┌───────────────────────────────┐
│ Chrome OS RMA Shim Bootloader │
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1) Info
2) Explanation
3) Prebuilt Images
4) Build Instructions
5) Usage
6) Copyright
For more detailed information, please see the project's README.
Info:
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.
Explanation:
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 in which 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, however, as it boots to 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 does a pivot_root to enter the rootfs, where it runs 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.
Prebuilt Images:
Prebuilt images are available here. If there is not a prebuilt image for your board, you must manually build the shimboot image.
For these images, the name of the default user is "user" and its password is "user" as well. You should change these credentials as soon as possible.
Build Instructions:
1. Find the board name of your Chromebook. You can search for the model name on chrome100.dev.
1. Clone this repository and cd into it.
2. Run `sudo ./build_complete.sh [board_name]` to download the required data and build the disk image.
See the README for full instructions.
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 expand_rootfs`.
7. Change your own password by running `passwd user`. The root user is disabled by default.
Copyright:
Shimboot is licensed under the GNU GPL v3. Unless otherwise indicated, all code has been written by me, ading2210.
The source code for this project is available here: https://github.com/ading2210/shimboot
Credit for fixing the original bug in systemd goes to @r58playz.
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/>.