Trusted Substrate is a meta-layer in OpenEmbedded aimed toward board makers who want to produce an Arm SystemReady compliant firmware and ensure consistent behaviour, tamper protection and common features across platforms. In a nutshell, TrustedSubstrate is building firmware for devices which verifies the running software hasn’t been tampered with. It does so by utilizing a well-known set of standards.
UEFI secure boot enabled by default UEFI Secure Boot is a verification mechanism for ensuring that code launched by a computer’s UEFI firmware is trusted. It is designed to protect a system against malicious code being loaded and executed early in the boot process before the operating system has been loaded.
Measured boot. With a discrete or firmware, TPM Measured Boot is a method where each of the software layers in the boot sequence of the device, measures the next layer in the execution order and extends the value in a designated TPM PCR. Measured boot further validates the boot process beyond Secure Boot.
Dual banked firmware updates with rollback and bricking protection provides protection to the firmware update mechanism and shield the device against bricking as well as rollback attacks.
Requirements and specifications
Trusted Substrate interfaces rely on a combination of existing standards:
SystemReady BBSR compliance (optional in the main case)
PSA Firmware Framework A specification compliance (Trust Services)
Global Platform TEE compliance
You can find a complete list of the requirements here
The following diagram shows how Trusted Substrate can be seen from upper layers:
Trusted Substrate exists in two flavours that build on SystemReady counterparts: TrustedSubstrate-IR and TrustedSubstrate-ES:
TrustedSubstrate-IR implementation is built on Trusted Firmware A, OP-TEE, and U-Boot and uses Device Tree as hardware description.
TrustedSubstrate-ES implementation is built on EDK2, OP-TEE and uses ACPI as hardware description (the main difference with typical data center firmware is the presence of OP-TEE). There are discussions to extend U-Boot to offer full ACPI support in this context.
The primary goal of the project is to upstream all necessary technologies in a number of open-source projects to seed SystemReady compliance. Linaro Edge & Fog computing group hardware as well as Qemu (64 & 32 bits) will be used as reference platforms for the development.
Development of Trusted Substrate is "feature orientated" rather than upstream project orientated. In other words, when a feature is planned, activities in relevant upstream projects is identified and monitored for completion as a whole. Each upstream project has its own roadmap that is not related to SystemReady compliance and is independent from other projects. So if you are evaluating what community to join, the decision criteria is whether your goal is holistic or just focused at an individual project.
Related Linaro software development projects
The Trusted Substrate project covers a wide range of software components as stated above. To orchestrate engineering activities in manageable pieces, the development is split between the following projects (Trusted Substrate project leadership ensure coherency and completeness across projects):
Dependable Boot - ensure SystemReady boot flow conformance across firmware projects (TF-A, OP-TEE, U-Boot, Linux kernel). This project collates work from different teams in Linaro (Kernel Working Group, Security Working Group, LEDGE). This is the main part of the Trusted Substrate project.
https://linaro.atlassian.net/wiki/spaces/DTE - efforts to create a System Device Tree that covers asymmetric computing platforms and to change the lifecycle of Device Tree so that it is provided by firmware to operating systems. Linaro does not carry out major development in Device Tree right now. Instead the work is done in other projects where needed.
https://linaro.atlassian.net/wiki/spaces/LOC - while most OP-TEE activities related to SystemReady are guided by Dependable Boot, some long term changes such as Trusted Application lifecycle and distribution scenarios may actually be driven by this project.