chromebooks.md 7.9 KB
title: Chromebook flashing instructions x-toc-enable: true ...
This page attempts to give a brief, general overview of how to flash custom firmware on ChromeOS devices. This guide usually refers to all of them as "Chromebook"s since it's the most common form factor.
Flashrom
A special fork of flashrom, maintained by Google, is required for flashing these Chromebook devices. See:
https://chromium.googlesource.com/chromiumos/third_party/flashrom/
You must then compile this from source, and run it.
Enable ChromeOS "Developer Mode"
Chromebooks are locked-down by default to only run ChromeOS. Most things
you will want to do on these require you unlock it by enabling their
Developer Mode.
On most devices, you would press the Escape + Refresh + Power key
combination to restart into the Recovery Mode, then press Ctrl + D and
finally confirm enabling Developer Mode with Enter.
On your next boot, it will show you an "OS Verification is disabled"
screen. Waiting for 30 seconds or pressing Ctrl + D on this screen will
proceed to boot into ChromeOS, which then erases all data on the device
and reboots again into a clean ChromeOS installation.
With Developer Mode enabled, you can launch a terminal emulator inside
ChromeOS by pressing the Ctrl + Alt + T key combination. Run shell
inside the resulting crosh prompt to actually get to a bash session
where you can run programs. Most of the root file system is read-only,
except for /usr/local and any mounted drives under /media/removable.
Identify your device
It's more common to refer to ChromeOS boards by their codenames, and many compatible devices can share a single codename. Libreboot ROM images also use these, you should only use the one corresponding to your device's. There are a number of ways to find it, some are:
- Check the "Model" on the Recovery Mode or Developer Mode screens
- Visit
chrome://versionin ChromeOS and check the "Firmware Version" - Run
crossystem hwidorcrossystem fwidin a terminal
Back up stock firmware
The stock firmware on your device comes with some irreplaceable data that is unique to your device. This can include the serial number and hardware ID of the device, network MAC address, HDCP keys, maybe more. The stock firmware is also the only one that will properly boot and run ChromeOS.
Make sure you back up the original firmware before trying to replace it.
The version of flashrom in ChromeOS understands host as a programmer
to flash firmware internally. To back up stock firmware you can run:
sudo flashrom -p host -r depthcharge.rom
sudo flashrom -p host -v depthcharge.rom
Keep the resulting depthcharge.rom file safe and properly backed up on
another device.
If you can already boot a conventional Linux distro on your Chromebook,
you may be able to use flashrom -p linux_mtd on that system instead.
Check external flashability
If a ROM image you flash is broken, you may need to restore the stock
firmware to fix the board to get internal flashing working. Refer to the
external flashing guide, and check that the result of
flashrom -r matches what you get when you run it from the device.
Chromebooks may have 1.8V as the supply voltage for the SPI NOR chip, be
extra careful about that.
On newer Chromebooks, there is a root-of-trust chip providing a Closed Case Debugging mechanism that lets you flash externally using a special USB debugging cable. However, most boards that Libreboot supports do not have this.
Disable write protection
Chromebooks have the SPI flash chip partially write-protected by default, but thankfully this protection can be disabled by the device owner. How to do so depends on the board, refer to the ChromiumOS documentation on Write Protection for more info. You will usually need to do this only once for the board's lifetime, unless you manually enable it again.
On most boards that Libreboot supports, write-protection is enforced by a physical screw. When screwed in, it forms an electrical connection that asserts the WP pin on the flash chip. The screw can be identified by the fact that it bridges electrical contacts, but finding and removing it might require you to disassemble most of the board.
Newer boards have a root-of-trust chip enforcing write-protection. The Closed Case Debugging mechanism should be used to disable hardware write-protection. Opening the case and disconnecting the battery might also disable it.
Disabling the write-protect signal doesn't directly make the chip stop protecting its data, it just allows you to disable its write-protection in software. That also needs to be done in ChromeOS afterwards:
sudo flashrom -p host --wp-status
sudo flashrom -p host --wp-disable
sudo flashrom -p host --wp-range 0x0,0x0
The --wp arguments are only available on the
ChromiumOS fork of flashrom.
If you are using another OS or an external flasher, you may need to
compile and use that flashrom fork to disable write-protection. There is
no lbmk support yet for automatically building it.
Prepare the ROM image
Libreboot ROM image layouts are currently incompatible with the regions that should be carried over from the stock firmware. However, the released images should still be somewhat usable, since the Chromebooks supported so far don't require any non-redistributable blobs to be injected by the end user.
Future Libreboot versions will likely require post-processing to preserve irreplaceable data in the firmware image. For now, make sure to keep backups of the original firmware.
TODO: Instructions to preserve vital data when FMAPs are compatible.
Flash the ROM image
WARNING: Although none are supported yet, make sure not to flash ROM images on x86 Chromebooks without injecting non-redistributable blobs first (like Intel ME firmware). This is not yet documented here.
You can flash the ROM image both internally and externally. For the latter, see the external flashing guide and the ChromiumOS Closed Case Debugging documentation if your board supports it.
To flash the entire ROM image internally, run within ChromeOS:
sudo flashrom -p host -w libreboot.rom
sudo flashrom -p host -v libreboot.rom
If you can already boot a conventional Linux distro on your Chromebook,
you may be able to use flashrom -p linux_mtd on that system instead.
Install an operating system
Libreboot currently lacks any good documentation about this, but there is this page which refers to Debian:
In general, ARM-compatible distros targeting U-boot can be used. You can follow their instructions, once you have Libreboot up and running.