building/trusted_applications.rst

.. _build_trusted_applications:

####################
Trusted Applications
####################
This document tells how to implement a Trusted Application for OP-TEE, using
OP-TEE's so called `TA-devkit` to both build and sign the Trusted Application
binary. In this document, a `Trusted Application` running in the OP-TEE os is
referred to as a `TA`.
Note that in the default setup a private key generated by Linaro and
distributed along with the :ref:`optee_os` source is used for signing Trusted
Applications.
See TASign_ for more details, including offline signing of TAs.

TA Mandatory files
******************
The Makefile for a Trusted Application must be written to rely on OP-TEE
TA-devkit resources in order to successfully build the target application.
TA-devkit is built when one builds :ref:`optee_os`.

.. todo::

    Joakim: We need to add CMake instructions also.

To build a TA, one must provide:

    - **Makefile**, a make file that should set some configuration variables and
      include the TA-devkit make file.

    - **sub.mk**, a make file that lists the sources to build (local source
      files, subdirectories to parse, source file specific build directives).

    - **user_ta_header_defines.h**, a specific ANSI-C header file to define most
      of the TA properties.

    - An implementation of at least the TA entry points, as extern functions:
      ``TA_CreateEntryPoint()``, ``TA_DestroyEntryPoint()``,
      ``TA_OpenSessionEntryPoint()``, ``TA_CloseSessionEntryPoint()``,
      ``TA_InvokeCommandEntryPoint()``

TA file layout example
======================
As an example, :ref:`hello_world` looks like this:

.. code-block:: none

    hello_world/
    ├── ...
    └── ta
        ├── Makefile                  BINARY=<uuid>
        ├── Android.mk                Android way to invoke the Makefile
        ├── sub.mk                    srcs-y += hello_world_ta.c
        ├── include
        │   └── hello_world_ta.h      Header exported to non-secure: TA commands API
        ├── hello_world_ta.c          Implementation of TA entry points
        └── user_ta_header_defines.h  TA_UUID, TA_FLAGS, TA_DATA/STACK_SIZE, ...

TA Makefile Basics
******************
Required variables
==================
The main TA-devkit make file is located in :ref:`optee_os` at
``ta/mk/ta_dev_kit.mk``. The make file supports make targets such as ``all`` and
``clean`` to build a TA or a library and clean the built objects.

The make file expects a couple of configuration variables:

TA_DEV_KIT_DIR
    Base directory of the TA-devkit. Used by the TA-devkit itself to locate its tools.

BINARY and LIBNAME
    These are exclusive, meaning that you cannot use both at the same time. If
    building a TA, ``BINARY`` shall provide the TA filename used to load the TA.
    The built and signed TA binary file will be named ``${BINARY}.ta``. In
    native OP-TEE, it is the TA UUID, used by tee-supplicant to identify TAs. If
    one is building a static library (that will be later linked by a TA), then
    ``LIBNAME`` shall provide the name of the library. The generated library
    binary file will be named ``lib${LIBNAME}.a``

CROSS_COMPILE and CROSS_COMPILE32
    Cross compiler for the TA or the library source files. ``CROSS_COMPILE32``
    is optional. It allows to target AArch32 builds on AArch64 capable systems.
    On AArch32 systems, ``CROSS_COMPILE32`` defaults to ``CROSS_COMPILE``.

Optional variables
==================
Some optional configuration variables can be supported, for example:

O
    Base directory for build objects filetree. If not set, TA-devkit defaults to
    **./out** from the TA source tree base directory.

