TF-A BL2 Trusted Board Boot

Applicable for STM32MP13x lines, STM32MP15x lines

1 Article purpose[edit]

The main purpose of this article is to explain how the Trusted Board Boot feature support is managed in the Trusted Firmware-A component and the STM32 MPU specific implementation.

2 Overview[edit]

Trusted Firmware-A has a strong focus on security management. It defines a reference implementation of secure software and implements the Trusted Board Boot requirements[1] specified by Arm®. TF-A BL2 implements an authentication framework that uses a defined Chain of Trust (CoT) based on Arm® TBBR requirements to achieve a secure boot.

The authentication framework is enabled using a dedicated Trusted Firmware-A build flag in the BL2 command line:
TRUSTED_BOARD_BOOT=1

Some specific libraries are required to build tools and enable the TRUSTED_BOARD_BOOT : Prerequisites software library.

Warning white.png Warning
The TRUSTED_BOARD_BOOT=1 is mandatory as soon as the MPU is in Secure Close state.

3 Chain of Trust (CoT)[edit]

To manage the Trusted Board Boot requirements, Trusted Firmware-A manages a public key Infrastructure (PKI) model so called Chain of Trust.

The Chain of Trust is following X509.v3[2] certificate standard adding some specific extensions and relies on a public key infrastructure (PKI) generating self-signed certificates. The Root of Trust of this chain is based on a Root of Trust Public Key (ROTPK).

Chain of Trust manages two certificate types:

  • Key certificate used to verify public keys, which have been used to sign content certificates.
  • Content certificate used to store the hash of a bootloader image.

Both certificates contain a non-volatile counter value that can be used for anti-rollback protection.

TF-A BL2 implements a default certificate chain to describe the PKI topology. It uses the firmware configuration framework to register into TF-A BL2 the chaining implementation based on default device tree include fdts/cot_descriptors.dtsi . It describes the chain of trust following the specified bindings.

Once generated, all the certificates are part of the FIP, which are updatable independently.

The certificate generation uses the default Trusted Firmware-A certificate creation tool.

4 Authentication framework[edit]

Trusted Firmware-A design an authentication framework to centralize all secure mechanism relying on an authentication module (AM), image parser module (IPM) and Crypto Module (CM). This generic framework based on the certificate chain of trust is responsible of the following:

  • allocate memory for the loaded image
  • identify the image and load it
  • check the integrity
  • authenticate / [ Decrypt ] the image
  • extract the CoT information if needed to authenticate next image in CoT

A reference implementation has been integrated in Trusted Firmware-A using mbedTLS[3] for:

  • the Image Parser Module(IPM) using x509.v3 certificate (Image Parser Library (IPL))
  • the Crypto Module(CM) with a dedicated Cryptographic Library (CL).

5 STM32 MPU implementation[edit]

5.1 CoT[edit]

STM32 MPU uses the standard Arm®TBBR PKI implementation by including the generic fdts/cot_descriptors.dtsi in the TF-A BL2 device tree board file.

Warning white.png Warning
This CoT description is defined in the BL2 device tree so it cannot be changed without any TF-A BL2 update

5.1.1 Define a custom CoT[edit]

It is possible to update the CoT by changing the chain of certificate. This must be done accordingly at:

5.2 RoT[edit]

The Root of Trust Public Key (ROTPK) used as reference for the chain of trust.
The STM32MP1 platform plat_get_rotpk_info implementation is made in : plat/st/common/stm32mp_trusted_boot.c .

The ROTPK is given in the high-level certificate and verified against a hash store OTP value:

  • On STM32MP15x lines More info.png, the hash of the public key used is located in the PKH section of OTP.
  • On STM32MP13x lines Warning.png, multiple root keys are available on the platform which allow a key revocation mechanism. The multiple public key hashes are used to generate the final hash store in the PKHTH section of OTP which is controlled to confirm key integrity. The public key hash used is located in the Trusted Firmware-A BL2 STM32 header which is located in SRAM3.
Warning white.png Warning
The default implementation uses the same public key as the ROM Code.

5.2.1 Define a custom RoT[edit]

It is possible to change the ROTPK to have a dedicated public key for the CoT by changing the following:

 static int get_rotpk_hash(void *cookie, uint8_t *hash, size_t len)
 {
   if (cookie != NULL) {
      return -EINVAL;
   }
   return copy_hash_from_otp(<New_OTP_define>, hash, len);
 }

5.3 Non-volatile counters[edit]

Each certificate embeds a non-volatile counter value that is checked to control anti-rollback mechanism.
There are two non-volatile counters: - Trusted non-volatile counter - Non trusted volatile counter

On STM32MP1, TAMP monotonic counter is used to store the backup value, which requires backup battery to maintain the content. It is mandatory to align the same value between trusted and non-trusted value as only one counter is used as reference.

5.4 STM32 firmware encryption[edit]

On STM32MP13x lines Warning.png, it is possible to encrypt binary in the FIP.

The binary that can be encrypted must be defined in the plat/st/common/stm32mp_fconf_io.c .

Trusted Firmware-A provides a tool to manage the firmware encryption.

The current implementation uses the same key as the ROM Code which is located in EDMK section of OTP.

5.4.1 Define a custom encryption key[edit]

A new decryption key can be defined by updating:

5.5 STM32 Cryptographic Library (CL)[edit]

The STM32MP1 platform supports ECDSA based certificate with SHA256 signature.
A dedicated Cryptographic Library was implemented for STM32 MPU to use hardware accelerators plat/st/common/stm32mp_crypto_lib.c . It requires mbedTLS API to parse ASN1 certificate.

The Cryptographic Library (CL) defined the functions to:

  • On STM32MP15x lines More info.png:
    • Verify a digital signature : ECDSA based authentication (P256NIST or BRAINPOOL_P256T1) uses HASH and ROM Code secure services.
    • Verify a HASH : HASH calculation uses HASH
  • On STM32MP13x lines Warning.png:
    • ECDSA based authentication (P256NIST or BRAINPOOL_P256T1) uses HASH and PKA hardware accelerators.
    • Verify a HASH : HASH calculation uses HASH
    • Decrypt and verify a binary using SAES (Limited to AES GCM mode)

5.6 STM32 MPU build options[edit]

To manage the Trusted Board Boot, TF-A_BL2 must be built using TRUSTED_BOARD_BOOT=1.

When TRUSTED_BOARD_BOOT=1 has been set, the FIP must embed all the certificates.

It is possible to directly generate the FIP with all the necessary certificates using the specific build flag GENERATE_COT=1.

It is also possible to specify external keys. Otherwise, missing keys will be generated by the tool.

Description Makefile Flag certificate creation tool args
Root public key ROT_KEY --rot-key
Algorithm used for key generation KEY_ALG --key-alg
Algorithm used for hash HASH_ALG --hash-alg
Trusted word key TRUSTED_WORLD_KEY --trusted-world-key
Non Trusted word key NON_TRUSTED_WORLD_KEY --non-trusted-world-key
Trusted Firmware Non volatile counter TFW_NVCTR_VAL --tfw-nvctr
Non Trusted Firmware Non volatile counter NTFW_NVCTR_VAL --ntfw-nvctr
Trusted key certificate TRUSTED_KEY_CERT --trusted-key-cert


Warning white.png Warning
The content of each certificate is different and because of the dependencies, it requires all long arguments list to regenerate a single certificate properly. By default, The FIP will manage to randomly generate the constructed certificates with the correct output.

6 References[edit]