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-documentation for Discrete TPM drivers. -documentation for a proof of concept on rpi3; Measured Boot using Discrete TPM. Signed-off-by: Abhi Singh <abhi.singh@arm.com> Change-Id: If8e7c14a1c0b9776af872104aceeff21a13bd821
119 lines
4.9 KiB
ReStructuredText
119 lines
4.9 KiB
ReStructuredText
Discrete TPM drivers
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====================
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This section focuses on the design and functionality of Discrete TPM drivers
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in |TF-A|. The |TPM| technology is designed to provide
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a dedicated, hardware-based solution for storing cryptographic keys and
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performing security-related operations.
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Discrete TPMs are separate, standalone hardware components that are physically
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isolated from the system's main processor. This isolation helps protect
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sensitive information, such as encryption keys and platform credentials, from
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being accessed or tampered with by malicious software or unauthorized users.
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When a Discrete TPM interface is implemented correctly, the risk of software
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based attacks is reduced, further reducing the attack surface.
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TPM measurements establish the security posture of a system and are used for
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attestation. Performing measurements using a TPM in TF-A is beneficial from
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a security standpoint because it ensures hardware-backed attestation earlier
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in the boot flow, reducing the risk of a compromised root of trust.
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The design implemented in TF-A supports multiple types of TPM hardware interfaces
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and hardware bus types in order to be compatible with different platforms.
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Platforms opt to use a specific messaging interface, such as |CRB| or |FIFO|,
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and a specific hardware bus interface, such as |I2C| or |SPI|.
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Driver architecture
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-------------------
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The Discrete TPM drivers are split up into four layers, each serving a distinct
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purpose in the overall architecture:
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- **Command Layer**: This layer provides various TPM commands based on the
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`TCG TPM 2.0 Library Specification`_. It allows a system to initialize the
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TPM interface, perform a TPM startup, set up a locality for operations like
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PCR extend and read, and release the locality when finished.
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- **Interface Layer**: This layer handles sending and receiving TPM commands
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via a specific TPM interface like FIFO or CRB. It also includes functions
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such as getting information, requesting access, and relinquishing access,
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tailored to the specific interface.
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- **Link Layer**: Discrete TPMs may appear as a SPI, I2C, or memory mapped
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device. The link layer maps the command passed from the interface layer to
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the appropriate bus type. It includes hardware link read and write functions
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that use the platform bus interface to transfer commands.
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- **Platform Layer**: The platform layer implements the details of how to
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communicate to the TPM chip for a specific platform. The link layer uses the
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platform layer to read and write to the TPM.
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.. note::
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The command, interface, and link layers are implemented in common code in
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TF-A. The platform layer is implemented in platform specific code.
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The following diagram illustrates the Discrete TPM driver stack for the Raspberry
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Pi 3 platform.
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|rpi3 dtpm driver stack|
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Header files
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^^^^^^^^^^^^
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- TPM Drivers: ``include/drivers/tpm``
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Source files
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^^^^^^^^^^^^
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- TPM Drivers: ``drivers/tpm``
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Build time config options
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-------------------------
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- ``MBOOT_TPM_HASH_ALG``: The hash algorithm to be used by the TPM, currently
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the only supported algorithm is ``sha256``. As additional Discrete TPMs are
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tested and integrated in TF-A, support for more algorithms will become
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available.
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- ``DISCRETE_TPM``: Boolean flag to enable Discrete TPM support. Depending
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on the selected TPM interface, the appropriate drivers will be built and
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packaged into firmware.
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- ``TPM_INTERFACE``: This flag is required when ``DISCRETE_TPM=1``,
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currently the only supported interface is ``FIFO_SPI``.
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Ideally there should be four options:
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.. code:: shell
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FIFO_I2C
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FIFO_SPI
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FIFO_MMIO
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CRB
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.. note::
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``MBOOT_TPM_HASH_ALG`` will automatically overwrite ``MBOOT_EL_HASH_ALG``.
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This is to ensure the event log and the TPM are using the same hash
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algorithm.
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Discrete TPM Initialization
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---------------------------
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The TPM needs to be initialized based on the platform, the hardware interfaces
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need to be set up independently, and once they are setup, the TPM commands
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``tpm_interface_init()`` and subsequently ``tpm_startup()`` can be called.
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``tpm_startup()`` only needs to be called once after startup, or if the system
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is reset.
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An example of platform specific TPM hardware initialization for the rpi3 can be
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found in ``plat/rpi/rpi3/rpi3_bl1_setup.c`` and ``plat/rpi/rpi3/rpi3_bl1_mboot.c``
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Discrete TPM PCR Extend
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-----------------------
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Once the TPM is setup, the TPM ``pcr_extend`` operation can be used to extend
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hashes and store them in PCR 0.
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An example of ``pcr_extend`` that is used during rpi3 measured boot can be found
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in ``plat/rpi/rpi3/rpi3_bl1_mboot.c`` and ``plat/rpi/rpi3/rpi3_bl2_mboot.c``.
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*Copyright (c) 2025, Arm Limited. All rights reserved.*
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.. |rpi3 dtpm driver stack| image::
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../resources/diagrams/rpi3_dtpm_driver.png
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.. _TCG TPM 2.0 Library Specification: https://trustedcomputinggroup.org/resource/tpm-library-specification/
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