docs/design_documents/drtm_poc.rst

DRTM Proof of Concept
=====================

Dynamic Root of Trust for Measurement (DRTM) begins a new trust environment
by measuring and executing a protected payload.

Static Root of Trust for Measurement (SRTM)/Measured Boot implementation,
currently used by TF-A covers all firmwares, from the boot ROM to the normal
world bootloader. As a whole, they make up the system's TCB. These boot
measurements allow attesting to what software is running on the system and
enable enforcing security policies.

As the boot chain grows or firmware becomes dynamically extensible,
establishing an attestable TCB becomes more challenging. DRTM  provides a
solution to this problem by allowing measurement chains to be started at
any time. As these measurements are stored separately from the boot-time
measurements, they reduce the size of the TCB, which helps reduce the attack
surface and the risk of untrusted code executing, which could compromise
the security of the system.

Components
~~~~~~~~~~

   - **DCE-Preamble**: The DCE Preamble prepares the platform for DRTM by
     doing any needed configuration, loading the target payload image(DLME),
     and preparing input parameters needed by DRTM. Finally, it invokes the
     DL Event to start the dynamic launch.

   - **D-CRTM**: The D-CRTM is the trust anchor (or root of trust) for the
     DRTM boot sequence and is where the dynamic launch starts. The D-CRTM
     must be implemented as a trusted agent in the system. The D-CRTM
     initializes the TPM for DRTM and prepares the environment for the next
     stage of DRTM, the DCE. The D-CRTM measures the DCE, verifies its
     signature, and transfers control to it.

   - **DCE**: The DCE executes on an application core. The DCE verifies the
     system’s state, measures security-critical attributes of the system,
     prepares the memory region for the target payload, measures the payload,
     and finally transfers control to the payload.

   - **DLME**: The protected payload is referred to as the Dynamically Launched
     Measured Environment, or DLME. The DLME begins execution in a safe state,
     with a single thread of execution, DMA protections, and interrupts
     disabled. The DCE provides data to the DLME that it can use to verify the
     configuration of the system.

In this proof of concept, DCE and D-CRTM are implemented in BL31 and
DCE-Preamble and DLME are implemented in UEFI application. A DL Event is
triggered as a SMC by DCE-Preamble and handled by D-CRTM, which launches the
DLME via DCE.

This manual provides instructions to build TF-A code with pre-buit EDK2
and DRTM UEFI application.

Building the PoC for the Arm FVP platform
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

(1) Use the below command to clone TF-A source code -

.. code:: shell

   $ git clone https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git

(2) There are prebuilt binaries required to execute the DRTM implementation
    in the `prebuilts-drtm-bins`_.
    Download EDK2  *FVP_AARCH64_EFI.fd* and UEFI DRTM application *test-disk.img*
    binary from `prebuilts-drtm-bins`_.

(3) Build the TF-A code using below command

.. code:: shell

   $ make CROSS_COMPILE=aarch64-none-elf- ARM_ROTPK_LOCATION=devel_rsa
     DEBUG=1 V=1 BL33=</path/to/FVP_AARCH64_EFI.fd> DRTM_SUPPORT=1
     MBEDTLS_DIR=</path/to/mbedTLS-source> USE_ROMLIB=1 all fip

Running DRTM UEFI application on the Armv8-A AEM FVP
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To run the DRTM test application along with DRTM implementation in BL31,
you need an FVP model. Please use the version of FVP_Base_RevC-2xAEMvA model
advertised in the TF-A documentation.

.. code:: shell

    FVP_Base_RevC-2xAEMvA \
    --data cluster0.cpu0=</path/to/romlib.bin>@0x03ff2000 \
    --stat \
    -C bp.flashloader0.fname=<path/to/fip.bin> \
    -C bp.secureflashloader.fname=<path/to/bl1.bin> \
    -C bp.ve_sysregs.exit_on_shutdown=1 \
    -C bp.virtioblockdevice.image_path=<path/to/test-disk.img> \
    -C cache_state_modelled=1 \
    -C cluster0.check_memory_attributes=0 \
    -C cluster0.cpu0.etm-present=0 \
    -C cluster0.cpu1.etm-present=0 \
    -C cluster0.cpu2.etm-present=0 \
    -C cluster0.cpu3.etm-present=0 \
    -C cluster0.stage12_tlb_size=1024 \
    -C cluster1.check_memory_attributes=0 \
    -C cluster1.cpu0.etm-present=0 \
    -C cluster1.cpu1.etm-present=0 \
    -C cluster1.cpu2.etm-present=0 \
    -C cluster1.cpu3.etm-present=0 \
    -C cluster1.stage12_tlb_size=1024 \
    -C pctl.startup=0.0.0.0 \
    -Q 1000 \
    "$@"

The bottom of the output from *uart1* should look something like the
following to indicate that the last SMC to unprotect memory has been fired
successfully.

.. code-block:: shell

 ...

 INFO:    DRTM service handler: version
 INFO:    ++ DRTM service handler: TPM features
 INFO:    ++ DRTM service handler: Min. mem. requirement features
 INFO:    ++ DRTM service handler: DMA protection features
 INFO:    ++ DRTM service handler: Boot PE ID features
 INFO:    ++ DRTM service handler: TCB-hashes features
 INFO:    DRTM service handler: dynamic launch
 WARNING: DRTM service handler: close locality is not supported
 INFO:    DRTM service handler: unprotect mem

--------------

*Copyright (c) 2022, Arm Limited. All rights reserved.*

.. _prebuilts-drtm-bins: https://downloads.trustedfirmware.org/tf-a/drtm
.. _DRTM-specification: https://developer.arm.com/documentation/den0113/a