. In such case, the Kconfig configurations in the Nordic Security Backend control the features enabled through TF-M.

You can configure what crypto modules to include in TF-M by using the TFM_CRYPTO_ Kconfig options found in file zephyr/modules/trusted-firmware-m/Kconfig.tfm.crypto_modules.

TF-M utilizes hardware unique keys when the PSA Crypto key derivation APIs are used, and psa_key_derivation_setup is called with the algorithm TFM_CRYPTO_ALG_HUK_DERIVATION.

Minimal build

The default configuration of TF-M has all supported features enabled, which results in a significant memory footprint. A minimal version of the TF-M secure application is provided in nRF Connect SDK to show how to configure a reduced version of TF-M.

The secure services supported by this minimal version allow for generating random numbers, hashing with SHA-256, and using tfm_platform_mem_read. This corresponds to the feature set provided by the Secure Partition Manager.

The minimal version of TF-M is enabled by setting the CONFIG_TFM_MINIMAL option.

When CONFIG_TFM_MINIMAL is set, the configurability of TF-M is severely limited. Hence, it is not possible to modify the TF-M minimal configuration to create your own variant of the minimal configuration. Instead, the default configuration must be used as a starting point.

Programming

The procedure for programming an application with TF-M is the same as for other multi-image applications in nRF Connect SDK.

After building the application, a merged.hex file is created that contains MCUboot, TF-M, and the application. The merged.hex file can be then programmed using SES. When using the command line, the file is programmed automatically when you call ninja flash or west flash.

Logging

TF-M employs two UART interfaces for logging: one for the secure part (MCUboot and TF-M), and one for the non-secure application. The logs arrive on different COM ports on the host PC.

On the nRF5340 DK, you must connect specific wires on the kit to receive secure logs on the host PC. Wire the pins P0.25 and P0.26 to RxD and TxD respectively.

Limitations

Application code that uses SPM Secure Services cannot use TF-M because the interface to TF-M is different and, at this time, not all SPM functions are available in TF-M.