Adding Secure Partition

Terms and abbreviations

This document uses the following terms and abbreviations.

Table 35 term table

Term	Meaning
FF-M	Firmware Framework for M
ID	Identifier
IPC	Interprocess communication
IPC model	The secure IPC framework
irqs	Interrupt requests
MMIO	Memory Mapped I/O
PSA	Platform Security Architecture
RoT	Root of Trust
SFN	Secure Function
SFN model	Secure Function model
SID	RoT Service ID
SP	Secure Partition
SPM	Secure Partition Manager
TF-M	Trusted firmware M

Introduction

Secure Partition is an execution environment that provides the following functions to Root of Trust (RoT) Services:

• Access to resources, protection of its own code and data.

• Mechanisms to interact with other components in the system.

Each Secure Partition is a single thread of execution and the smallest unit of isolation.

This document mainly describes how to add a secure partition in TF-M and focuses on the configuration, manifest, implement rules. The actual source-level implementation is not included in this document.

Note

If not otherwise specified, the steps are identical for IPC and SFN model.

The IPC and SFN model conforms to the PSA Firmware Framework for M (FF-M) v 1.1 changes. Refer to PSA Firmware Framework specification and Firmware Framework for M 1.1 Extensions for details.

Process

The main steps to add a secure partition are as follows:

• Add source folder

• Add manifest

• Add configuration

• Implement the RoT services

Add source folder

Add a source folder under <TF-M base folder>/secure_fw/partitions for the new secure partition (Let’s take example as the folder name):

This folder should include those parts:

• Manifest file

• CMake configuration files

• Source code files

Add manifest

Each Secure Partition must have resource requirements declared in a manifest file. The Secure Partition Manager (SPM) uses the manifest file to assemble and allocate resources within the SPE. The manifest includes the following:

• A Secure Partition name.

• A list of implemented RoT Services.

• Access to other RoT Services.

• Memory requirements.

• Scheduling hints.

• Peripheral memory-mapped I/O regions and interrupts.

Note

The current manifest format in TF-M is “yaml” which is different from the requirement of PSA FF.

Here is a manifest reference example for the IPC model:

Note

To use SFN model, the user needs to replace "model": "IPC" to "model": "SFN". The user also needs to remove the attribute "entry_point", and optionally replace it with "entry_init".

{
  "psa_framework_version": 1.1,
  "name": "TFM_SP_EXAMPLE",
  "type": "APPLICATION-ROT",
  "priority": "NORMAL",
  "model": "IPC",
  "entry_point": "tfm_example_main",
  "stack_size": "0x0200",
  "services" : [
    {
      "name": "ROT_A",
      "sid": "0x000000E0",
      "non_secure_clients": true,
      "connection_based": true,
      "version": 1,
      "version_policy": "STRICT"
      "mm_iovec": "disable"
    }
  ],
  "mmio_regions": [
    {
      "name": "TFM_PERIPHERAL_A",
      "permission": "READ-WRITE"
    }
  ],
  "irqs": [
    {
      "source": "TFM_A_IRQ",
      "name": "A_IRQ",
      "handling": "SLIH"
    }
  ]
  "dependencies": [
    "TFM_CRYPTO",
    "TFM_INTERNAL_TRUSTED_STORAGE_SERVICE"
  ]
}

Secure Partition ID Distribution

Every Secure Partition has an identifier (ID). TF-M will generate a header file that includes definitions of the Secure Partition IDs. The header file is <TF-M build folder>generated/interface/include/psa_manifest/pid.h. Each definition uses the name attribute in the manifest as its name and the value is allocated by SPM.

The Partition ID can be set to a fixed value or omitted to be auto allocated.

#define name id-value

Table 36 PID table

Secure Partitions	PID Range
TF-M Internal Partitions	0 - 255
PSA and user Partitions	256 - 2999
TF-M test Partitions	3000 - 4999
Firmware Framework test Partitions	5000 - 5999
Reserved	6000 -

Please refer to <TF-M base folder>/tools/tfm_manifest_list.yaml, <TF-M test repo>/test/secure_fw/tfm_test_manifest_list.yaml and <TF-M base folder>/tools/tfm_psa_ff_test_manifest_list.yaml for the detailed PID allocations.

About where to add the definition, please refer to the chapter Add configuration.

RoT Service ID (SID) Distribution

An RoT Service is identified by its RoT Service ID (SID). A SID is a 32-bit number that is associated with a symbolic name in the Secure Partition manifest. The bits [31:12] uniquely identify the vendor of the RoT Service. The remaining bits [11:0] can be used at the discretion of the vendor.

Here is the RoT Service ID table used in TF-M.

