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Trusted Firmware-M Overview

Trusted Firmware-M (TF-M) is a reference implementation of the Platform Security Architecture (PSA) IoT Security Framework. It defines and implements an architecture and a set of software components that aim to address some of the main security concerns in IoT products.

Zephyr RTOS has been PSA Certified since Zephyr 2.0.0 with TF-M 1.0, and is currently integrated with TF-M 1.4.1.

What Does TF-M Offer?

Through a set of secure services and by design, TF-M provides:

  • Isolation of secure and non-secure resources

  • Embedded-appropriate crypto

  • Management of device secrets (keys, etc.)

  • Firmware verification (and encryption)

  • Protected off-chip data storage and retrieval

  • Proof of device identity (device attestation)

  • Audit logging

Build System Integration

When using TF-M with a supported platform, TF-M will be automatically built and link in the background as part of the standard Zephyr build process. This build process makes a number of assumptions about how TF-M is being used, and has certain implications about what the Zephyr application image can and can not do:

  • The secure processing environment (secure boot and TF-M) starts first

  • Resource allocation for Zephyr relies on choices made in the secure image.

Architecture Overview

A TF-M application will, generally, have the following three parts, from most to least trusted, left-to-right, with code execution happening in the same order (secure boot > secure image > ns image).

While the secure bootloader is optional, it is enabled by default, and secure boot is an important part of providing a secure solution:

+-------------------------------------+           +--------------+
| Secure Processing Environment (SPE) |           |     NSPE     |
| +----------++---------------------+ |           | +----------+ |
| |          ||                     | |           | |          | |
| | bl2.bin  ||  tfm_s_signed.bin   | |           | |zephyr.bin| |
| |          ||                     | | <- PSA -> | |          | |
| |  Secure  || Trusted Firmware-M  | |    APIs   | |  Zephyr  | |
| |   Boot   ||   (Secure Image)    | |           | |(NS Image)| |
| |          ||                     | |           | |          | |
| +----------++---------------------+ |           | +----------+ |
+-------------------------------------+           +--------------+

Communication between the (Zephyr) Non-Secure Processing Environment (NSPE) and the (TF-M) Secure Processing Environment image happens based on a set of PSA APIs, and normally makes use of an IPC mechanism that is included as part of the TF-M build, and implemented in Zephyr (see modules/trusted-firmware-m/interface).

Root of Trust (RoT) Architecture

TF-M is based upon a Root of Trust (RoT) architecture. This allows for hierarchies of trust from most, to less, to least trusted, providing a sound foundation upon which to build or access trusted services and resources.

The benefit of this approach is that less trusted components are prevented from accessing or compromising more critical parts of the system, and error conditions in less trusted environments won’t corrupt more trusted, isolated resources.

The following RoT hierarchy is defined for TF-M, from most to least trusted:

  • PSA Root of Trust (PRoT), which consists of:

    • PSA Immutable Root of Trust: secure boot

    • PSA Updateable Root of Trust: most trusted secure services

  • Application Root of Trust (ARoT): isolated secure services

The PSA Immutable Root of Trust is the most trusted piece of code in the system, to which subsequent Roots of Trust are anchored. In TF-M, this is the secure boot image, which verifies that the secure and non-secure images are valid, have not been tampered with, and come from a reliable source. The secure bootloader also verifies new images during the firmware update process, thanks to the public signing key(s) built into it. As the name implies, this image is immutable.

The PSA Updateable Root of Trust implements the most trusted secure services and components in TF-M, such as the Secure Partition Manager (SPM), and shared secure services like PSA Crypto, Internal Trusted Storage (ITS), etc. Services in the PSA Updateable Root of Trust have access to other resources in the same Root of Trust.

The Application Root of Trust is a reduced-privilege area in the secure processing environment which, depending on the isolation level chosen when building TF-M, has limited access to the PRoT, or even other ARoT services at the highest isolation levels. Some standard services exist in the ARoT, such as Protected Storage (PS), and generally custom secure services that you implement should be placed in the ARoT, unless a compelling reason is present to place them in the PRoT.

These divisions are distinct from the untrusted code, which runs in the non-secure environment, and has the least privilege in the system. This is the Zephyr application image in this case.

Isolation Levels

At present, there are three distinct isolation levels defined in TF-M, with increasingly rigid boundaries between regions. The isolation level used will depend on your security requirements, and the system resources available to you.

  • Isolation Level 1 is the lowest isolation level, and the only major boundary is between the secure and non-secure processing environment, usually by means of Arm TrustZone on Armv8-M processors. There is no distinction here between the PSA Updateable Root of Trust (PRoT) and the Application Root of Trust (ARoT). They execute at the same privilege level. This isolation level will lead to the smallest combined application images.

  • Isolation Level 2 builds upon level one by introducing a distinction between the PSA Updateable Root of Trust and the Application Root of Trust, where ARoT services have limited access to PRoT services, and can only communicate with them through public APIs exposed by the PRoT services. ARoT services, however, are not strictly isolated from one another.

