ArmoredWitness Trusted OS

Introduction

TODO.

Supported hardware

The following table summarizes currently supported SoCs and boards.

SoC	Board	SoC package	Board package
NXP i.MX6UL	USB armory Mk II LAN	imx6ul	usbarmory/mk2
NXP i.MX6ULL	USB armory Mk II	imx6ul	usbarmory/mk2

Purpose

This trusted OS is a TamaGo unikernel intended to run on the board(s) listed above in the TrustZone Secure World system mode, to be used in conjuction with the counterpart witness trusted applet unikernel running in the Secure World user mode.

The GoTEE syscall interface is implemented for communication between the Trusted OS and Trusted Applet.

The trusted OS can be also executed under QEMU emulation, including networking support (requires a tap0 device routing the Trusted Applet IP address).

⚠️ emulated runs perform partial tests due to lack of full hardware support by QEMU.

make DEBUG=1 FAKE_STORAGE=1 BEE=0 trusted_os_embedded_applet log_os qemu
...
00:00:00 tamago/arm • TEE security monitor (Secure World system/monitor)
00:00:00 SM applet verification
00:00:01 SM applet verified
00:00:01 SM loaded applet addr:0x90000000 entry:0x9007751c size:14228514
00:00:01 SM starting mode:USR sp:0xa0000000 pc:0x9007751c ns:false
00:00:02 tamago/arm • TEE user applet
00:00:02 TA MAC:1a:55:89:a2:69:41 IP:10.0.0.1 GW:10.0.0.2 DNS:8.8.8.8:53
00:00:02 TA requesting SM status
00:00:02 ----------------------------------------------------------- Trusted OS ----
00:00:02 Secure Boot ............: false
00:00:02 Runtime ................: tamago/arm
00:00:02 Link ...................: false
00:00:02 TA starting ssh server (SHA256:eeMIwwN/zw1ov1BvO6sW3wtYi463sq+oLgKhmAew1WE) at 10.0.0.1:22

Trusted OS signing

For an overview of the firmware authentication process please see https://github.com/transparency-dev/armored-witness/tree/main/docs/firmware_auth.md.

To maintain the chain of trust the Trusted OS must be signed and logged. To this end, two note signing keys must be generated.

$ go run github.com/transparency-dev/serverless-log/cmd/generate_keys@HEAD \
  --key_name="DEV-TrustedOS-1" \
  --out_priv=armored-witness-os-1.sec \
  --out_pub=armored-witness-os-1.pub
$ go run github.com/transparency-dev/serverless-log/cmd/generate_keys@HEAD \
  --key_name="DEV-TrustedOS-2" \
  --out_priv=armored-witness-os-2.sec \
  --out_pub=armored-witness-os-2.pub

The corresponding public key files will be built into the bootloader to verify the OS.

Trusted Applet authentication

To maintain the chain of trust the OS performs trusted applet authentication before executing it. This includes verifying signatures and Firmware Transparency artefacts produced when the applet was built.

Firmware transparency

All ArmoredWitness firmware artefacts need to be added to a firmware transparency log.

The provided Makefile has support for maintaining a local firmware transparency log on disk. This is intended to be used for development only.

In order to use this functionality, a log key pair can be generated with the following command:

$ go run github.com/transparency-dev/serverless-log/cmd/generate_keys@HEAD \
  --key_name="DEV-Log" \
  --out_priv=armored-witness-log.sec \
  --out_pub=armored-witness-log.pub

Building and executing on ARM targets

Download and install the latest TamaGo binary release.

Building the OS

Ensure the following environment variables are set:

Variable	Description
OS_PRIVATE_KEY1	Path to OS firmware signing key 1. Used by the Makefile to sign the OS.
OS_PRIVATE_KEY2	Path to OS firmware signing key 2. Used by the Makefile to sign the OS.
APPLET_PUBLIC_KEY	Path to applet firmware verification key. Embedded into the OS to verify the applet at run-time.
LOG_PUBLIC_KEY	Path to log verification key. Embedded into the OS to verify at run-time that the applet is correctly logged.
LOG_ORIGIN	FT log origin string. Embedded into the OS to verify applet firmware transparency.
LOG_PRIVATE_KEY	Path to log signing key. Used by Makefile to add the new OS firmware to the local dev log.
DEV_LOG_DIR	Path to directory in which to store the dev FT log files.

The OS firmware image can then be built, signed, and logged with the following command:

# The trusted_os target builds the firmware image, and log_os target adds it
# to the local firmware transparency log.
make trusted_os log_os

The final executable, trusted_os.elf is created in the bin subdirectory, and should be used for loading through armored-witness-boot.

Firmware transparency artefacts will be written into ${DEV_LOG_DIR}.

Development builds

To aid in development, it is also possible to build the OS with the Trusted Applet directly embedded within it:

make trusted_os_embedded_applet

The resulting bin/trusted_os.elf may be seral booted directly to the device with the imx_boot tool, or similar. Note that since this OS image is not being loaded via the bootloader, it does not need to be added to the FT log.

Optional development environment variables

The following environment variables may be set when compiling the Trusted OS image in order to add extra debugging support to the resulting image:

Variable	Description
DEBUG	When set to 1, enables output of logging, stdout, etc. See #debugging section below for more details
FAKE_STORAGE	When set to 1, provides a storage card implementation which passes-through to SDCard on native hardware, but uses RAM to emulate an SDCard otherwise.

Encrypted RAM support

Only on i.MX6UL P/Ns, the BEE environment variable must be set to match armored-witness-boot compilation options in case AES CTR encryption for all external RAM, using TamaGo bee package, is configured at boot.

The following targets are available:

TARGET	Board	Executing and debugging
usbarmory	UA-MKII-LAN	usbarmory/mk2

The targets support native (see relevant documentation links in the table above) as well as emulated execution (e.g. make qemu).

Debugging

An optional Serial over USB console can be used to access Trusted OS and Trusted Applet logs, it can be enabled when compiling with the DEBUG environment variable set:

make DEBUG=1 trusted_os

The Serial over USB console can be accessed from a Linux host as follows:

picocom -b 115200 -eb /dev/ttyACM0 --imap lfcrlf

QEMU

The Trusted OS image can be executed under emulation as follows:

make qemu

Note that emulated MMC storage is not currently supported, so the Trusted OS should be built with the FAKE_STORAGE environment variable set to 1:

make DEBUG=1 FAKE_STORAGE=1 trusted_os

The emulation run network connectivity should be configured as follows (Linux example with tap0):

ip addr add 10.0.0.2/24 dev tap0
ip link set tap0 up
ip tuntap add dev tap0 mode tap group <your user group>

The emulated target can be debugged with GDB using make qemu-gdb, this will make qemu waiting for a GDB connection that can be launched as follows:

arm-none-eabi-gdb -ex "target remote 127.0.0.1:1234" example

Breakpoints can be set in the usual way:

b ecdsa.GenerateKey
continue

Trusted Applet installation

Installing the various firmware images onto the device can be accomplished using the provision tool.

LED status

The USB armory Mk II LEDs are used, in sequence, as follows:

Boot sequence	Blue	White
0. initialization	off	off
1. trusted applet verified	off	on
2. trusted applet execution	on	on