usbboot

The owner can control Trustzone if the device is shipped with unfused OTP registers. On Raspberry Pi for example, you can write the hash of your own public key to locations 47-54 of the OTP memory block: https://www.raspberrypi.com/documentation/computers/raspberr... Here's the QuickStart for the entire process: https://github.com/raspberrypi/usbboot/blob/master/secure-bo... Note that the Raspberry Pi does not have a full TrustZone implementation to protect secure mode memory, etc. But it is a widely available device with good documentation and allows developers to experiment with and learn about the basics of TrustZone architecture.

This isn't possible. The docs[1] state once revoke_devkey has been set, you can't upgrade to a bootloader not supporting secure-boot and downgrading is disabled. [1] https://github.com/raspberrypi/usbboot/blob/master/secure-bo...

The Pi4 has true hardware support for secure boot. If set up correctly, you won't be able to boot anything not properly signed. An incomplete overview of how this works is: * Instead of having all boot related files (start4.elf, kernel.img, ...) on the first partition of the SD card, you instead have a single boot.img FAT image containing those files instead. * You sign that file with your own RSA 2048 key and place a boot.sig containing the signature next to the boot.img file. * You flash the Pi4 EEPROM and include your public key and some additional EEPROM settings. * You instruct the EEPROM to burn the hash of your public key into the Pi's OTP memory. Once that's done, the key cannot be changed and the Pi will not boot into anything not signed with your key. * Optionally you can also place keys for disk encryption into the OTP memory a

On the Pi Zero and Pi CM (maybe also others) you don't even need an SD card to boot it. You can boot it via rpi-boot https://github.com/raspberrypi/usbboot so no need for qemu for testing. You can just test it on real hardware in no time.