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238 lines
9.1 KiB
238 lines
9.1 KiB
Trusted Board Boot Design Guide
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===============================
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.. section-numbering::
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:suffix: .
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.. contents::
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The Trusted Board Boot (TBB) feature prevents malicious firmware from running on
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the platform by authenticating all firmware images up to and including the
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normal world bootloader. It does this by establishing a Chain of Trust using
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Public-Key-Cryptography Standards (PKCS).
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This document describes the design of Trusted Firmware-A (TF-A) TBB, which is
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an implementation of the Trusted Board Boot Requirements (TBBR) specification,
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Arm DEN0006C-1. It should be used in conjunction with the `Firmware Update`_
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design document, which implements a specific aspect of the TBBR.
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Chain of Trust
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--------------
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A Chain of Trust (CoT) starts with a set of implicitly trusted components. On
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the Arm development platforms, these components are:
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- A SHA-256 hash of the Root of Trust Public Key (ROTPK). It is stored in the
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trusted root-key storage registers.
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- The BL1 image, on the assumption that it resides in ROM so cannot be
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tampered with.
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The remaining components in the CoT are either certificates or boot loader
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images. The certificates follow the `X.509 v3`_ standard. This standard
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enables adding custom extensions to the certificates, which are used to store
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essential information to establish the CoT.
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In the TBB CoT all certificates are self-signed. There is no need for a
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Certificate Authority (CA) because the CoT is not established by verifying the
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validity of a certificate's issuer but by the content of the certificate
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extensions. To sign the certificates, the PKCS#1 SHA-256 with RSA Encryption
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signature scheme is used with a RSA key length of 2048 bits. Future version of
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TF-A will support additional cryptographic algorithms.
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The certificates are categorised as "Key" and "Content" certificates. Key
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certificates are used to verify public keys which have been used to sign content
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certificates. Content certificates are used to store the hash of a boot loader
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image. An image can be authenticated by calculating its hash and matching it
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with the hash extracted from the content certificate. The SHA-256 function is
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used to calculate all hashes. The public keys and hashes are included as
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non-standard extension fields in the `X.509 v3`_ certificates.
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The keys used to establish the CoT are:
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- **Root of trust key**
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The private part of this key is used to sign the BL2 content certificate and
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the trusted key certificate. The public part is the ROTPK.
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- **Trusted world key**
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The private part is used to sign the key certificates corresponding to the
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secure world images (SCP\_BL2, BL31 and BL32). The public part is stored in
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one of the extension fields in the trusted world certificate.
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- **Non-trusted world key**
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The private part is used to sign the key certificate corresponding to the
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non secure world image (BL33). The public part is stored in one of the
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extension fields in the trusted world certificate.
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- **BL3-X keys**
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For each of SCP\_BL2, BL31, BL32 and BL33, the private part is used to
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sign the content certificate for the BL3-X image. The public part is stored
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in one of the extension fields in the corresponding key certificate.
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The following images are included in the CoT:
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- BL1
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- BL2
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- SCP\_BL2 (optional)
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- BL31
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- BL33
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- BL32 (optional)
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The following certificates are used to authenticate the images.
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- **BL2 content certificate**
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It is self-signed with the private part of the ROT key. It contains a hash
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of the BL2 image.
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- **Trusted key certificate**
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It is self-signed with the private part of the ROT key. It contains the
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public part of the trusted world key and the public part of the non-trusted
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world key.
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- **SCP\_BL2 key certificate**
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It is self-signed with the trusted world key. It contains the public part of
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the SCP\_BL2 key.
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- **SCP\_BL2 content certificate**
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It is self-signed with the SCP\_BL2 key. It contains a hash of the SCP\_BL2
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image.
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- **BL31 key certificate**
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It is self-signed with the trusted world key. It contains the public part of
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the BL31 key.
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- **BL31 content certificate**
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It is self-signed with the BL31 key. It contains a hash of the BL31 image.
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- **BL32 key certificate**
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It is self-signed with the trusted world key. It contains the public part of
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the BL32 key.
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- **BL32 content certificate**
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It is self-signed with the BL32 key. It contains a hash of the BL32 image.
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- **BL33 key certificate**
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It is self-signed with the non-trusted world key. It contains the public
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part of the BL33 key.
