Permalink
382 lines (332 sloc) 17.8 KB

Porting guidelines for OP-TEE

  1. Introduction
  2. Add a new platform
  3. Hardware Unique Key
  4. Secure Clock
  5. Root and Chain of Trust
  6. Hardware Crypto IP
  7. Power Management / PSCI
  8. Memory firewalls / TZASC
  9. Trusted Application private/public keypair

1. Introduction


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 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.

2. 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.

2.1 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:

$ 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:

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 Platform-specific configuration and flags in the build system documentation.

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.

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):

#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:

#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.

2.2 Devices officially 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.

2.2.1 Update README.md

There is a section (3. Platforms Supported) 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.

2.2.2 Update travis.xml

Since we are using Travis to test pull request etc, we would like that you also all your device to the travis file, so that it will at least be built when someone is doing a pull request. Add a line saying:

- PLATFORM=gendev  PLATFORM_FLAVOR=gendev-flav  make -j8 all -s

2.2.3 Maintainer

If you are submitting the board support upstream and cannot give Linaro maintainers a device, then 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.

2.2.4 Update build.git

This isn't strictly necessary, but we are trying to create repo setup(s) for the device(s) that we are in charge of. That makes it very easy for newcomers to get started with a certain platform. So please consider creating a new manifest for the device you have added to OP-TEE.

3. 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.

4. 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).

5. 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 ARM Trusted Firmware 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 ARM Trusted Firmware (see optee_with_auth_framework.md for the details).

6. Hardware Crypto IP

By default OP-TEE uses a software crypto library (currently 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 we have in our hands do have hardware crypto IP's, but due to NDA's etc it has not been possible to enable it. If you have a device capable of doing crypto operations on a dedicated crypto block and you prefer to use that in favor for the software implementation, then you will need to implement relevant functions defined in core/include/crypto/crypto.h, the Crypto API, and write the low level driver that communicates with the device. Our crypto.md file describes how the Crypto API is integrated. Since the communication with crypto blocks tends to be quite different depending on what kind of crypto block you have, we have not written how that should be done. It might be that we do that in the future when get hold of a device where we can use the crypto block.

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.

7. Power Management / PSCI

In section 2 when we talked about the file main.c, we added a couple of handlers related to power management, we are talking about the following lines:

	.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 ARM Trusted Firmware 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.

8. 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 ARM Trusted Firmware 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 ARM-TF). 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.

9. 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.