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crdyboot

Pronounced CUR-dee-boot.

Crdyboot is a UEFI bootloader for ChromeOS Flex. It is not yet in use.

Crdyboot acts as a bridge between UEFI firmware and the Chromebook style of booting. It uses vboot to select and validate an appropriate kernel partition, then launches that kernel using the Linux EFI stub.

Features

  • Well documented and as simple as possible.
  • Broad hardware support. Any amd64 machine with UEFI should be able to use crdyboot. This includes 32-bit UEFI environments.
  • Uses vboot to:
    • Verify that both the kernel and the kernel command-line have been signed with a trusted key, which in turn allows verifying that the rootfs has not been modified. (Note that this can only be fully relied on if using custom Secure Boot keys, otherwise a different OS signed with the Microsoft keys could be used to avoid verifying the rootfs.)
    • Automatically roll back from a bad OS update by swapping between the A and B partitions.

License

BSD

Code layout

The project is organized as a Rust workspace containing several packages:

  • The vboot package is a thin wrapper around the C vboot library. It also exposes a DiskIo trait through which it can read and write blocks to a disk. This package is no_std, and can be built for both the UEFI targets and the host target. Building for the host allows tests to be run on the host.
  • The libcrdy package is where most of the bootloader is implemented. It implements the DiskIo trait using the uefi crate, and uses the vboot package to load and verify a kernel. It then boots into that kernel using the EFI stub. This package is also no_std and can also be built for both UEFI targets and the host target for testing purposes.
  • The crdyboot package provides the actual bootloader executable. It contains the embedded key used to verify the kernel data, the SBAT data used for revocation, and sets up logging and allocation. Then it uses libcrdy to load, verify, and run the kernel.
  • The xtask package contains a host executable that provides the various xtask commands shown below. It's like a fancy Makefile for running various dev and test operations.
  • The enroller subdirectory contains a small UEFI application that enrolls a test key in the PK, KEK, and db variables. This is used to set up the test VM, and can also be used on real hardware (see the “Testing on real hardware” section).

Dependencies

Install Rust: https://rustup.rs

Install tools used for image signing and running in a VM:

sudo apt install efitools gdisk llvm ovmf ovmf-ia32 qemu-system-x86 sbsigntool

After installing qemu, add your user to the kvm group. You will need to log out and back in for this to take effect:

sudo adduser ${USER} kvm

Building and testing

Before running any other commands in the repository, run this setup command:

cargo xtask setup <reven-verity-image-path>

This will copy the reven image to a local directory and run various setup commands. The image must have rootfs verification enabled (i.e. build_image must be run without -r or --no-enable-rootfs-verification). Any kind of image (base, dev, or test) is allowed.

To check formatting, lint, test, build crdyboot, and install to the image:

cargo xtask check

To just build crdyboot and install to the image (a quicker subset of check):

cargo xtask build

Then run it in QEMU:

cargo xtask qemu [--ia32] [--secure-boot]

Testing on real hardware

To test secure boot with real hardware you will need to enroll custom keys. Write workspace/enroller.bin to a USB, and write workspace/disk.bin to a second USB, e.g. using writedisk.

Boot the DUT and enter the boot setup. Find the secure boot settings and change it to setup mode. (The details will vary from one vendor to another.)

Plug in the enroller USB and reboot. Use the boot menu to select the USB and wait for it to complete.

Unplug the enroller USB and plug in the cloudready USB, then reboot. Use the boot menu to select the USB.