USB device boot code

This is the USB MSD boot code which should work on the Raspberry Pi model A, Compute Module, Compute Module 3, Compute Module 4 and Raspberry Pi Zero.

The default behaviour when run with no arguments is to boot the Raspberry Pi with special firmware so that it emulates USB Mass Storage Device (MSD). The host OS will treat this as a normal USB mass storage device allowing the file-system to be accessed. If the storage has not been formatted yet (default for Compute Module) then the Raspberry Pi Imager App can be used to install a new operating system.

Since RPIBOOT is a generic firmware loading interface it is possible to load other versions of the firmware by passing the -d flag to specify the directory where the firmware should be loaded from. E.g. The firmware in the msd can be replaced with newer/older versions.

For more information run rpiboot -h

Building

Linux / Cygwin / WSL

Clone this on your Pi or a Linux machine.
Make sure that the system date is set correctly, otherwise Git may produce an error.

sudo apt install git libusb-1.0-0-dev
git clone --depth=1 https://github.com/raspberrypi/usbboot
cd usbboot
make
sudo ./rpiboot

sudo isn't required if you have write permissions for the /dev/bus/usb device.

macOS

From a macOS machine, you can also run usbboot, just follow the same steps:

1. Clone the usbboot repository
2. Install libusb (brew install libusb)
3. Install pkg-config (brew install pkg-config)
4. Build using make
5. Run the binary

git clone --depth=1 https://github.com/raspberrypi/usbboot
cd usbboot
brew install libusb
brew install pkg-config
make
sudo ./rpiboot

Running

Compute Module 3

Fit the EMMC-DISABLE jumper on the Compute Module IO board before powering on the board or connecting the USB cable.

Compute Module 4

On Compute Module 4 EMMC-DISABLE / nRPIBOOT (GPIO 40) must be fitted to switch the ROM to usbboot mode. Otherwise, the SPI EEPROM bootloader image will be loaded instead.

Compute Module 4 extensions

In addition to the MSD functionality, there are a number of other utilities that can be loaded via RPIBOOT on Compute Module 4.

Directory	Description
recovery	Updates the bootloader EEPROM on a Compute Module 4
rpi-imager-embedded	Runs the embedded version of Raspberry Pi Imager on the target device
mass-storage-gadget	Replacement for MSD firmware. Uses Linux USB gadgetfs drivers to export all block devices (e.g. NVMe, EMMC) as MSD devices
secure-boot-recovery	Scripts that extend the recovery process to enable secure-boot, sign images etc
secure-boot-msd	Scripts for signing the MSD firmware so that it can be used on a secure-boot device
secure-boot-example	Simple Linux initrd with a UART console.

Booting Linux

The RPIBOOT protocol provides a virtual file-system to the Raspberry Pi bootloader and GPU firmware. It's therefore possible to boot Linux. To do this, you will need to copy all of the files from a Raspberry Pi boot partition plus create your own initramfs. On Raspberry Pi 4 / CM4 the recommended approach is to use a boot.img which is a FAT disk image containing the minimal set of files required from the boot partition.

Secure Boot

Secure Boot requires the latest stable bootloader image. WARNING: If the revoke_devkey option is used to revoke the ROM development key then it will not be possible to downgrade to a bootloader older than 2022-01-06 OR disable secure-boot mode.

Host setup

Secure boot require a 2048 bit RSA asymmetric keypair and the Python pycrytodomex module to sign the EEPROM config and boot image.

Install Python Crypto support (the pycryptodomex module)

python3 -m pip install pycryptodomex
# or
pip install pycryptodomex

Create an RSA key-pair using OpenSSL. Must be 2048 bits

cd $HOME
openssl genrsa 2048 > private.pem

Secure Boot - configuration

• Please see the secure boot EEPROM guide to enable via rpiboot recovery.bin.
• Please see the secure boot MSD guide for instructions about to mount the EMMC via USB mass-storage once secure-boot has been enabled.

Secure Boot - image creation

Secure boot requires a boot.img FAT image to be created. This plus a signature file (boot.sig) must be placed in the boot partition of the Raspberry Pi.

The contents of the boot.img are the files normally present in the Raspberry Pi OS boot partition i.e. firmware, DTBs and kernel image. However, in order to reduce boot time it is advisable to remove unused files e.g. firmware or kernel images for Pi models.

The firmware must be new enough to support secure boot. The latest firmware APT package supports secure boot. To download the firmware files directly.

git clone --depth 1 --branch stable https://github.com/raspberrypi/firmware

A helper script (make-boot-image) is provided to automate the image creation process. This script depends upon the mkfs.fat and losetup tools and only runs on Linux.

Root file-system

Normally, the Kernel modules and overlays for a secure-boot system would be provided in an initramfs and built with (buildroot)[https://buildroot.org/] or (yocto)[https://www.yoctoproject.org/].

This ensures that all of the kernel modules and device tree dependencies are covered by the secure-boot signature.

Since the initramfs is part of the boot.img it is possible to replace GPU firmware, kernel and dependencies in a single file update.

Alternatively, for test/development the following instructions explain how a normal Raspberry Pi OS install can be modified to be booted with the secure-boot loader.

Clone the Raspberry Pi OS boot files

Copy the contents of /boot to a local directory called secure-boot-files

Set the kernel root device

Since the boot file-system for the firmware is now in a signed disk image the OS cannot write to this. Therefore, any changes to cmdline.txt must be made before the boot.img file is signed.

• Verify that cmdline.txt in secure-boot-files points to the correct UUID for the root file-system. Alternatively, for testing, you can specify the root device name e.g. root=/dev/mmcblk0p2.

• Remove init-resize.sh from cmdline.txt

Create the boot image

The -b product argument (pi4,pi400,cm4) tells the script to discard files which are not required by that product. This makes the image smaller and reduces the time taken to calculate the hash of the image file thereby reducing the boot time.

sudo ../tools/make-boot-image -d secure-boot-files -o boot.img -b pi4

The maximum supported size for boot.img is currently 96 megabytes.

Verify the boot image

To verify that the boot image has been created correctly use losetup to mount the .img file.

sudo su
mkdir -p boot-mount
LOOP=$(losetup -f)
losetup -f boot.img
mount ${LOOP} boot-mount/

 echo boot.img contains
find boot-mount/

umount boot-mount
losetup -d ${LOOP}
rmdir boot-mount

Sign the boot image

For secure-boot, rpi-eeprom-digest extends the current .sig format of sha256 + timestamp to include an hex format RSA bit PKCS#1 v1.5 signature. The key length must be 2048 bits.

../tools/rpi-eeprom-digest -i boot.img -o boot.sig -k "${KEY_FILE}"

Hardware security modules

rpi-eeprom-digest is a shell script that wraps a call to openssl dgst -sign. If the private key is stored withing a hardware security module instead of a .PEM file the openssl command will need to be replaced with the appropriate call to the HSM.

rpi-eeprom-digest called by update-pieeprom.sh to sign the EEPROM config file.

The RSA public key must be stored within the EEPROM so that it can be used by the bootloader. By default, the RSA public key is automatically extracted from the private key PEM file. Alternatively, the public key may be specified separately via the -p argument to update-pieeprom.sh and rpi-eeprom-config.

To extract the public key in PEM format from a private key PEM file, run:

openssl rsa -in private.pem -pubout -out public.pem`

Copy the secure boot image to the boot partition on the Raspberry Pi.

Copy boot.img and boot.sig to the chosen boot filesystem. Secure boot images can be loaded from any of the normal boot devices (e.g. SD, USB, Network).