USB 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.

This version of rpiboot has been modified to work from directories which contain the booting firmware. There is a msd/ directory which contains bootcode.bin and start.elf to turn the Raspberry Pi device into a USB Mass Storage Device (MSD). If run without arguments embedded versions of bootcode.bin and start.elf are used to enable the MSD behaviour.

For more information run 'rpiboot -h'

Building

Ubuntu

Clone this on your Pi or an Ubuntu linux machine

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

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

Note: You might see an OS warning message about a new disk that it can't access, click "ignore", this likely means that the storage is empty and has no filesystem. From here I recommend installing an OS using the Raspberry Pi Imager App, or using any other means like dd.

Running your own (not MSD) build

If you would like to boot the Raspberry Pi with a standard build you just need to copy the FAT partition files into a subdirectory (it must have at the minimum bootcode.bin and start.elf). If you take a standard firmware release then this will at the very least boot the linux kernel which will then stop (and possibly crash!) when it looks for a filesystem. To provide a filesystem there are many options, you can build an initramfs into the kernel, add an initramfs to the boot directory or provide some other interface to the filesystem.

sudo ./rpiboot -d boot

This will serve the boot directory to the Raspberry Pi Device.

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.

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.

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 filesystem 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).