Chromebook flashing instructions

Return to index

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. Canoeboot 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://version in ChromeOS and check the “Firmware Version”
Run crossystem hwid or crossystem fwid in 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 GNU+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 Canoeboot 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 Canoeboot 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 cbmk support yet for automatically building it.

Prepare the ROM image

Canoeboot 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 Canoeboot 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 canoeboot.rom
sudo flashrom -p host -v canoeboot.rom

If you can already boot a conventional GNU+Linux distro on your Chromebook, you may be able to use flashrom -p linux_mtd on that system instead.

Install an operating system (experimental research)

In general, ARM-compatible distros targeting U-boot can be used. There are three general methods for installing that vary depending on the distribution:

EFI - common u-boot methodology used by both arm64 and amd64 systems.
boot.scr - an older u-boot specific script used by some distributions.
extlinux.conf - a newer flat, bootloader-spec text file that typically lives in /boot/extlinux/extlinux.conf