Table 37 SID table

Partitions	Vendor ID(20 bits)	Function ID(12 bits)
initial_attestation	0x00000	0x020-0x03F
platform	0x00000	0x040-0x05F
protected_storage	0x00000	0x060-0x06F
internal_trusted_storage	0x00000	0x070-0x07F
crypto	0x00000	0x080-0x09F
firmware_update	0x00000	0x0A0-0x0BF
tfm_secure_client	0x0000F	0x000-0x01F
tfm_ipc_client	0x0000F	0x060-0x07F
tfm_ipc_service	0x0000F	0x080-0x09F
tfm_slih_test_service	0x0000F	0x0A0-0x0AF
tfm_flih_test_service	0x0000F	0x0B0-0x0BF
tfm_ps_test_service	0x0000F	0x0C0-0x0DF
tfm_secure_client_2	0x0000F	0x0E0-0x0FF
tfm_sfn_test_service_1	0x0000F	0x100-0x11F
tfm_sfn_test_service_2	0x0000F	0x120-0x13F
tfm_attest_test_service	0x0000F	0x140-0x15F

RoT Service Stateless Handle Distribution

A Secure partition may include stateless services. They are distinguished and referenced by stateless handles. In manifest, a stateless_handle attribute is set for indexing stateless services. It must be either "auto" or a number in the range [1, 32] in current implementation and may extend. Also the connection-based attribute must be set to false for stateless services.

The indexes of stateless handles are divided into two ranges for different usages. Indexes [1, 16] are assigned to TF-M Secure Partitions. The rest indexes [17, 32] are reserved for any other Secure Partitions, for example Secure Partitions in tf-m-tests and tf-m-extras.

The following table summaries the stateless handle allocation for the TF-M Secure Partitions.

Table 38 Stateless Handle table

Partition name	Stateless Handle
TFM_SP_CRYPTO	1
TFM_SP_PS	2
TFM_SP_ITS	3
TFM_SP_INITIAL_ATTESTATION	4
TFM_SP_FWU	5
TFM_SP_PLATFORM	6

For the indexes of other Secure Partitions, please refer to their manifests or documentations.

stack_size

The stack_size is required to indicate the stack memory usage of the Secure Partition. The value of this attribute must be a decimal or hexadecimal value in bytes. It can also be a build configurable with default value defined in config_base.cmake. The value of the configuration can be overridden to fit different use cases.

heap_size

This attribute is optional. The default value is 0. It indicates the heap memory usage of the Secure Partition. The allowed values are the same as the stack_size.

mmio_regions

This attribute is a list of MMIO region objects which the Secure Partition needs access to. TF-M only supports the named_region current. Please refer to PSA FF for more details about it. The user needs to provide a name macro to indicate the variable of the memory region.

TF-M uses the below structure to indicate a peripheral memory.

struct platform_data_t {
  uint32_t periph_start;
  uint32_t periph_limit;
  int16_t periph_ppc_bank;
  int16_t periph_ppc_loc;
};

Note

This structure is not expected by TF-M, it’s only that the current implementations are using. Other peripherals that need different information to create isolation need to define a different structure with the same name.

Here is an example for it:

struct platform_data_t tfm_peripheral_A;
#define TFM_PERIPHERAL_A                 (&tfm_peripheral_A)

Add configuration

The following configuration tasks are required for the newly added secure partition:

Add CMake configure files

• <TF-M base folder>/secure_fw/partitions/example/CMakeLists.txt, which is the compilation configuration for this module. Add library tfm_app_rot_partition_example and associated source files.

Here is a reference example for CMakeLists.txt

Note

The secure partition must be built as a standalone static library, and the name of the library must follow this pattern, as it affects how the linker script will lay the partition in memory: - tfm_psa_rot_partition* in case of a PSA RoT partition - tfm_app_rot_partition* in case of an Application RoT partition

The current CMake configuration should also be updated, by updating <TF-M base folder>/config/config_base.cmake to include the CMake configuration variable of the newly added Secure Partition, e.g, TFM_PARTITION_EXAMPLE and adding the relevant subdirectory in <TF-M base folder>/secure_fw/CMakeLists.txt, e.g. add_subdirectory(partitions/example). Please refer to the source code of TF-M for more detail.

Update manifest list

The <TF-M base folder>/tools/tfm_manifest_list.yaml is used to collect necessary information of secure partition. The manifest tool tools/tfm_parse_manifest_list.py processes it and generates necessary files while building.

Please refer to the Manifest List for the format of manifest lists.

Reference configuration example:

{
  "description": "TFM Example Partition",
  "manifest": "secure_fw/partitions/example/tfm_example_partition.yaml",
  "conditional": "@TFM_PARTITION_EXAMPLE@",
  "output_path": "partitions/example",
  "version_major": 0,
  "version_minor": 1,
  "pid": 290,
  "linker_pattern": {
    "library_list": [
      "*tfm_*partition_example*"
     ]
  }
}

TF-M also supports out-of-tree Secure Partition build where you can have your own manifest lists. Please refer to Out-of-tree Secure Partition build for details.

