Introduction

This TamaGo based unikernel acts as a primary boot loader for the USB armory Mk II, allowing boot of kernel images (e.g. Linux) from either the eMMC card or an external microSD card.

This repository also provides a command line utility to load imx executables through USB using SDP.

Compiling

Build the TamaGo compiler (or use the latest binary release):

wget https://github.com/usbarmory/tamago-go/archive/refs/tags/latest.zip
unzip latest.zip
cd tamago-go-latest/src && ./all.bash
cd ../bin && export TAMAGO=`pwd`/go

The BOOT environment variable must be set to either uSD or eMMC to configure the bootloader media for /boot/armory-boot.conf, as well as kernel images, location.

The START environment variable must be set to the offset of the first valid ext4 partition where /boot/armory-boot.conf is located (typically 5242880 for USB armory Mk II default pre-compiled images).

The CONSOLE environment variable may be set to on to enable serial logging when a debug accessory is connected.

Build the armory-boot.imx application executable:

git clone https://github.com/usbarmory/armory-boot && cd armory-boot
make imx BOOT=uSD START=5242880

Docker

For convenience a docker configuration is provided.

Ensure docker daemon is running and build the armory-boot docker image, this needs to be done only the first time:

docker build --build-arg UID=$UID --build-arg GID=$UID -t armory-boot docker

You can enter to the container as follows:

docker run -it --rm -v $PWD:/build armory-boot

Installing

The armory-boot.imx file can be flashed on the internal eMMC card or an external micro SD card as shown in these instructions.

Configuration

The bootloader expects a single configuration file to read information on the image and parameters to boot.

The bootloader is configured via a single configuration file, and can boot either an ARM kernel image or an ELF unikernel (e.g. tamago-example). The required elements in the configuration file differ depending on the type of image being loaded, examples for both are given below.

It is an error specify both unikernel and kernel config parameters in the same configuration file.

Linux kernel boot

To load a Linux kernel, the bootloader requires that you provide the paths to the kernel image and the Device Tree Blob file, along with their respective SHA256 hashes (only used with configuration signature verification, see Secure Boot), as well as the kernel command line.

An optional initial ramdisk can be passed with the initrd parameter.

Example /boot/armory-boot.conf configuration file for loading a Linux kernel:

{
  "kernel": [
    "/boot/zImage-5.4.51-0-usbarmory",
    "aceb3514d5ba6ac591a7d5f2cad680e83a9f848d19763563da8024f003e927c7"
  ],
  "dtb": [
    "/boot/imx6ulz-usbarmory-default-5.4.51-0.dtb",
    "60d4fe465ef60042293f5723bf4a001d8e75f26e517af2b55e6efaef9c0db1f6"
  ],
  "initrd": [
    "/boot/initrd.img-5.4.51-0-usbarmory",
    "64119096fd329e89f062cb5e0fc5b8e66f98081aef987e0bc7a92a05f4452540"
  ],
  "cmdline": "console=ttymxc1,115200 root=/dev/mmcblk0p1 rootwait rw"
}

TamaGo unikernel boot

To load a TamaGo unikernel, the bootloader only needs the path to the ELF binary along with its SHA256 hash (only used with configuration signature verification, see Secure Boot).

Example /boot/armory-boot.conf configuration file for loading a TamaGo unikernel:

{
  "unikernel": [
    "/boot/tamago-example",
    "e6de9214249dd7989b4056372424e84b273ff4e5d2410fa12ac230ddaf22690a"
  ]
}

Secure Boot

On secure booted systems the imx_signed target should be used instead with the relevant HAB_KEYS set.

Additionally, to maintain the chain of trust, the PUBLIC_KEY environment variable must be set with either a signify or minisign public key to enable configuration file signature verification.

Example key generation (signify):

signify -G -p armory-boot.pub -s armory-boot.sec

Example key generation (minisign):

minisign -G -p armory-boot.pub -s armory-boot.sec

Compilation with embedded key:

make imx_signed BOOT=uSD START=5242880 PUBLIC_KEY=<last line of armory-boot.pub> HAB_KEYS=<path>

When armory-boot is compiled with the PUBLIC_KEY variable, a signature for the configuration file must be created in /boot/armory-boot.conf.sig using with the corresponding secret key.

Example signature generation (signify):

signify -S -s armory-boot.sec -m armory-boot.conf -x armory-boot.conf.sig

Example signature generation (minisign):

minisign -S -s armory-boot.sec -m armory-boot.conf -x armory-boot.conf.sig

LED status

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

Boot sequence	Blue	White
0. initialization	off	off
1. boot media detected	on	off
2. kernel verification complete	on	on
3. jumping to kernel image	off	off

Serial Download Protocol utility

The armory-boot-usb command line utility allows to load an imx executable through USB using SDP, useful for testing or initial provisioning purposes.

You can automatically download, compile and install the utility, under your GOPATH, as follows:

go install github.com/usbarmory/armory-boot/cmd/armory-boot-usb@latest

Alternatively you can manually compile it from source:

git clone https://github.com/usbarmory/armory-boot
cd armory-boot && make armory-boot-usb

The utility can be cross compiled for Windows as follows:

make armory-boot-usb.exe

Pre-compiled binaries for Linux and Windows are released here.

The utility is meant to be used on devices running in USB SDP mode:

sudo armory-boot-usb -i armory-boot.imx
found device 15a2:0080 Freescale SemiConductor Inc  SE Blank 6ULL
parsing armory-boot.imx
loading DCD at 0x00910000 (952 bytes)
loading imx to 0x9000f400 (2182144 bytes)
jumping to 0x9000f400
serial download complete

Authors

Andrea Barisani
andrea.barisani@withsecure.com | andrea@inversepath.com

License

armory-boot | https://github.com/usbarmory/armory-boot
Copyright (c) WithSecure Corporation

These source files are distributed under the BSD-style license found in the LICENSE file.

ckahlo/armory-boot