Stateless Root of Trust Services Reference

Author:: Mingyang Sun
Organization:: Arm Limited
Contact:: mingyang.sun@arm.com

Introduction

This document describes the implementation for the FF-M v1.1 feature - ‘Stateless RoT Service’, and the related references when developing RoT services.

It is recommended to refer to the FF-M v1.0 specification [1] and FF-M v1.1 extension [2] for background and rationale details.

Implementation Details

This chapter describes the implementation-defined items, including stateless handle value definition, tooling update, and programming API changes.

Stateless Handle Value Definition

The index, stateless indicator, and service version information are encoded into a handle by the manifest tooling, and then generated to header file sid.h.

Table 41 Bit Fields of Stateless Handle
Bits	Field Description
bit 31	reserved
bit 30	stateless handle indicator bit, always 1
bit 29 - bit 16	reserved
bit 15 - bit 8	service version requested by client - for client version check
bit 7 - bit 5	reserved
bit 4 - bit 0	the handle index, [0, 31]

Since connection-based services and stateless services share the same PSA API psa_call(), an indicator bit is set in the handle indicate the type of the handle. If it is set, the handle is stateless, and definition is as described in the table above. Maximum connection-based handle is 0x3FFFFFFF, thus the indicator bit is always 0.

The index is used for organizing stateless services in manifest tool and locating a stateless service in SPM logic. A range of index [0, 31] is the initial implementation. Future expansion is possible.

Tooling Support

TF-M provides a tool (tools/tfm_parse_manifest_list.py) to generate source header files required by partition and services. For example, the generated sid.h contains service ID and version. The tooling is extended to generate stateless handle from partition manifests automatically.

The stateless_handle attribute in manifest is only supported by partitions with firmware framework version 1.1.

If stateless_handle in manifest is set to an integer, the index is stateless_handle - 1.
If it is auto or not set, the first empty index in range [0, 31] is assigned.
Other settings - tooling reports an error.

Finally, the tooling encodes the handle according to definitions in Stateless Handle Value Definition section, and writes them to sid.h header file.

Changes in Programming API

This chapter describes the changes in programming API for stateless services. The following APIs’ behavious and message data structure members are updated to support the stateless service.

psa_connect()

According to FF-M v1.1, client calling psa_connect() with the SID of a stateless RoT Service is a PROGRAMMER_ERROR.

psa_close()

According to FF-M v1.1, client passing a stateless handle to call this API is a PROGRAMMER_ERROR.

psa_call()

psa_status_t psa_call(psa_handle_t handle, int32_t type,
                      const psa_invec *in_vec, size_t in_len,
                      psa_outvec *out_vec, size_t out_len)

API parameters and behaviour change:

The handle parameter must be a stateless handle defined in psa_manifest/sid.h when requesting a stateless service.
This API validates stateless handle, decodes index and service version information from it. SPM uses the index to know which stateless service is requested.
This API performs some operations as psa_connect() does, such as the authorization check, service and client version check, and handle space allocation.

Service behaviour change during a “psa_call”:

Service does not accept connection and disconnection messages. After a “psa_call” request is serviced, it calls psa_reply(), frees the connection handle to handle pool.

psa_set_rhandle()

According to FF-M v1.1, stateless service calling this API on a message is a PROGRAMMER_ERROR and it will never return.

psa_msg_t type

The rhandle member of a psa_msg_t type received by a stateless RoT Service is always NULL.

Application Recommendation

There are particular services that do not need sessions. The access to the service is a one-shot connection. These services provide standalone operations that do not require any non-volatile state of resources to be maintained by the RoT service itself or do not expose any kind of context or session to the caller. Such services are recommended to be implemented as stateless, to provide quick access and to avoid extra overheads.

In this case, rhandle access would be prohibited as it is used for saving state or non-volatile resources while stateless services do not need them.

Update Feasibility of Existing Services

TF-M native services are used widely. They only need standalone operations and do not need to keep state between sessions. For example, the service in Crypto partition does not do anything during psa_connect() or psa_close() process. Same for services in other partitions, thus all of them can be implemented as stateless.

Analysis for them:

Table 42 TF-M Partition Services Update Possibility
Partition	Number of Services	Can be Stateless
ITS	4	All
PS	5	All
Crypto	1	All
FWU	6	All
Platform	4	All
Initial Attestation	2	All

Other services are not analyzed here.

Grouping Services

Stateless service table is stored statically, and TF-M supports 32 stateless services currently.

Similar stateless services in a partition could be grouped, and assign one SID for the group. The type parameter in psa_call() could be extended to identify the service in group. In this case, it is recommended to use consecutive value for type.

It is recommended that each Seccure Partition declares one stateless service and uses the type parameter to distinguish different stateless services. Therefore, more stateless services can be supported.

Migrating to Stateless Services

Please refer to Chapter 4 “Stateless Root of Trust services”, Appendix B.3.2 “Using a stateless RoT Service”, and Appendix D “Implementing session-less RoT Services” in FF-M v1.1 document for details on which kind of service can be stateless and how to implement a stateless service.