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minor fixes, improve readme

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ading2210 2023-10-21 05:06:52 -04:00
parent df47933f4c
commit a53331a422
3 changed files with 33 additions and 13 deletions
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39
README.md
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@ -1,13 +1,13 @@
# Chrome OS RMA Shim Bootloader
This is a set of scripts for patching a Chrome OS RMA shim to serve as a bootloader for a standard Linux disto.
This is a set of scripts for patching a Chrome OS RMA shim to serve as a bootloader for a standard Linux distribution.
## About:
Chrome OS RMA shims are disk images which are bootable from 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 verfied. This lets us replace the rootfs with anything we want, including a full Linux distribution.
Chrome OS RMA shims are disk images which are bootable from Chromebooks which are designed to run a variety of diagnostic utilities, and they'll work even if the device is enterprise enrolled. Unfortunately for Google, there exists a [security flaw](https://sh1mmer.me/) 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 seperate bootloader is required to transition from the shim environment to the main rootfs. This bootloader then does `pivot_root` to enter the rootfs, where it then starts the init system.
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 does `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 workaroudn is to [apply a patch](https://github.com/ading2210/chromeos-systemd) to systemd, and then it can be recompiled and hosted at a [repo somewhere](https://shimboot.ading.dev/debian/).
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](https://github.com/ading2210/chromeos-systemd) to systemd, and then it can be recompiled and hosted at a [repo somewhere](https://shimboot.ading.dev/debian/).
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.
@ -19,7 +19,24 @@ After copying all the firmware from the recovery image and shim to the rootfs, w
Note that rootfs partitions have to be named `shimboot_rootfs:<partname>` for the bootloader to recognize them.
## Current Development Roadmap:
## Status:
### What Works:
- Systmed
- X11
- XFCE
- Backlight
- Touchscreen
- 3D acceleration
- Bluetooth
- Wifi (partially)
- Suspend (partially)
### What Doesn't Work:
- Audio
- Zram
### Development Roadmap:
- ~~build the image automatically~~
- ~~boot to a shell~~
- ~~switch_root into an actual rootfs~~
@ -52,12 +69,12 @@ Note that rootfs partitions have to be named `shimboot_rootfs:<partname>` for th
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](https://chromiumdash.appspot.com/serving-builds?deviceCategory=ChromeOS) for your board.
3. Clone this repository and cd into it.
4. Run `mkdir -p data/rootfs` to make a directory for 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`.
8. Flash the generated image to a USB drive or SD card.
9. Plug the USB into your Chromebook and enter recovery mode. It should detect the USB and enter the shimboot bootloader.
4. Run `sudo ./build_rootfs.sh data/rootfs bookworm` to build the base rootfs.
5. Run `sudo ./patch_rootfs.sh path_to_shim path_to_reco data/rootfs` to patch the base rootfs and add any needed drivers.
6. Run `sudo ./build.sh image.bin path_to_shim data/rootfs` to generate a disk image at `image.bin`.
7. Flash the generated image to a USB drive or SD card.
8. Enable developer mode on your Chromebook. Even if it's enrolled and dev mode is blocked, it'll still work for running shimboot.
9. Plug the USB into your Chromebook and enter recovery mode. It should detect the USB and run the shimboot bootloader.
Note that these instructions are currently incomplete.

2
build_rootfs.sh
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@ -41,7 +41,7 @@ fi
rootfs_dir=$(realpath "${1}")
release_name="${2}"
debootstrap $release_name $rootfs_dir http://deb.debian.org/debian/
debootstrap --arch amd64 $release_name $rootfs_dir http://deb.debian.org/debian/
cp -ar rootfs/* $rootfs_dir
chroot_mounts="proc sys dev run"

5
patch_rootfs.sh
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@ -28,7 +28,10 @@ copy_modules() {
local reco_rootfs=$(realpath $2)
local target_rootfs=$(realpath $3)
cp -r "${shim_rootfs}/lib/modules/"* "${target_rootfs}/lib/modules/"
rm -rf "${target_rootfs}/lib/modules"
cp -r "${shim_rootfs}/lib/modules" "${target_rootfs}/lib/modules"
mkdir -p "${target_rootfs}/lib/firmware"
cp -r --remove-destination "${shim_rootfs}/lib/firmware/"* "${target_rootfs}/lib/firmware/"
cp -r --remove-destination "${reco_rootfs}/lib/firmware/"* "${target_rootfs}/lib/firmware/"