Hi,

In Arm CCA the Security Model strongly recommends implementing the CCA HES functionality to ensure the system security properties. A way to achieve this is to add a trusted subsystem to the system, which behaves like a secure enclave. In ARM reference design this trusted subsystem is called to Runtime Security Subsystem (RSS). RSS can execute a firmware component that implements the functional requirements of the HES. But its firmware is not restricted to be only the HES, other tenants are also allowed. The goal of the CCA HES is to provide fundamental services to the AP to ensure its security properties. These fundamental services include secure boot, measured boot and attestation, etc. You can find more information about the role of CCA HES and about its functional requirements in the Arm CCA Security Model [1]. In ARM reference design the CCA HES is going to be executed by RSS. CCA HES is based on TF-M.

In this patch series [2] the AP side support of the CCA HES functionalities is going to be introduced:

- Communication over an MHU channel between the AP and RSS.

- Communication abstracted by the PSA API. So, AP can leverage standard PSA calls to invoke these services on the RSS.

- CCA HES  provides a measured boot backend. Measurements are taken during AP boot can be stored by RSS and retrieved as part of the CCA Platform Attestation token.

- CCA Platform Attestation token can be requested from RSS.

Currently, there is no publicly available FVP platform to test these patches, but it will be available later this year. Some limited testing is available on the AEM FVP. Here the RSS based measured boot backend is enabled and mocked version of the measured boot and attestation APIs are available. Due to the lack of RSS in the FVP, the APIs do not communicate to RSS, instead just print the measurements to the console and return to a hard-coded attestation token.

[1] https://developer.arm.com/documentation/DEN0096/latest

[2] https://review.trustedfirmware.org/q/topic:%2522rss/mboot-attest%2522

Best regards,

Tamas Ban