Example Makefile
================
A typical Makefile for a TA looks something like this

.. code-block:: Makefile

    # Append specific configuration to the C source build (here log=info)
    # The UUID for the Trusted Application
    BINARY=8aaaf200-2450-11e4-abe2-0002a5d5c51b

    # Source the TA-devkit make file
    include $(TA_DEV_KIT_DIR)/mk/ta_dev_kit.mk

.. _build_trusted_applications_submk:

sub.mk directives
=================
The make file expects that current directory contains a file ``sub.mk`` that is
the entry point for listing the source files to build and other specific build
directives. Here are a couple of examples of directives one can implement in a
sub.mk make file:

.. code-block:: Makefile

    # Adds /hello_world_ta.c from current directory to the list of the source
    # file to build and link.
    srcs-y += hello_world_ta.c

    # Includes path **./include/** from the current directory to the include
    # path.
    global-incdirs-y += include/

    # Adds directive -Wno-strict-prototypes only to the file hello_world_ta.c
    cflags-hello_world_ta.c-y += -Wno-strict-prototypes

    # Removes directive -Wno-strict-prototypes from the build directives for
    # hello_world_ta.c only.
    cflags-remove-hello_world_ta.c-y += -Wno-strict-prototypes

    # Adds the static library foo to the list of the linker directive -lfoo.
    libnames += foo

    # Adds the directory path to the libraries pathes list. Archive file
    # libfoo.a is expected in this directory.
    libdirs += path/to/libfoo/install/directory

    # Adds the static library binary to the TA build dependencies.
    libdeps += path/to/greatlib/libgreatlib.a

Android Build Environment
*************************
.. todo::

    Joakim: Move this to the AOSP page?

OP-TEE's TA-devkit supports building in an Android build environment. One can
write an ``Android.mk`` file for the TA (stored side by side with the Makefile).
Android's build system will parse the ``Android.mk`` file for the TA which in
turn will parse a TA-devkit Android make file to locate TA build resources. Then
the Android build will execute a ``make`` command to built the TA through its
generic Makefile file.

A typical ``Android.mk`` file for a TA looks like this (``Android.mk`` for
:ref:`hello_world` is used as an example here).

.. code-block:: Makefile

    # Define base path for the TA sources filetree
    LOCAL_PATH := $(call my-dir)

    # Define the module name as the signed TA binary filename.
    local_module := 8aaaf200-2450-11e4-abe2-0002a5d5c51b.ta

    # Include the devkit Android make script
    include $(OPTEE_OS_DIR)/mk/aosp_optee.mk

TA Mandatory Entry Points
*************************
A TA must implement a couple of mandatory entry points, these are:

.. code-block:: c

    TEE_Result TA_CreateEntryPoint(void)
    {
        /* Allocate some resources, init something, ... */
        ...

        /* Return with a status */
        return TEE_SUCCESS;
    }

    void TA_DestroyEntryPoint(void)
    {
        /* Release resources if required before TA destruction */
        ...
    }

    TEE_Result TA_OpenSessionEntryPoint(uint32_t ptype,
                                        TEE_Param param[4],
                                        void **session_id_ptr)
    {
        /* Check client identity, and alloc/init some session resources if any */
        ...

        /* Return with a status */
        return TEE_SUCCESS;
    }

    void TA_CloseSessionEntryPoint(void *sess_ptr)
    {
        /* check client and handle session resource release, if any */
        ...
    }

    TEE_Result TA_InvokeCommandEntryPoint(void *session_id,
                                          uint32_t command_id,
                                          uint32_t parameters_type,
                                          TEE_Param parameters[4])
    {
        /* Decode the command and process execution of the target service */
        ...

        /* Return with a status */
        return TEE_SUCCESS;
    }

.. _build_ta_properties:

TA Properties
*************
Trusted Application properties shall be defined in a header file named
``user_ta_header_defines.h``, which should contain:

    - ``TA_UUID`` defines the TA uuid value
    - ``TA_FLAGS`` define some of the TA properties
    - ``TA_STACK_SIZE`` defines the RAM size to be reserved for TA stack
    - ``TA_DATA_SIZE`` defines the RAM size to be reserved for TA heap (TEE_Malloc()
      pool)

Refer to :ref:`ta_properties` to understand how to configure these macros.