Implement the RoT services

To implement RoT services, the partition needs a source file which contains the implementations of the services, as well as the partition entry point. The user can create this source file under <TF-M base folder>/secure_fw/partitions/example/tfm_example_partition.c.

As an example, the RoT service with SID ROT_A will be implemented.

Entry point for IPC Model Partitions

This function must have a loop that repeatedly waits for input signals and then processes them, following the Secure Partition initialization.

#include "psa_manifest/tfm_example.h"
#include "psa/service.h"

void tfm_example_main(void)
{
    psa_signal_t signals = 0;

    /* Secure Partition initialization */
    example_init();

    /*
     * Continually wait for one or more of the partition's RoT Service or
     * interrupt signals to be asserted and then handle the asserted
     * signal(s).
     */
    while (1) {
        signals = psa_wait(PSA_WAIT_ANY, PSA_BLOCK);
        if (signals & ROT_A_SIGNAL) {
            rot_A();
        } else {
            /* Should not come here */
            psa_panic();
        }
    }
}

Entry init for SFN Model Partitions

In the SFN model, the Secure Partition consists of one optional initialization function, which is declared as the entry_init symbol as mentioned in section Add manifest. After initialization, the entry_init function returns the following values:

• Return PSA_SUCCESS if initialization succeeds.

• Return PSA_SUCCESS if initialization is partially successful, and you want some SFNs to receive messages. RoT Services that are non-operational must respond to connection requests with PSA_ERROR_CONNECTION_REFUSED.

• Return an error status if the initialization failed, and no SFNs within the Secure Partition must be called.

Service implementation for IPC Model

The service is implemented by the rot_A() function, which is called upon an incoming signal. This implementation is up to the user, however an example service has been included for reference. The following example sends a message “Hello World” when called.

#include "psa_manifest/tfm_example.h"
#include "psa/service.h"

static void rot_A(void)
{
    const int BUFFER_LEN = 32;
    psa_msg_t msg;
    int i;
    uint8_t rec_buf[BUFFER_LEN];
    uint8_t send_buf[BUFFER_LEN] = "Hello World";

    psa_get(ROT_A_SIGNAL, &msg);
    switch (msg.type) {
    case PSA_IPC_CONNECT:
    case PSA_IPC_DISCONNECT:
        /*
         * This service does not require any setup or teardown on connect
         * or disconnect, so just reply with success.
         */
        psa_reply(msg.handle, PSA_SUCCESS);
        break;
    case PSA_IPC_CALL:
        for (i = 0; i < PSA_MAX_IOVEC; i++) {
            if (msg.in_size[i] != 0) {
                psa_read(msg.handle, i, rec_buf, BUFFER_LEN);
            }
            if (msg.out_size[i] != 0) {
                psa_write(msg.handle, i, send_buf, BUFFER_LEN);
            }
        }
        psa_reply(msg.handle, PSA_SUCCESS);
        break;
    default:
        /* cannot get here [broken SPM] */
        psa_panic();
        break;
    }
}

Service implementation for SFN Model

SFN model consists of a set of Secure Functions (SFN), one for each RoT Service. The connection, disconnection and request messages do not cause a Secure Partition signal to be asserted for SFN Secure Partitions. Instead, the Secure Function (SFN) for the RoT Service is invoked by the framework, with the message details provided as a parameter to the SFN. To add a secure function (SFN) to process messages for each RoT Service, each SFN will have following prototype.

psa_status_t <<name>>_sfn(const psa_msg_t *msg);

A connection-based example service has been included for reference which sends a message “Hello World” when called.

#include "psa_manifest/tfm_example.h"
#include "psa/service.h"

psa_status_t rot_a_sfn(const psa_msg_t *msg)
{
    const int BUFFER_LEN = 32;
    int i;
    uint8_t rec_buf[BUFFER_LEN];
    uint8_t send_buf[BUFFER_LEN] = "Hello World";

    switch (msg->type) {
    case PSA_IPC_CONNECT:
    case PSA_IPC_DISCONNECT:
        /*
         * This service does not require any setup or teardown on connect
         * or disconnect, so just reply with success.
         */
        return PSA_SUCCESS;
    case PSA_IPC_CALL:
        for (i = 0; i < PSA_MAX_IOVEC; i++) {
            if (msg->in_size[i] != 0) {
                psa_read(msg->handle, i, rec_buf, BUFFER_LEN);
            }
            if (msg.->out_size[i] != 0) {
                psa_write(msg->handle, i, send_buf, BUFFER_LEN);
            }
        }
        return PSA_SUCCESS;
    default:
        /* cannot get here [broken SPM] */
        return PSA_ERROR_PROGRAMMER_ERROR;
    }
}

Test suites and test partitions

A regression test suite can be added to verify whether the new secure partition works as expected. Refer to Adding TF-M Regression Test Suite for the details of adding a regression test suite.

