TF-A BL2 Trusted Board Boot

Applicable for

STM32MP13x lines, STM32MP15x lines

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1. Article purpose

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

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 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

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

3. Chain of Trust (CoT)

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 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 defined in fdts/cot_descriptors.dtsi . It is used by the firmware configuration framework to register into TF-A BL2 the chaining implementation based on generic CoT DTSI file. It describes the chain of trust following the specified bindings. STM32MPU platforms use a specific STM32MPU device tree fdts/stm32mp1-cot-descriptors.dtsi which described a specific topology for ST needs described in the following chapter. Based on this example, the PKI topology can be customized by customers.

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

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 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

5.1. CoT

STM32 MPU is based on the standard Arm^®TBBR PKI implementation. STM32MPU defines a specific fdts/stm32mp1-cot-descriptors.dtsi in the TF-A BL2 device tree board file.

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

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

• Certificate creation tool platform level plat/st/stm32mp1/stm32mp1_tbb_cert.c .
• Custom device tree descriptor level fdts/stm32mp1-cot-descriptors.dtsi following the bindings.

5.2. RoT

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 , the hash of the public key used is located in the PKH section of OTP.
  

• On STM32MP13x lines , 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

The default implementation uses the same public key as the ROM Code.

5.2.1. Define a custom RoT

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

• Use the STM32MP15x lines  direct access to OTP value in plat/st/common/stm32mp_trusted_boot.c
• Define a new dedicated OTP value in the plat/st/stm32mp1/stm32mp1_def.h
• Add a new entry in the board device tree in the fuse section with to defined name and OTP section.
• Get the value from the new defined OTP updating the plat/st/common/stm32mp_trusted_boot.c

 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

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 non-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. This implies generating the certificate version number for each FIP image must match the common platform unique TAMP rollback counter. That is using the same argument value for options --tfw-nvctr and --ntfw-nvctr when generating the authenticated FIP image.

5.4. STM32 firmware encryption

On STM32MP13x lines , 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

A new decryption key can be defined by updating:

• Redefine the plat_get_enc_key_info function to access the required OTP value in plat/st/common/stm32mp_crypto_lib.c
• Define a new dedicated OTP value in the plat/st/stm32mp1/stm32mp1_def.h
• Add a new entry in the board device tree in the fuse section aligned with the defined name of OTP section.

5.5. STM32 Cryptographic Library (CL)

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 :
  • 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 :
  • 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

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
STM32MP configuration certificate	STM32MP_CFG_CERT	--stm32mp-cfg-cert

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.

Microprocessor unit

Arm^® is a registered trademark of Arm Limited (or its subsidiaries) in the US and/or elsewhere.

Trusted firmware for Arm^® Cortex^®-A

Boot loader stage 2

Chain of trust

One time programmed

Public key hash, used for secure boot - NEW

Public key hash table hash, used for secure boot - NEW

Firmware image package is a packaging format used by TF-A

Tamper

Encryption Decryption master key, used for secure boot - NEW

Elliptic curve digital signature algorithm

Application programming interface

Advanced encryption standard