.. hint::

    UUIDs can be generated using python

    .. code-block:: python

        python -c 'import uuid; print(uuid.uuid4())'

    or in most Linux systems using either

    .. code-block:: bash

        cat /proc/sys/kernel/random/uuid # Linux only
        uuidgen # available from the util-linux package in most distributions



.. _user_ta_header_defines_h:

Example of a property header file
=================================

.. code-block:: c

    #ifndef USER_TA_HEADER_DEFINES_H
    #define USER_TA_HEADER_DEFINES_H

    #define TA_UUID
        { 0x8aaaf200, 0x2450, 0x11e4, \
            { 0xab, 0xe2, 0x00, 0x02, 0xa5, 0xd5, 0xc5, 0x1b} }

    #define TA_FLAGS			(TA_FLAG_EXEC_DDR | \
                            TA_FLAG_SINGLE_INSTANCE | \
                            TA_FLAG_MULTI_SESSION)
    #define TA_STACK_SIZE			(2 * 1024)
    #define TA_DATA_SIZE			(32 * 1024)

    #define TA_CURRENT_TA_EXT_PROPERTIES \
        { "gp.ta.description", USER_TA_PROP_TYPE_STRING, "Foo TA for some purpose." }, \
        { "gp.ta.version", USER_TA_PROP_TYPE_U32, &(const uint32_t){ 0x0100 } }

    #endif /* USER_TA_HEADER_DEFINES_H */

.. note::

    It is recommended to use the ``TA_CURRENT_TA_EXT_PROPERTIES`` as above to
    define extra properties of the TA.

.. note::

    Generating a fresh UUID with suitable formatting for the header file can be
    done using:

    .. code-block:: python

        python -c "import uuid; u=uuid.uuid4(); print(u); \
           n = [', 0x'] * 11; \
           n[::2] = ['{:12x}'.format(u.node)[i:i + 2] for i in range(0, 12, 2)]; \
           print('\n' + '#define TA_UUID\n\t{ ' + \
                 '0x{:08x}'.format(u.time_low) + ', ' + \
                 '0x{:04x}'.format(u.time_mid) + ', ' + \
                 '0x{:04x}'.format(u.time_hi_version) + ', \\ \n\n\t\t{ ' + \
                 '0x{:02x}'.format(u.clock_seq_hi_variant) + ', ' + \
                 '0x{:02x}'.format(u.clock_seq_low) + ', ' + \
                 '0x' + ''.join(n) + '} }')"


Checking TA parameters
**********************
GlobalPlatforms TEE Client APIs ``TEEC_InvokeCommand()`` and
``TEE_OpenSession()`` allow clients to invoke a TA with some invocation
parameters: values or references to memory buffers. It is mandatory that TA's
verify the parameters types before using the parameters themselves. For this a
TA can rely on the macro ``TEE_PARAM_TYPE_GET(param_type, param_index)`` to get
the type of a parameter and check its value according to the expected parameter.

For example, if a TA expects that command ID 0 comes with ``params[0]`` being a
input value, ``params[1]`` being a output value, and ``params[2]`` being a
in/out memory reference (buffer), then the TA should implemented the following
sequence:

.. code-block:: c

    TEE_Result handle_command_0(void *session, uint32_t cmd_id,
                                uint32_t param_types, TEE_Param params[4])
    {
        if ((TEE_PARAM_TYPE_GET(param_types, 0) != TEE_PARAM_TYPE_VALUE_IN) ||
            (TEE_PARAM_TYPE_GET(param_types, 1) != TEE_PARAM_TYPE_VALUE_OUT) ||
            (TEE_PARAM_TYPE_GET(param_types, 2) != TEE_PARAM_TYPE_MEMREF_INOUT) ||
            (TEE_PARAM_TYPE_GET(param_types, 3) != TEE_PARAM_TYPE_NONE)) {
            return TEE_ERROR_BAD_PARAMETERS
        }

