secure-boot-example: Add step by step example / quick start

The secure-boot-example/boot.img has been replaced with the mass-storage-gadget Linux OS image because it already supported login, hdmi and ethernet.

secure-boot-example: Add step by step example / quick start
The secure-boot-example/boot.img has been replaced with the mass-storage-gadget Linux OS image because it already supported login, hdmi and ethernet.
Tim Gover · Tim Gover
1 parent 69bec5e8
Showing 6 changed files with 162 additions and 6 deletions
Readme.md
secure-boot-example/README.md
secure-boot-example/boot.img
secure-boot-example/boot.sig
secure-boot-example/bootcode4.bin
secure-boot-example/config.txt
@@ -91,6 +91,9 @@ 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.
+### Tutorial
+Creating a secure-boot system from scratch can be quite complex. The [secure-boot tutorial](secure-boot-example/README.md) uses a minimal example OS image to demonstrate how the Raspberry Pi specific aspects of secure-boot work.
+
 ### Host setup
 Secure boot require a 2048 bit RSA asymmetric keypair and the Python `pycrytodomex` module to sign the EEPROM config and boot image.
-This directory contains an example secure boot image signed with the example private key in this directory.
+# Secure-boot quickstart
+
+## Overview
+Secure-boot is a mechanism for verifying the integrity of the kernel image and 
+other files required during boot by storing them in a signed ramdisk image.
+These files include the GPU firmware (start.elf etc), Kernel, initrd, device-tree tree
+and overlays.
+Secure-boot does not depend on particular OS, nor does it provide services
+to the OS after startup.  However, since the secure-boot will only load trusted 
+kernel + initrd images the kernel/initrd can safely verify or mount an
+an encrypted file-system e.g. by using a TPM.
+
+N.B The memory for the secure ramdisk is reclaimed as soon as the CPU is started.
+
+## Example OS
+This example includes a simple buildroot image for Compute Module 4 in order
+to demonstrate secure-boot on a simple but functional OS.
+
+The example does NOT modify the OTP or make other permanent changes to the system
+and the code-signing can be disabled by reflashing the default bootloader EEPROM.
+
+The tutorial does not cover how to create the `boot.img` file or permanently
+require secure-boot in OTP. Please see the top level [README](../Readme.md#building) and
+[secure-boot-recovery/README](../secure-boot-recovery/README.md) guides for
+more information.
+
+### Requirements for running this example
+* A Raspberry Pi Compute Module 4
+* Micro USB cable for `rpiboot` connection
+* USB serial cable (for debug logs)
+* Linux or Cygwin (Windows 10)
+* OpenSSL
+* Python3
+* Python `cryptodomex`
+
+```bash
+python3 -m pip install pycryptodomex
+# or
+pip install pycryptodomex
+```
+
+### Clean configuration
+Before starting it's advisable to create a fresh clone of the `usbboot` repo
+to ensure that there are no stale configuration files.
+
+```bash
+git clone https://github.com/raspberrypi/usbboot secure-boot
+cd secure-boot
+make
+```
+See the top-level [README](../Readme.md) for build instructions.
+
+### Hardware setup for `rpiboot` mode
+Prepare the Compute Module for `rpiboot` mode.
+
+* Set the `nRPIBOOT` jumper which is labelled `Fit jumper to disable eMMC Boot' on the Compute Module 4 IO board.
+* Connect the micro USB cable to the `USB slave` port on the CM4 IO board.
+* Power cycle the CM4 IO board.
+* Connect the USB serial adapter to [GPIO 14/15](https://www.raspberrypi.com/documentation/computers/os.html#gpio-and-the-40-pin-header) on the 40-pin header.
+
+### Reset the Compute Module EEPROM
+Enable `rpiboot` mode and flash the latest EEPROM image
+```bash
+./rpiboot -d recovery
+```
+
+### Boot the example image
+Enable `rpiboot` mode and load the OS via `rpiboot` without enabling code-signing.
+```bash
+./rpiboot -d secure-boot-example
+```
+The OS should load and show activity on the boot UART and HDMI console.
