architecture/porting_guidelines.rst

.. _porting_guidelines:

##################
Porting guidelines
##################
This document serves a dual purpose:

* Serve as a base for getting OP-TEE up and running on a new device with initial
  xtest validation passing. This is the first part of this document (section 2).

* Highlight the missing pieces if you intend to make a real secure product, that
  is what the second part of this document is about.

We are trying our best to implement full end to end security in OP-TEE in a
generic way, but due to the nature of devices being different, NDA etc, it is
not always possible for us to do so and in those cases, we most often try to
write a generic API, but we will just stub the code. This porting guideline
highlights the missing pieces that must be addressed in a real secure consumer
device. Hopefully we will sooner or later get access to devices where we at
least can make reference implementations publicly available to everyone for the
missing pieces we are talking about here.

.. _add_a_new_platform:

Add a new platform
******************
The first thing you need to do after you have decided to port OP-TEE to another
device is to add a new platform device. That can either be adding a new platform
variant (``PLATFORM_FLAVOR``) if it is a device from a family already supported,
or it can be a brand new platform family (``PLATFORM``). Typically this initial
setup involve configuring UART, memory addresses etc. For simplicity let us call
our fictive platform for "gendev" just so we have something to refer to when
writing examples further down.

core/arch/arm
=================
In ``core/arch/arm`` you will find all the currently supported devices. That is
where you are supposed to add a new platform or modify an existing one.
Typically you will find this set of files in a specific platform folder:

.. code-block:: bash

    $ ls
    conf.mk  main.c  platform_config.h  sub.mk

So for the gendev platform it means that the files should be placed in this
folder:

.. code-block:: bash

    core/arch/arm/plat-gendev

**conf.mk**

This is the device specific makefile where you define configurations unique to
your platform. This mainly comprises two things:

- OP-TEE configuration variables (``CFG_``), which may be assigned values in
  two ways. ``CFG_FOO ?= bar`` should be used to provide a default value that
  may be modified at compile time. On the other hand, variables that must be
  set to some value and cannot be modified should be set by: ``$(call
  force,CFG_FOO,bar)``.

- Compiler flags for the TEE core, the user mode libraries and the Trusted
  Applications, which may be added to macros used by the build system. Please
  see :ref:`configuration_and_flags` and similar sections on that page.

It is recommended to use a existing platform configuration file as a starting
point. For instance, `core/arch/arm/plat-hikey/conf.mk`_.

The platform ``conf.mk`` file should at least define the default platform flavor
for the platform, the core configurations (architecture and number of cores),
the main configuration directives (generic boot, arm trusted firmware support,
generic time source, console driver, etc...) and some platform default
configuration settings.

.. code-block:: make

    PLATFORM_FLAVOR ?= hikey
    
    include core/arch/arm/cpu/cortex-armv8-0.mk
    
    $(call force,CFG_TEE_CORE_NB_CORE,8)
    $(call force,CFG_GENERIC_BOOT,y)
    $(call force,CFG_PL011,y)
    $(call force,CFG_PM_STUBS,y)
    $(call force,CFG_SECURE_TIME_SOURCE_CNTPCT,y)
    $(call force,CFG_WITH_ARM_TRUSTED_FW,y)
    $(call force,CFG_WITH_LPAE,y)
    
    ta-targets = ta_arm32
    ta-targets += ta_arm64
    
    CFG_NUM_THREADS ?= 8
    CFG_CRYPTO_WITH_CE ?= y
    CFG_WITH_STACK_CANARIES ?= y
    CFG_CONSOLE_UART ?= 3
    CFG_DRAM_SIZE_GB ?= 2

**main.c**

This platform specific file will contain power management handlers and code
related to the UART. We will talk more about the information related to the
handlers further down in this document. For our gendev device it could look like
this (here we are excluding the necessary license header to save some space):

.. code-block:: c

    #include <console.h>
    #include <drivers/serial8250_uart.h>
    #include <kernel/generic_boot.h>
    #include <kernel/panic.h>
    #include <kernel/pm_stubs.h>
    #include <mm/core_mmu.h>
    #include <platform_config.h>
    #include <stdint.h>
    #include <tee/entry_fast.h>
    #include <tee/entry_std.h>
    
    static void main_fiq(void)
    {
    	panic();
    }
    
    static const struct thread_handlers handlers = {
    	.std_smc = tee_entry_std,
    	.fast_smc = tee_entry_fast,
    	.nintr = main_fiq,
    	.cpu_on = cpu_on_handler,
    	.cpu_off = pm_do_nothing,
    	.cpu_suspend = pm_do_nothing,
    	.cpu_resume = pm_do_nothing,
    	.system_off = pm_do_nothing,
    	.system_reset = pm_do_nothing,
    };
    
    const struct thread_handlers *generic_boot_get_handlers(void)
    {
    	return &handlers;
    }
    