        /* process command */
        ...
    }

    TEE_Result TA_InvokeCommandEntryPoint(void *session, uint32_t command_id,
                          uint32_t param_types, TEE_Param params[4])
    {
        switch (command_id) {
        case 0:
            return handle_command_0(session, param_types, params);

        default:
            return TEE_ERROR_NOT_SUPPORTED;
        }
    }

.. _TASign:

Signing of TAs
**************

All :ref:`REE Filesystem Trusted Applications<ree_fs_ta>` need to be signed. The
signature is verified by :ref:`optee_os` upon loading of the TA. Within the
:ref:`optee_os` source is a directory ``keys``. The public part of
``keys/default_ta.pem`` will be compiled into the :ref:`optee_os` binary and the
signature of each TA will be verified against this key upon loading. Currently
``keys/default_ta.pem`` must contain an RSA key.

.. warning::

    :ref:`optee_os` comes with a default **private** key in its source to
    facilitate easy development, testing, debugging and QA. Never deploy an
    :ref:`optee_os` binary with this key in production. Instead replace this key
    as soon as possible with a public key and keep the private part of the key
    offline, preferably on an HSM.

.. note::

    Currently only a single key for signing TAs is supported by :ref:`optee_os`.

TAs are signed using the ``sign.py`` script referenced from
``ta/mk/ta_dev_kit.mk`` in :ref:`optee_os`. Its default behaviour is to sign a
compiled TA binary and attach the signature to form a complete TA for
deployment. For **offline** signing, a three-step process is required: In a
first step a digest of the compiled binary has to be generated, in the second
step this digest is signed offline using the private key and finally in the
third step the binary and its signature are stitched together into the full TA.

Offline Signing of TAs
======================

The TA dev kit does sign an application as last step of the linking process. For
example, the file ``ta/arch/arm/link.mk`` in the :ref:`optee_os` source tree
contains the statement

.. code-block:: sh

    $(q)$(SIGN) --key $(TA_SIGN_KEY) --uuid $(user-ta-uuid) \
	    --in $$< --out $$@

To avoid build errors when signing offline, this make script needs to be
adopted. The signing script can be found at
``$(TA_DEV_KIT_DIR)/../scripts/sign.py``

Overall, offline signing is done with the following sequence of steps:

0. (Preparation) Generate a 2048 bit RSA key for signing in a secure, offline
environment. Extract the public key and copy it to the ``keys`` directory in the
:ref:`optee_os` source tree. Adjust ``TA_SIGN_KEY`` for different file/path
names. (Copy and) modify the ``link.mk`` file for the default linking step to
produce a digest of the TA binary instead of the full TA.

1. Manually (or with the modified linking script) generate a digest of the TA
binary using

.. code-block:: sh

    sign.py digest --key $(TA_SIGN_KEY) --uuid $(user-ta-uuid)

2. Sign this digest offline, e.g. with OpenSSL

.. code-block:: sh

   base64 --decode digestfile | \
   openssl pkeyutl -sign -inkey $TA_SIGN_KEY \
     -pkeyopt digest:sha256 -pkeyopt rsa_padding_mode:pkcs1 | \
   base64 > sigfile

or with pkcs11-tool using a Nitrokey HSM

.. code-block:: sh

   echo "0000: 3031300D 06096086 48016503 04020105 000420" | \
     xxd -c 19 -r > /tmp/sighdr
   cat /tmp/sighdr $(base64 --decode digestfile) > /tmp/hashtosign
   pkcs11-tool --id $key_id -s --login -m RSA-PKCS \
     --input-file /tmp/hashtosign | \
     base64 > sigfile

3. Manually (or with an extra make target) stitch the TA together using

.. code-block:: sh

    sign.py stitch --key $(TA_SIGN_KEY) --uuid $(user-ta-uuid)

By default the UUID is taken as the base file name for all files. Different file
names and paths can be set through additional options to ``sign.py``. Consult
``sign.py --help`` for a full list of options and parameters.