Adding Secure Partition

Terms and abbreviations

This document uses the following terms and abbreviations.

term table
Term	Meaning
FF	Firmware Framework
ID	Identifier
IPC	Interprocess communication
IPC model	The secure IPC framework
irqs	Interrupt requests
Library model	The secure function call framework
MMIO	Memory Mapped I/O
PSA	Platform Security Architecture
RoT	Root of Trust
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 library and IPC model.

The IPC model conforms the PSA Firmware Framework (FF) v 1.0.0. Refer to PSA Firmware Framework specification 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: EXAMPLE.yaml
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, please refer to Library model support for the library extend:

{
  "psa_framework_version": 1.0,
  "name": "TFM_SP_EXAMPLE",
  "type": "PSA-ROT",
  "priority": "HIGH",
  "entry_point": "example_main",
  "stack_size": "0x0200",
  "services" : [
    {
      "name": "ROT_A",
      "sid": "0x0000F000",
      "non_secure_clients": true,
      "version": 1,
      "version_policy": "STRICT"
    }
  ],
  "mmio_regions": [
    {
      "name": "TFM_PERIPHERAL_A",
      "permission": "READ-WRITE"
    }
  ],
  "irqs": [
    {
      "source": "TFM_A_IRQ",
      "signal": "SPM_CORE_A_IRQ",
    }
  ],
  "linker_pattern": {
    "object_list": [
      "*EXAMPLE.*"
    ]
  }
}

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 base folder>/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

Please refer to <TF-M base folder>/tools/tfm_manifest_list.yaml for the 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.

SID table
Services	Vendor ID(20 bits)	Function ID(12 bits)
audit_logging	0x00000	0x000-0x01F
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
test_secure_service	0x0000F	0x000-0x01F
core_test	0x0000F	0x020-0x03F
core_test_2	0x0000F	0x040-0x05F
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

RoT Service Stateless Handle Distribution

A RoT Service may include stateless services. They are distinguished and referenced by stateless handles. In manifest, a stateless_handle attribute is set for indexing stateless services. The valid range is [1, 32] in current implementation and may extend.

Here is the stateless handle allocation for partitions in TF-M. Partitions not listed are not applied to stateless mechanism yet.

Stateless Handle table
Partition name	Stateless Handle
TFM_SP_CRYPTO	1
TFM_SP_PS	2
TFM_SP_ITS	3
TFM_SP_INITIAL_ATTESTATION	4

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 a example for it:

struct platform_data_t tfm_peripheral_A;
#define TFM_PERIPHERAL_A                 (&tfm_peripheral_A)

linker_pattern

linker_pattern is a legacy region which contains the minimum information required to link a Secure Partition’s compiled static objects. Now, it is required as ‘IMPLEMENTATION DEFINED’ in PSA FF 1.0.0.

Library model support

For the library model, the user needs to add a secure_functions item. The main difference between secure_function and services is the extra signal key for secure function entry.

The signal must be the upper case of the secure function name.

"secure_functions": [
  {
    "name": "TFM_EXAMPLE_A",
    "signal": "EXAMPLE_A_FUNC",
    "sid": "0x00000000",
    "non_secure_clients": true,
    "version": 1,
    "version_policy": "STRICT"
  },

Add configuration

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

Add CMake configure files

CMakeLists.txt, which is the compilation configuration for this module.

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 config_default.cmake to include the definition of the newly introduced partition and adding the relevant subdirectoy in secure_fw/CMakeLists.txt. 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.

name: The name string of the secure partition.
short_name: should be the same as the name in the secure partition manifest file.
manifest: the relative path of the manifest file to TF-M root. In out-of-tree secure partition, manifest can be an absolute path or the relative path to the current manifest list file.
conditional: Optional. Configure control macro for this partition.
version_major: major version the partition manifest.
version_minor: minor version the partition manifest.
pid: Secure Partition ID value distributed in chapter Secure Partition ID Distribution.
output_dir: Optional. Secure Partition can specify a relative path to <build_dir>/generated to hold the generated files. It enables Secure Partition to select a generated path independent from its source code path, for example in out-of-tree Secure Parition build.

Reference configuration example:

{
  "name": "Example Service",
  "short_name": "TFM_SP_EXAMPLE",
  "manifest": "secure_fw/partitions/EXAMPLE/tfm_example.yaml",
  "conditional": "TFM_PARTITION_EXAMPLE",
  "version_major": 0,
  "version_minor": 1,
  "pid": 256
}

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/EXAMPLE.c. The linker detects source files according to the pattern matching defined by the “linker_pattern” attribute in the tfm_manifest_list.yaml file.

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

Entry point function

This function acts as a main() function for the partition. On incoming signals for service calls, the entry point function handles signals by calling the relevant service function. An example entry point is given

void example_main(void)
{
    psa_signal_t signals = 0;

    while (1) {
        signals = psa_wait(PSA_WAIT_ANY, PSA_BLOCK);
        if (signals & ROT_A_SIGNAL) {
            rot_A();
        } else {
            /* Should not come here */
            psa_panic();
        }
    }
}

Service implementation

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.

static void rot_A(void)
{
    const int BUFFER_LEN = 32;
    psa_msg_t msg;
    psa_status_t r;
    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:
        if (service_in_use & ROT_A_SIGNAL) {
            r = PSA_ERROR_CONNECTION_REFUSED;
        } else {
            service_in_use |= ROT_A_SIGNAL;
            r = PSA_SUCCESS;
        }
        psa_reply(msg.handle, r);
        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;
    case PSA_IPC_DISCONNECT:
        assert((service_in_use & ROT_A_SIGNAL) != 0);
        service_in_use &= ~ROT_A_SIGNAL;
        psa_reply(msg.handle, PSA_SUCCESS);
        break;
    default:
        /* cannot get here [broken SPM] */
        psa_panic();
        break;
    }
}

Test connection

To test that the service has been implemented correctly, the user needs to call it from somewhere. One option is to create a new testsuite, such as <TF-M-test base folder>/test/suites/example/non_secure/example_ns_interface_testsuite.c.

static void tfm_example_test_1001(struct test_result_t *ret)
{
    char str1[] = "str1";
    char str2[] = "str2";
    char str3[128], str4[128];
    struct psa_invec invecs[2] = {{str1, sizeof(str1)},
                                  {str2, sizeof(str2)}};
    struct psa_outvec outvecs[2] = {{str3, sizeof(str3)},
                                    {str4, sizeof(str4)}};
    psa_handle_t handle;
    psa_status_t status;
    uint32_t version;

    version = psa_version(ROT_A_SID);
    TEST_LOG("TFM service support version is %d.\r\n", version);
    handle = psa_connect(ROT_A_SID, ROT_A_VERSION);
    status = psa_call(handle, PSA_IPC_CALL, invecs, 2, outvecs, 2);
    if (status >= 0) {
        TEST_LOG("psa_call is successful!\r\n");
    } else {
        TEST_FAIL("psa_call is failed!\r\n");
        return;
    }

    TEST_LOG("outvec1 is: %s\r\n", outvecs[0].base);
    TEST_LOG("outvec2 is: %s\r\n", outvecs[1].base);
    psa_close(handle);
    ret->val = TEST_PASSED;
}

Once the test and service has been implemented, the project can be built and executed. The user should see the “Hello World” message in the console as received by the testsuite.

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/manifes_list.yaml>;<Absolute-path-part-2-folder/manifes_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