    /*
     * Register the physical memory area for peripherals etc. Here we are
     * registering the UART console.
     */
    register_phys_mem(MEM_AREA_IO_NSEC, CONSOLE_UART_BASE, SERIAL8250_UART_REG_SIZE);
    
    static struct serial8250_uart_data console_data;
    
    void console_init(void)
    {
    	serial8250_uart_init(&console_data, CONSOLE_UART_BASE,
    			     CONSOLE_UART_CLK_IN_HZ, CONSOLE_BAUDRATE);
    	register_serial_console(&console_data.chip);
    }

**platform_config.h**

This is a mandatory header file for every platform, since there are several
files relaying upon the existence of this particular file. This file is where
you will find the major differences between different platforms, since this is
where you do the memory configuration, define base addresses etc. we are going
to list a few here, but it probably makes more sense to have a look at the
already existing ``platform_config.h`` files for the other platforms. Our
fictive gendev could look like this:

.. code-block:: c

    #ifndef PLATFORM_CONFIG_H
    #define PLATFORM_CONFIG_H
    
    /* Make stacks aligned to data cache line length */
    #define STACK_ALIGNMENT		64
    
    /* 8250 UART */
    #define CONSOLE_UART_BASE	0xcafebabe /* UART0 */
    #define CONSOLE_BAUDRATE	115200
    #define CONSOLE_UART_CLK_IN_HZ	19200000
    
    /* Optional: when used with CFG_WITH_PAGER, defines the device SRAM */
    #define TZSRAM_BASE		0x3F000000
    #define TZSRAM_SIZE		(200 * 1024)
    
    /* Mandatory main secure RAM usually DDR */
    #define TZDRAM_BASE		0x60000000
    #define TZDRAM_SIZE		(32 * 1024 * 1024)
    
    /* Mandatory TEE RAM location and core load address */
    #define TEE_RAM_START		TZDRAM_BASE
    #define TEE_RAM_PH_SIZE		TEE_RAM_VA_SIZE
    #define TEE_RAM_VA_SIZE		(4 * 1024 * 1024)
    #define TEE_LOAD_ADDR		(TZDRAM_BASE + 0x20000)
    
    /* Mandatory TA RAM (external less secure RAM) */
    #define TA_RAM_START		(TZDRAM_BASE + TEE_RAM_VA_SIZE)
    #define TA_RAM_SIZE		(TZDRAM_SIZE - TEE_RAM_VA_SIZE)
    
    /* Mandatory: for static SHM, need a hardcoded physical address */
    #define TEE_SHMEM_START		0x08000000
    #define TEE_SHMEM_SIZE		(4 * 1024 * 1024)
    
    #endif /* PLATFORM_CONFIG_H */

This is minimal amount of information in the ``platform_config.h`` file. I.e,
the memory layout for on-chip and external RAM. Note that parts of the DDR
typically will need to be shared with normal world, so there is need for some
kind of memory firewall for this (more about that further down). As you can see
we have also added the UART configuration here, i.e., the ``DEVICE0_xyz`` part.

Official board support in OP-TEE?
=================================
We do encourage everyone to submit their board support to the OP-TEE project
itself, so it becomes part of the official releases and will be maintained by
the OP-TEE community itself. If you intend to do so, then there are a few more
things that you are supposed to do.

**Update platforms supported**

There is a section at the :ref:`platforms_supported` page that lists all devices
officially supported in OP-TEE, that is where you also shall list your device.
It should contain the name of the platform, then composite ``PLATFORM`` flag and
whether the device is publicly available or not. If there is a product page on
the internet for the device, please also create a link when writing the device
name.

**Update .shippable.yml**

Since we are using Shippable to test pull requests etc, we would like that you
also add your device to the `.shippable.yml
<https://github.com/OP-TEE/optee_os/blob/master/.shippable.yml>`_ file, so that
it will at least be built when someone is doing a pull request. Add a line at
the end of file:

.. code-block:: xml

    - _make PLATFORM=<platform-name>_

**Maintainer**

If you are submitting the board support upstream we are going to ask you to
become the maintainer for the device you have added. This means that you should
also update the MAINTAINERS.md_ file accordingly. By being a maintainer for a
device you are responsible to keep it up to date and you will be asked every
quarter as part of the OP-TEE release schedule to test your device running the
latest OP-TEE software.

**Update build.git and manifest.git**

This isn't strictly necessary, but we are trying to create and maintain OP-TEE
developer builds that should make it easy to setup, build and deploy OP-TEE on
various devices. We encourage all maintainers to do the same for the boards they
are in charge of. Therefore please consider creating a new :ref:`manifest` (and
a new ``*.mk`` in :ref:`build`) for the device you have added to OP-TEE.