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- **BL33 content certificate**
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It is self-signed with the BL33 key. It contains a hash of the BL33 image.
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The SCP\_BL2 and BL32 certificates are optional, but they must be present if the
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corresponding SCP\_BL2 or BL32 images are present.
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Trusted Board Boot Sequence
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---------------------------
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The CoT is verified through the following sequence of steps. The system panics
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if any of the steps fail.
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- BL1 loads and verifies the BL2 content certificate. The issuer public key is
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read from the verified certificate. A hash of that key is calculated and
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compared with the hash of the ROTPK read from the trusted root-key storage
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registers. If they match, the BL2 hash is read from the certificate.
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Note: the matching operation is platform specific and is currently
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unimplemented on the Arm development platforms.
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- BL1 loads the BL2 image. Its hash is calculated and compared with the hash
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read from the certificate. Control is transferred to the BL2 image if all
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the comparisons succeed.
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- BL2 loads and verifies the trusted key certificate. The issuer public key is
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read from the verified certificate. A hash of that key is calculated and
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compared with the hash of the ROTPK read from the trusted root-key storage
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registers. If the comparison succeeds, BL2 reads and saves the trusted and
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non-trusted world public keys from the verified certificate.
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The next two steps are executed for each of the SCP\_BL2, BL31 & BL32 images.
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The steps for the optional SCP\_BL2 and BL32 images are skipped if these images
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are not present.
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- BL2 loads and verifies the BL3x key certificate. The certificate signature
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is verified using the trusted world public key. If the signature
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verification succeeds, BL2 reads and saves the BL3x public key from the
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certificate.
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- BL2 loads and verifies the BL3x content certificate. The signature is
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verified using the BL3x public key. If the signature verification succeeds,
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BL2 reads and saves the BL3x image hash from the certificate.
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The next two steps are executed only for the BL33 image.
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- BL2 loads and verifies the BL33 key certificate. If the signature
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verification succeeds, BL2 reads and saves the BL33 public key from the
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certificate.
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- BL2 loads and verifies the BL33 content certificate. If the signature
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verification succeeds, BL2 reads and saves the BL33 image hash from the
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certificate.
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The next step is executed for all the boot loader images.
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- BL2 calculates the hash of each image. It compares it with the hash obtained
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from the corresponding content certificate. The image authentication succeeds
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if the hashes match.
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The Trusted Board Boot implementation spans both generic and platform-specific
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BL1 and BL2 code, and in tool code on the host build machine. The feature is
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enabled through use of specific build flags as described in the `User Guide`_.
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On the host machine, a tool generates the certificates, which are included in
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the FIP along with the boot loader images. These certificates are loaded in
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Trusted SRAM using the IO storage framework. They are then verified by an
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Authentication module included in TF-A.
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The mechanism used for generating the FIP and the Authentication module are
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described in the following sections.
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Authentication Framework
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------------------------
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The authentication framework included in TF-A provides support to implement
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the desired trusted boot sequence. Arm platforms use this framework to
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implement the boot requirements specified in the TBBR-client document.
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More information about the authentication framework can be found in the
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`Auth Framework`_ document.
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Certificate Generation Tool
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---------------------------
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The ``cert_create`` tool is built and runs on the host machine as part of the
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TF-A build process when ``GENERATE_COT=1``. It takes the boot loader images
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and keys as inputs (keys must be in PEM format) and generates the
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certificates (in DER format) required to establish the CoT. New keys can be
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generated by the tool in case they are not provided. The certificates are then
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passed as inputs to the ``fiptool`` utility for creating the FIP.
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The certificates are also stored individually in the in the output build
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directory.
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The tool resides in the ``tools/cert_create`` directory. It uses OpenSSL SSL
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library version 1.0.1 or later to generate the X.509 certificates. Instructions
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for building and using the tool can be found in the `User Guide`_.
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--------------
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*Copyright (c) 2015-2018, Arm Limited and Contributors. All rights reserved.*
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.. _Firmware Update: firmware-update.rst
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.. _X.509 v3: http://www.ietf.org/rfc/rfc5280.txt
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.. _User Guide: user-guide.rst
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.. _Auth Framework: auth-framework.rst
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