Some regression tests require a dedicated RoT service. The implementations of the RoT service for test are similar to secure partition addition. Refer to Adding partitions for regression tests to get more information.

Out-of-tree Secure Partition build

TF-M supports out-of-tree Secure Partition build, whose source code folders are maintained outside TF-M repo. Developers can configure TFM_EXTRA_MANIFEST_LIST_FILES and TFM_EXTRA_PARTITION_PATHS in build command line to include out-of-tree Secure Partitions.

• TFM_EXTRA_MANIFEST_LIST_FILES

  A list of the absolute path(s) of the manifest list(s) provided by out-of-tree Secure Partition(s). Use semicolons ; to separate multiple manifest lists. Wrap the multiple manifest lists with double quotes.

• TFM_EXTRA_PARTITION_PATHS

  A list of the absolute directories of the out-of-tree Secure Partition source code folder(s). TF-M build system searches CMakeLists.txt of partitions in the source code folder(s). Use semicolons ; to separate multiple out-of-tree Secure Partition directories. Wrap the multiple directories with double quotes.

A single out-of-tree Secure Partition folder can be organized as the figure below.

secure partition folder
      ├── CMakeLists.txt
      ├── manifest_list.yaml
      ├── out_of_tree_partition_manifest.yaml
      └── source code

In the example above, TFM_EXTRA_MANIFEST_LIST_FILES and TFM_EXTRA_PARTITION_PATHS in the build command can be configured as listed below.

-DTFM_EXTRA_MANIFEST_LIST_FILES=<Absolute-path-sp-folder/manifest_list.yaml>
-DTFM_EXTRA_PARTITION_PATHS=<Absolute-path-sp-folder>

Multiple out-of-tree Secure Partitions can be organized in diverse structures. For example, multiple Secure Partitions can be maintained under the same directory as shown below.

top-level folder
      ├── Partition 1
      │       ├── CMakeLists.txt
      │       ├── partition_1_manifest.yaml
      │       └── source code
      ├── Partition 2
      │       └── ...
      ├── Partition 3
      │       └── ...
      ├── manifest_list.yaml
      └── Root CMakeLists.txt

In the example above, a root CMakeLists.txt includes all the partitions’ CMakLists.txt, for example via add_subdirectory(). The manifest_list.yaml lists all partitions’ manifest files. TFM_EXTRA_MANIFEST_LIST_FILES and TFM_EXTRA_PARTITION_PATHS in build command line can be configured as listed below.

-DTFM_EXTRA_MANIFEST_LIST_FILES=<Absolute-path-top-level-folder/manifest_list.yaml>
-DTFM_EXTRA_PARTITION_PATHS=<Absolute-path-top-level-folder>

Alternatively, out-of-tree Secure Partitions can be separated in different folders.

partition 1 folder                    partition 2 folder
    ├── CMakeLists.txt                    ├── CMakeLists.txt
    ├── manifest_list.yaml                ├── manifest_list.yaml
    ├── partition_1_manifest.yaml         ├── partition_2_manifest.yaml
    └── source code                       └── source code

In the example above, each Secure Partition manages its own manifest files and CMakeLists.txt. TFM_EXTRA_MANIFEST_LIST_FILES and TFM_EXTRA_PARTITION_PATHS in build command line can be configured as listed below. Please note those input shall be wrapped with double quotes.

-DTFM_EXTRA_MANIFEST_LIST_FILES="<Absolute-path-part-1-folder/manifest_list.yaml>;<Absolute-path-part-2-folder/manifest_list.yaml>"
-DTFM_EXTRA_PARTITION_PATHS="<Absolute-path-part-1-folder>;<Absolute-path-part-2-folder>"

Note

Manifest list paths in TFM_EXTRA_MANIFEST_LIST_FILES do NOT have to be one-to-one mapping to Secure Partition directories in TFM_EXTRA_PARTITION_PATHS. The orders don’t matter either.

Further Notes

• In the IPC model, Use PSA FF proposed memory accessing mechanism. SPM provides APIs and checking between isolation boundaries, a free accessing of memory can cause program panic.

• In the IPC model, the memory checking inside partition runtime is unnecessary. SPM handles the checking while memory accessing APIs are called.

• In the IPC model, the client ID had been included in the message structure and secure partition can get it when calling psa_get() function. The secure partition does not need to call tfm_core_get_caller_client_id() to get the caller client ID anymore.

• In the IPC model, SPM will check the security policy and partition dependence between client and service. So the service does not need to validate the secure caller anymore.

Reference

PSA Firmware Framework specification

Firmware Framework for M 1.1 Extensions

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