.. _hardware_unique_key:

Hardware Unique Key
*******************
Most devices have some kind of Hardware Unique Key (HUK) that is mainly used to
derive other keys. The HUK could for example be used when deriving keys used in
secure storage etc. The important thing with the HUK is that it needs to be well
protected and in the best case the HUK should never ever be readable directly
from software, not even from the secure side. There are different solutions to
this, crypto accelerator might have support for it or, it could involve another
secure co-processor.

In OP-TEE the HUK **is** just **stubbed** and you will see that in the function
called ``tee_otp_get_hw_unique_key(...)`` in
`core/include/kernel/tee_common_otp.h`_. In a real secure product you **must**
replace this with something else. If your device lacks the hardware support for
a HUK, then you must at least change this to something else than just zeroes.
But, remember it is not good secure practice to store a key in software,
especially not the key that is the root for everything else, so this is not
something we recommend that you should do.

Secure Clock
************
The Time API in GlobalPlatform Internal Core API specification defines three
sources of time; system time, TA persistent time and REE time. The REE time is
by nature considered as an unsecure source of time, but the other two should in
a fully trustable hardware make use of trustable source of time, i.e., a secure
clock. Note that from GlobalPlatform point of view it is not required to make
use of a secure clock, i.e., it is OK to use time from REE, but the level of
trust should be reflected by the ``gpd.tee.systemTime.protectionLevel`` property
and the ``gpd.tee.TAPersistentTime.protectionLevel`` property (100=REE
controlled clock, 1000=TEE controlled clock). So the functions that one needs to
pay attention to are ``tee_time_get_sys_time(...)`` and
``tee_time_get_ta_time(...)``. If your hardware has a secure clock, then you
probably want to change the implementation there to instead use the secure clock
(and then you would also need to update the property accordingly, i.e.,
``tee_time_get_sys_time_protection_level()`` and the variable
``ta_time_prot_lvl`` in ``tee_svc.c``).

Root and Chain of Trust
***********************
To be able to assure that your devices are running the (untampered) binaries you
intended to run you will need to establish some kind of trust anchor on the
devices.

The most common way of doing that is to put the root public key in some read
only memory on the device. Quite often SoC's/OEM's stores public key(s) directly
or the hash(es) of the public key(s) in OTP_. When the boot ROM (which indeed
needs to be ROM) is about to load the first stage bootloader it typically reads
the public key from the software binary itself, hash the key and compare it to
the key in OTP_. If they are matching, then the boot ROM can be sure that the
first stage bootloader was indeed signed with the corresponding private key.

In OP-TEE you will not find any code at all related to this and this is a good
example when it is hard for us to do this in a generic way since device
manufacturers all tend to do this in their own unique way and they are not very
keen on sharing their low level boot details and security implementation with
the rest of the world. This is especially true on ARMv7-A. For ARMv8-A it looks
bit better, since Arm in Trusted Firmware A have implemented and defined how a
abstract the chain of trust (see auth-framework.rst_).
We have successfully verified OP-TEE by using the authentication framework from
Trusted Firmware A (see :ref:`secure_boot` for the details).

Hardware Crypto IP
******************
By default OP-TEE uses a software crypto library (currently mbed TLS and
LibTomCrypt) and you have the ability to enable Crypto Extensions that were
introduced with ARMv8-A (if the device is capable of that). Some of the devices
supported in OP-TEE OS repository have hardware crypto capabilities. A
framework, named drvcrypt has been designed to integrate them. The
`drvcrypt_register_*()`` API functions allow drivers to register support for
given cryptographic operations in OP-TEE core crypto API. Our
:ref:`cryptographic_implementation` page describes in detail how the Crypto API
is integrated.

Random Number Generator
***********************
By default OP-TEE is configured with a software PRNG. The entropy is added to
software PRNG at various places, but unfortunately it is still quite easy to
predict the data added as entropy. As a consequence, unless the RNG is based on
hardware the generated random will be quite weak.

If your platform has a hardware entropy source, you should set
``CFG_WITH_SOFTWARE_PRNG`` to ``n``, and provide an implementation for
``hw_get_random_bytes()``, which returns multiple bytes of entropy.

When ``CFG_WITH_SOFTWARE_PRNG=n``, the platform can enable a PTA service for
normal world to retrieve good quality random bytes. See configuration
switches ``CFG_HWRNG_PTA`` and ``CFG_HWRNG_QUALITY``, from 0 to 1024.

