Chrome OS RMA Shim Bootloader

Shimboot is a collection of scripts for patching a Chrome OS RMA shim to serve as a bootloader for a standard Linux distribution. It allows you to boot a full desktop Debian install on a Chromebook, without needing to unenroll it or modify the firmware.

Features:

• Run a full Debian installation on a Chromebook
• Does not modify the firmware
• Works on enterprise enrolled devices
• Can boot Chrome OS with no restrictions (useful for enrolled devices)
• Nearly full device compatibility
• Optional disk compression
• Multiple desktop environments supported

About:

Chrome OS RMA shims are bootable disk images which are designed to run a variety of diagnostic utilities on Chromebooks, and they'll work even if the device is enterprise enrolled. Unfortunately for Google, there exists a security flaw where the root filesystem of the RMA shim is not verified. This lets us replace the rootfs with anything we want, including a full Linux distribution.

Simply replacing the shim's rootfs doesn't work, as it boots in an environment friendly to the RMA shim, not regular Linux distros. To get around this, a separate bootloader is required to transition from the shim environment to the main rootfs. This bootloader then runs pivot_root to enter the rootfs, where it then starts the init system.

Another problem is encountered at this stage: the Chrome OS kernel will complain about systemd's mounts, and the boot process will hang. A simple workaround is to apply a patch to systemd, and then it can be recompiled and hosted at a repo somewhere.

After copying all the firmware from the recovery image and shim to the rootfs, we're able to boot to a mostly working XFCE desktop.

The main advantages of this approach are that you don't need to touch the device's firmware in order to run Linux. Simply rebooting and unplugging the USB drive will return the device to normal, which can be useful if the device is enterprise enrolled. However, since we are stuck with the kernel from the RMA shim, some features such as audio and suspend may not work.

Partition Layout:

1. 1MB dummy stateful partition
2. 32MB Chrome OS kernel
3. 20MB bootloader
4. The rootfs partitions fill the rest of the disk

Note that rootfs partitions have to be named shimboot_rootfs:<partname> for the bootloader to recognize them.

Status:

Driver support depends on the device you are using shimboot on. The patch_rootfs.sh script attempts to copy all the firmware and drivers from the shim and recovery image into the rootfs, so expect most things to work on other boards. ARM Chromebooks are not supported at the moment.

Device Compatibility Table:

Feature \ Board Name	dedede	octopus	nissa	reks	kefka	zork
X11	yes	yes	yes	no (kernel too old)	no (kernel too old)	yes
Wifi	yes	yes	yes	yes	yes	yes
Internal Audio	no	yes	no	untested	yes	no
Backlight	yes	yes	yes	untested	yes	untested
Touchscreen	yes	yes	yes	untested	untested	yes
3D Acceleration	yes	yes	yes	no	no	yes
Bluetooth	yes	yes	yes	untested	untested	yes
Webcam	yes	yes	yes	untested	untested	yes

This table is incomplete. If you want to contribute a device compatibility report please create a new issue on the Github repository.

On all devices, expect the following features to work:

• Zram (compressed memory)
• Disk compression with squashfs

On all devices, the following features will not work:

• Suspend (disabled by the kernel)
• Swap (disabled by the kernel)

TODO:

• Finish Python TUI rewrite (see the python branch if you want to help with this)
• Transparent disk compression
• Full disk encryption
• Support for more distros (Ubuntu and Arch maybe)
• Support for ARM based Chromebooks (see issue #8)
• Eliminate binwalk dependency
• Get audio to work on dedede
• Get kexec working

PRs and contributions are welcome to help implement these features.

Usage:

Prerequisites:

• A separate Linux PC for the build process (preferably something Debian-based)
  • WSL2 is supported if you are on Windows
  • Github Codespaces is not supported at the moment
• A USB drive that is at least 8GB in size
• At least 20GB of free disk space
• An x86-based Chromebook

Build Instructions:

1. Find the board name of your Chromebook. You can search for the model name on chrome100.dev.
2. Clone this repository and cd into it.
3. Run sudo ./build_complete.sh <board_name> to download the required data and build the disk image.

Alternatively, you can run each of the steps manually:

1. Grab a Chrome OS RMA Shim from somewhere. Most of them have already been leaked and aren't too difficult to find.
2. Download a Chrome OS recovery image for your board.
3. Unzip the shim and the recovery image if you have not done so already.
4. Run mkdir -p data/rootfs to create a directory to hold the rootfs.
5. Run sudo ./build_rootfs.sh data/rootfs bookworm to build the base rootfs.
6. Run sudo ./patch_rootfs.sh path_to_shim path_to_reco data/rootfs to patch the base rootfs and add any needed drivers.
7. Run sudo ./build.sh image.bin path_to_shim data/rootfs to generate a disk image at image.bin.

