CrOS security model

CrOS (Chromium OS/Chrome OS) devices such as Chromebooks implement a strict security model to ensure that these devices do not become compromised, that is implemented as the verified boot (vboot) reference, most of which is executed within depthcharge. A detailed overview of the CrOS security model is available on the dedicated page.

In spite of the CrOS security model, depthcharge won't allow booting kernels without verifying their signature and booting from external media or legacy payload unless explicitly allowed: see configuring verified boot parameters.

Developer mode screen

The developer mode screen can be accessed in depthcharge when developer mode is enabled. Developer mode can be enabled from the recovery mode screen.

It allows booting normally, booting from internal storage, booting from external media (when enabled), booting from legacy payload (when enabled), showing information about the device and disabling developer mode.

Holding the developer mode screen

As instructed on the developer mode screen, the screen can be held by pressing Ctrl + H in the first 3 seconds after the screen is shown. After that delay, depthcharge will resume booting normally.

Booting normally

As instructed on the developer mode screen, a regular boot will happen after 3 seconds (if developer mode screen is not held).

The default boot medium (internal storage, external media, legacy payload) is shown on screen.

Booting from different mediums

Depthcharge allows booting from different mediums, when they are allowed (see configuring verified boot parameters to enable or disable boot mediums).

As instructed on the developer mode screen, booting from various mediums can be triggered by pressing various key combinations:

Internal storage: Ctrl + D
External media: Ctrl + U (when enabled)
Legacy payload: Ctrl + L (when enabled)

Showing device information

As instructed on the developer mode screen, showing device information can be triggered by pressing Ctrl + I or Tab. Various information is shown, including vboot non-volatile data, TPM status, GBB flags and key hashes.

Warnings

The developer mode screen will show warnings when:

Booting kernels without verifying their signature is enabled
Booting from external media is enabled
Booting legacy payloads is enabled

Recovery mode screen

The recovery mode screen can be accessed in depthcharge, by pressing Escape + Refresh + Power when the device is off.

It allows recovering the device from a bad state by booting from a trusted recovery media. When accessed with the device in a good state, it also allows enabling developer mode.

Recovering from a bad state

When the device fails to verify the signature of a piece of the boot software or when an error occurs, it is considered to be in a bad state and will instruct the user to reboot to recovery mode.

Recovery mode boots using only software located in write-protected memory, that is considered to be trusted and safe.

Recovery mode then allows recovering the device by booting from a trusted recovery media, that is automatically detected when recovery mode starts. When no external media is found or when the recovery media is invalid, instructions are shown on screen.

Trusted recovery media are external media (USB drives, SD cards, etc) that hold a kernel signed with the recovery key.

Google provides images of such recovery media for Chrome OS (which are not advised to users as they contain proprietary software).

They are signed with Google's recovery keys, that are pre-installed on the device when it ships.

When replacing the full flash of the device, the pre-installed keys are replaced. When the recovery private key is available (e.g. when using self-generated keys), it can be used to sign a kernel for recovery purposes.

Enabling developer mode

As instructed on the recovery mode screen, developer mode can be enabled by pressing Ctrl + D. Instructions to confirm enabling developer mode are then shown on screen.

Configuring verified boot parameters

Depthcharge's behavior relies on the verified boot (vboot) reference implementation, that can be configured with parameters stored in the verified boot non-volatile storage.

These parameters can be modified with the crossystem tool, that requires sufficient privileges to access the verified boot non-volatile storage.

crossystem relies on mosys, that is used to access the verified boot non-volatile storage on some devices. crossystem and mosys are both free software and their source code is made available by Google: crossystem. mosys.

These tools are not distributed along with Libreboot yet. However, they are preinstalled on the device, with ChromeOS.

Some of these parameters have the potential of weakening the security of the device. In particular, disabling kernels signature verification, external media boot and legacy payload boot can weaken the security of the device.

The following parameters can be configured:

Kernels signature verification:

crossystem dev_boot_signed_only=1 # enable
crossystem dev_boot_signed_only=0 # disable

External media boot:

crossystem dev_boot_usb=1 # enable
crossystem dev_boot_usb=0 # disable

Legacy payload boot:

crossystem dev_boot_legacy=1 # enable 
crossystem dev_boot_legacy=0 # disable

Default boot medium:

crossystem dev_default_boot=disk # internal storage
crossystem dev_default_boot=usb # external media
crossystem dev_default_boot=legacy # legacy payload

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