Platform Service Integration Guide

Introduction

TF-M Platform service is a trusted service which allows secure partitions and non-secure applications to interact with some platform-specific components. There are a number of features which requires some interaction with platform-specific components which are at the same time essential for the security of the system. Therefore, those components need to be handled by a secure partition which is part of the trusted compute base.

These platform-specific components include system reset, power management, Debug, GPIO, etc.

TF-M Platform interfaces

The TF-M interfaces for the Platform service are located in interface/include/. The TF-M Platform service source files are located in secure_fw/partitions/platform.

TF-M Platform service

The Platform service interfaces and types are defined and documented in interface/include/tfm_platform_api.h

  • platform_sp.h/c : These files define and implement functionalities related to the platform service

  • tfm_platform_secure_api.c : This file implements tfm_platform_api.h functions to be called from the secure partitions. This is the entry point when the secure partitions request an action to the Platform service (e.g system reset).

Platform HAL

The Platform Service relies on a platform-specific implementation to perform some functionalities. Mandatory functionality (e.g. system reset) that are required to be implemented for a platform to be supported by TF-M have their dedicated HAL API functions. Additional platform-specific services can be provided using the IOCTL function call.

For API specification, please check: platform/include/tfm_platform_system.h

An implementation is provided in all the supported platforms. Please, check platform/ext/target/<SPECIFIC_TARGET_FOLDER>/services/src/tfm_platform_system.c for examples.

The API must be implemented by the system integrators for their targets.

IOCTL

A single entry point to platform-specific code across the HAL is provided by the IOCTL service and HAL function:

enum tfm_platform_err_t tfm_platform_hal_ioctl(tfm_platform_ioctl_req_t request,
                                               psa_invec *in_vec,
                                               psa_outvec *out_vec);

A request type is provided by the client, with additional parameters contained in the optional in_vec parameter. An optional output buffer can be passed to the service in out_vec. An IOCTL request type not supported on a particular platform should return TFM_PLATFORM_ERR_NOT_SUPPORTED

Non-Volatile counters

The Platform Service provides an abstracted service for exposing the NV counters to secure partitions or non-secure callers. The following operations are supported:

  • Increment a counter.

  • Read a counter value to a preallocated buffer.

enum tfm_platform_err_t
tfm_platform_nv_counter_increment(uint32_t counter_id);

enum tfm_platform_err_t
tfm_platform_nv_counter_read(uint32_t counter_id,
                             uint32_t size, uint8_t *val);

The range of counters id is defined in : platform/include/tfm_plat_nv_counters.h

For Level 2,3 isolation implementations, secure partitions in the Application Root of Trust, should have TFM_PLATFORM_SERVICE set as a dependency for access to the NV counter API.

Current Service Limitations

  • system reset - The system reset functionality is only supported in isolation level 1. Currently the mechanism by which PSA-RoT services should run in privileged mode in level 3 is not in place due to an ongoing work in TF-M Core. So, the NVIC_SystemReset call performed by the service is expected to generate a memory fault when it tries to access the SCB->AIRCR register in level 3 isolation.


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