Prebuilt images are available if you don't have a suitable device to run the build on.

Booting the Image:

1. Obtain a shimboot image by downloading a prebuilt one or building it yourself.
2. Flash the shimboot image to a USB drive or SD card. Use the Chromebook Recovery Utility or dd if you're on Linux.
3. Enable developer mode on your Chromebook. If the Chromebook is enrolled, follow the instructions on the sh1mmer website (see the "Executing on Chromebook" section).
4. Plug the USB into your Chromebook and enter recovery mode. It should detect the USB and run the shimboot bootloader.
5. Boot into Debian and log in with the username and password that you configured earlier. The default username/password for the prebuilt images is user/user.
6. Expand the rootfs partition so that it fills up the entire disk by running sudo growpart /dev/sdX 4 (replacing sdX with the block device corresponding to your disk) to expand the partition, then running sudo resize2fs /dev/sdX4 to expand the filesystem.
7. Change the user password by running passwd user. The root user is disabled by default.

FAQ:

I want to use a different Linux distribution. How can I do that?

Using any Linux distro is possible, provided that you apply the proper patches to systemd and recompile it. Most distros have some sort of bootstrapping tool that allows you to install it to a directory on your host PC. Then, you can just pass that rootfs directory into patch_rootfs.sh and build.sh.

Debian Sid (the rolling release version of Debian) is also supported if you just want newer packages, and you can install it by passing an argument to build_rootfs.sh:

sudo ./build_rootfs.sh data/rootfs unstable

How can I install a desktop environment other than XFCE?

You can pass the desktop argument to the build_complete.sh script, like this:

sudo ./build_complete.sh grunt desktop=lxde

The valid values for this argument are: gnome, xfce, kde, lxde, gnome-flashback, cinnamon, mate, and lxqt.

Will this prevent me from using Chrome OS normally?

Shimboot does not touch the internal storage at all, so you will be able to use Chrome OS as if nothing happened. However, if you are on an enterprise enrolled device, booting Chrome OS again will force a powerwash due to the attempted switch into developer mode.

Can I unplug the USB drive while using Debian?

By default, this is not possible. However, you can simply copy your Debian rootfs onto your internal storage by first using fdisk to repartition it, using dd to copy the partition, and resize2fs to have it take up the entire drive. In the future, loading the OS to RAM may be supported, but this isn't a priority at the moment. You can also just blindly copy the contents of your Shimboot USB to the internal storage without bothering to repartition:

#assuming the usb drive is on sda and internal storage is on mmcblk1
sudo dd if=/dev/sda of=/dev/mmcblk1 bs=1M oflag=direct status=progress
sudo growpart /dev/mmcblk1 4
sudo resize2fs /dev/mmcblk1p4

GPU acceleration isn't working, how can I fix this?

If your kernel version is too old, the standard Mesa drivers will fail to load. Instead, you must download and install the mesa-amber drivers. Run the following commands:

sudo apt install libglx-amber0 libegl-amber0
echo "MESA_LOADER_DRIVER_OVERRIDE=i965" | sudo tee -a /etc/environment

You may need to change i965 to i915 (or r100/r200 for AMD hardware), depending on what GPU you have.

Can the rootfs be compressed to save space?

Compressing the Debian rootfs with a squashfs is supported, and you can do this by running the regular Debian rootfs through ./build_squashfs.sh. For example:

sudo ./build_rootfs.sh data/rootfs bookworm
sudo ./build_squashfs.sh data/rootfs_compressed data/rootfs path_to_shim
sudo ./build.sh image.bin path_to_shim data/rootfs_compressed

Any writes to the squashfs will persist, but they will not be compressed when saved. For the compression to be the most effective, consider pre-installing most of the software you use with custom_packages= before building the squashfs.

On the regular XFCE4 image, this brings the rootfs size down to 1.2GB from 3.5GB.

Copyright:

Shimboot is licensed under the GNU GPL v3. Unless otherwise indicated, all code has been written by me, ading2210.

Copyright Notice:

ading2210/shimboot: Boot desktop Linux from a Chrome OS RMA shim.
Copyright (C) 2023 ading2210

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.