  • Isolation Level 3 is the highest isolation level, and builds upon level 2 by isolating ARoT services from each other, so that each ARoT is essentially silo’ed from other services. This provides the highest level of isolation, but also comes at the cost of additional overhead and code duplication between services.

The current isolation level can be checked via CONFIG_TFM_ISOLATION_LEVEL.

Secure Boot

The default secure bootloader in TF-M is based on MCUBoot, and is referred to as BL2 in TF-M (for the second-stage bootloader, potentially after a HW-based bootloader on the secure MCU, etc.).

All images in TF-M are hashed and signed, with the hash and signature verified by MCUBoot during the firmware update process.

Some key features of MCUBoot as used in TF-M are:

  • Public signing key(s) are baked into the bootloader

  • S and NS images can be signed using different keys

  • Firmware images can optionally be encrypted

  • Client software is responsible for writing a new image to the secondary slot

  • By default, uses static flash layout of two identically-sized memory regions

  • Optional security counter for rollback protection

When dealing with (optionally) encrypted images:

  • Only the payload is encrypted (header, TLVs are plain text)

  • Hashing and signing are applied over the un-encrypted data

  • Uses AES-CTR-128 or AES-CTR-256 for encryption

  • Encryption key randomized every encryption cycle (via imgtool)

  • The AES-CTR key is included in the image and can be encrypted using:

    • RSA-OAEP

    • AES-KW (128 or 256 bits depending on the AES-CTR key length)

    • ECIES-P256

    • ECIES-X25519

Key config properties to control secure boot in Zephyr are:

Secure Processing Environment

Once the secure bootloader has finished executing, a TF-M based secure image will begin execution in the secure processing environment. This is where our device will be initially configured, and any secure services will be initialised.

Note that the starting state of our device is controlled by the secure firmware, meaning that when the non-secure Zephyr application starts, peripherals may not be in the HW-default reset state. In case of doubts, be sure to consult the board support packages in TF-M, available in the platform/ext/target/ folder of the TF-M module (which is in modules/tee/tf-m/trusted-firmware-m/ within a default Zephyr west workspace.)

Secure Services

As of TF-M 1.4.1, the following secure services are generally available (although vendor support may vary):

  • Audit Logging (Audit)

  • Crypto (Crypto)

  • Firmware Update (FWU)

  • Initial Attestation (IAS)

  • Platform (Platform)

  • Secure Storage, which has two parts:

    • Internal Trusted Storage (ITS)

    • Protected Storage (PS)

A template also exists for creating your own custom services.

For full details on these services, and their exposed APIs, please consult the TF-M Documentation.

Key Management and Derivation

Key and secret management is a critical part of any secure device. You need to ensure that key material is available to regions that require it, but not to anything else, and that it is stored securely in a way that makes it difficult to tamper with or maliciously access.

The Internal Trusted Storage service in TF-M is used by the PSA Crypto service (which itself makes use of mbedtls) to store keys, and ensure that private keys are only ever accessible to the secure processing environment. Crypto operations that make use of key material, such as when signing payloads or when decrypting sensitive data, all take place via key handles. At no point should the key material ever be exposed to the NS environment.

One exception is that private keys can be provisioned into the secure processing environment as a one-way operation, such as during a factory provisioning process, but even this should be avoided where possible, and a request should be made to the SPE (via the PSA Crypto service) to generate a new private key itself, and the public key for that can be requested during provisioning and logged in the factory. This ensures the private key material is never exposed, or even known during the provisioning phase.

TF-M also makes extensive use of the Hardware Unique Key (HUK), which every TF-M device must provide. This device-unique key is used by the Protected Storage service, for example, to encrypt information stored in external memory. For example, this ensures that the contents of flash memory can’t be decrypted if they are removed and placed on a new device, since each device has its own unique HUK used while encrypting the memory contents the first time.

HUKs provide an additional advantage for developers, in that they can be used to derive new keys, and the derived keys don’t need to be stored since they can be regenerated from the HUK at startup, using an additional salt/seed value (depending on the key derivation algorithm used). This removes the storage issue and a frequent attack vector. The HUK itself it usually highly protected in secure devices, and inaccessible directly by users.

TFM_CRYPTO_ALG_HUK_DERIVATION identifies the default key derivation algorithm used if a software implementation is used. The current default algorithm is HKDF (RFC 5869) with a SHA-256 hash. Other hardware implementations may be available on some platforms.

Non-Secure Processing Environment

Zephyr is used for the NSPE, using a board that is supported by TF-M where the CONFIG_BUILD_WITH_TFM flag has been enabled.

Generally, you simply need to select the *_ns variant of a valid target (for example mps2_an521_ns), which will configure your Zephyr application to run in the NSPE, correctly build and link it with the TF-M secure images, sign the secure and non-secure images, and merge the three binaries into a single tfm_merged.hex file. The west flash command will flash tfm_merged.hex by default in this configuration.

At present, Zephyr can not be configured to be used as the secure processing environment.