crdyboot

Pronounced CUR-dee-boot.

Crdyboot is a UEFI bootloader for ChromeOS Flex.

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.
• The libcrdy package contains shared code that is used by both the crdyboot and crdyshim packages.
• The crdyboot package produces the actual crdyboot 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 loads, verifies, and runs the kernel.
• The crdyshim package produces an optional first-stage bootloader similar to shim. Its purpose is to load, verify, and run the second stage bootloader.
• 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 tools subdirectory contains a couple packages used for testing.

Dependencies

Install Rust: https://rustup.rs

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

sudo apt install clang curl efitools gdisk libnss3-dev libssl-dev ovmf \
    ovmf-ia32 pkg-config qemu-system-x86 sbsigntool swtpm

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

Googlers: see go/crdyboot-internal for additional recommendations.

Building and testing

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

cargo xtask setup [<disk-path>]

This will prepare a reven image to use with VM tests. By default a build of the public reven board is downloaded from a GS bucket. If you are a Googler, you can pass --reven-private to get a recent build of the private reven board. Alternatively, you can provide a file path to use a local image. The image should be a test image with verity enabled.

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

cargo xtask check [--vm-tests]

The --vm-tests option enables slow tests that run under QEMU.

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

cargo xtask build

Then run it in QEMU:

cargo xtask qemu [--ia32] [--no-secure-boot] [--tpm1|--tpm2]

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.

Documentation

See the docs subdirectory.