When ``CFG_WITH_SOFTWARE_PRNG=n``, the random number generator is made
available to OP-TEE drivers and frameworks, including Trusted Applications
(thoguh GP TEE Internal Core API) and normal world (when ``CFG_HWRNG_PTA=y``).

Power Management / PSCI
***********************
In the :ref:`add_a_new_platform` section where we talked about the file
``main.c``, we added a couple of handlers related to power management, we are
talking about the following lines:

.. code-block:: c

    .cpu_on = cpu_on_handler,
    .cpu_off = pm_do_nothing,
    .cpu_suspend = pm_do_nothing,
    .cpu_resume = pm_do_nothing,
    .system_off = pm_do_nothing,
    .system_reset = pm_do_nothing,

The only function that actually does something there is the ``cpu_on`` function,
the rest of them are stubbed. The main reason for that is because we think that
how to suspend and resume is a device dependent thing. The code in OP-TEE is
prepared so that callbacks etc from Trusted Firmware A will be routed to OP-TEE,
but since the function(s) are just stubbed we will not do anything and just
return. In a real production device, you would probably want to save and restore
CPU states, secure hardware IPs' registers and TZASC and other memory firewall
related setting when these callbacks are being called.

Memory firewalls / TZASC
************************
Arm have defined a system IP / SoC peripheral called TrustZone Address Space
Controller (TZASC, see TZC-380_ and TZC-400_). TZASC can be used to configure
DDR memory into separate regions in the physcial address space, where each
region can have an individual security level setting. After enabling TZASC, it
will perform security checks on transactions to memory or peripherals. It is not
always the case that TZASC is on a device, in some cases the SoC has developed
something equivalent. In OP-TEE this is very well reflected, i.e., different
platforms have different ways of protecting their memory. On ARMv8-A platforms
we are in most of the cases using Trusted Firmware A as the boot firmware and
there the secure bootloader is the one that configures secure vs non-secure
memory using TZASC (see plat_arm_security_setup_ in TF-A). The takeaway here is
that you must make sure that you have configured whatever memory firewall your
device has such that it has a secure and a non-secure memory area.

.. _core_pub_priv_keypair:

Trusted Application private/public keypair
******************************************
By default all Trusted Applications (TA's) are signed with the pre-generated
2048-bit RSA development key (private key). This key is located in the ``keys``
folder (in the root of optee_os.git) and is named ``default_ta.pem``. This key
**must** be replaced with your own key and you should **never ever** check-in
this private key in the source code tree when in use in a real product. The
recommended way to store private keys is to use some kind of HSM_ (Hardware
Security Module), but an alternative would be temporary put the private key on a
computer considered as secure when you are about to sign TA's intended to be
used in real products. Typically it is only a few number of people having access
to this type of key in company. The key handling in OP-TEE is currently a bit
limited since we only support a single key which is used for all TA's. We have
plans on extending this to make it a bit more flexible. Exactly when that will
happen has not been decided yet.

.. _platform_ports:

Platform ports
**************
OP-TEE is a reference implementation for developers and device manufacturers.
This also implies that there are certain configurations and settings that cannot
be done in OP-TEE reference code. In short, there are cases when the default
configuration hasn't enabled all necessary security features for the end
product. There are a couple of reasons for that.

- Chipmakers and Semiconductors might only share specifications telling how to
  securely configure their devices with partners who have signed an NDA with
  them.
- In some cases a setting might be perfectly fine when OP-TEE is used in one
  particular environment, but the same setting might be insecure in another
  environment.

Because of this we always urge companies and device manufacturers making the end
product to follow the security guidelines from the chipmaker they are basing
their products on. Refer also to :ref:`platform_documentation`


.. _core/arch/arm/plat-hikey/conf.mk: https://github.com/OP-TEE/optee_os/blob/master/core/arch/arm/plat-hikey/conf.mk
.. _core/include/crypto/crypto.h: https://github.com/OP-TEE/optee_os/blob/master/core/include/crypto/crypto.h
.. _core/include/kernel/tee_common_otp.h: https://github.com/OP-TEE/optee_os/blob/master/core/include/kernel/tee_common_otp.h


.. _auth-framework.rst: https://github.com/ARM-software/arm-trusted-firmware/blob/master/docs/auth-framework.rst
.. _HSM: https://en.wikipedia.org/wiki/Hardware_security_module
.. _MAINTAINERS.md: https://github.com/OP-TEE/optee_os/blob/master/MAINTAINERS
.. _OTP: https://en.wikipedia.org/wiki/Programmable_read-only_memory
.. _plat_arm_security_setup: https://github.com/ARM-software/arm-trusted-firmware/search?utf8=%E2%9C%93&q=plat_arm_security_setup&type=
.. _TZC-380: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0431c/index.html
.. _TZC-400: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.100325_0001_02_en/index.html