+
+### Generate a signing key
+Secure-boot requires a 2048 bit RSA private key. You can either use a pre-existing
+key or generate an specific key for this example. The `KEY_FILE` environment variable
+used in the following instructions must contain the absolute path to the RSA private key in
+PEM format.
+
+```bash
+openssl genrsa 2048 > private.pem
+export KEY_FILE=$(pwd)/private.pem
+```
+
+**In a production environment it's essential that this key-file is stored privately and securely.**
+
+### Update the EEPROM to require signed OS images
+Enable `rpiboot` mode and flash the bootloader EEPROM with updated setting enables code-signing.
+
+The `boot.conf` file sets the `SIGNED_BOOT` property `1` which instructs the bootloader to only
+load files (firmware, kernel, overlays etc) from `boot.img` instead of the normal boot partition and verify the `boot.img` signature `boot.sig` using the public key in the EEPROM.
+
+The `update-pieeprom.sh` generates the signed `pieeprom.bin` image.
+
+```bash
+cd secure-boot-recovery
+# Generate the signed EEPROM image.
+../tools/update-pieeprom.sh -k "${KEY_FILE}"
+cd ..
+./rpiboot -d secure-boot-recovery
+```
+
+At this stage OTP has not been modified and the signed image requirement can be reverted by flashing a default, unsigned image. 
+However, once the [OTP secure-boot flags](../secure-boot-recovery/README.md#locking-secure-boot-mode) are set then `SIGNED_BOOT` is permenantely enabled and cannot be overriden via the EEPROM config.
+
+
+### Update the signature for the example OS image
+```bash
+cd secure-boot-example
+../tools/rpi-eeprom-digest -i boot.img -o boot.sig -k "${KEY_FILE}"
+cd ..
+```
+
+### Launch the signed OS image
+Enable `rpiboot` mode and run the example OS as before. However, if the
+`boot.sig` signature does not match `boot.img` the bootloader will refuse to
+load the OS.
-To sign the image with a different key run
 ```bash
+./rpiboot -d secure-boot-example
+```
+
+This example OS image is minimal Linux ramdisk image. Login as `root` with the empty password.
+
+### Mount the CM4 SD/EMMC after enabling secure-boot
+Now that `SIGNED_BOOT` is enabled the bootloader will only load images signed with private key generated earlier. 
+To boot the Compute Module in mass-storage mode a signed version of this code must be generated.  
+  
+**This signed-image should not be made available for download because it gives access to the EMMC as a block device**
+
+
+#### Sign the mass storage firmware image
+Sign the mass-storage drivers in the `secure-boot-msd` directory. Please see the [top level README](../Readme.md#compute-module-4-extensions) for a description of the different `usbboot` firmware drivers.
+```bash
+cd secure-boot-msd
 ../tools/rpi-eeprom-digest -i boot.img -o boot.sig -k "${KEY_FILE}"
+cd ..
 ```
-Clearly, product releases should never be signed with `example-private.pem`.
+#### Enable MSD mode.
+A new mass-storage device should now be visible on the host-OS. On Linux check `dmesg` for something like '/dev/sda'
+```bash
+./rpiboot -d secure-boot-msd
+```
+
+### Loading `boot.img` from SD/EMMC.
+The bootloader can load a ramdisk `boot.img` from any of the bootable modes defined by the [BOOT_ORDER](https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#BOOT_ORDER) EEPROM config setting.
+
+For example.
+
+* Boot the CM4 in MSD mode as explained in the previous step.
+* Copy the `boot.img` and `boot.sig` files from the `secure-boot-example` stage to the mass-storage-drive. No other files are required.
+* Remove the `nRPIBOOT` jumper.
+* Power cycle the CM4 IO board.
+* The system should now boot into the OS.
+
+N.B. The example image is currently the same as the `mass-storage-gadget so the EMMC/SD will appear as block devices on the host PC if the micro USB cable is still connected.
+
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+# Load the boot files from the ramdisk which is signed in secure-boot mode
+boot_ramdisk=1
+uart_2ndstage=1