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% Depthcharge payload
This section relates to the depthcharge payload used in libreboot.
- [CrOS security model](#cros_security_model)
- [Developer mode screen](#developer_mode_screen)
- Holding the developer mode screen
- Booting normally
- Booting from different mediums
- Showing device information
- Warnings
- [Recovery mode screen](#recovery_mode_screen)
- [Recovering from a bad state](#recovering_bad_state)
- [Enabling developer mode](#enabling_developer_mode)
- [Configuring verified boot
parameters](#configuring_verified_boot_parameters)
CrOS security model {#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](#configuring_verified_boot_parameters).
Developer mode screen {#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](#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 {#holding_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 {#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 {#booting_different_mediums}
------------------------------
Depthcharge allows booting from different mediums, when they are allowed
(see [configuring verified boot
parameters](#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 {#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 {#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 {#recovering_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 {#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 {#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](https://chromium.googlesource.com/chromiumos/platform/vboot_reference/).
[mosys](https://chromium.googlesource.com/chromiumos/platform/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:
- Enabled with:
\# **crossystem dev\_boot\_signed\_only=1**
- Disabled with:
\# **crossystem dev\_boot\_signed\_only=0**
- External media boot:
- Enabled with:
\# **crossystem dev\_boot\_usb=1**
- Disabled with:
\# **crossystem dev\_boot\_usb=0**
- Legacy payload boot:
- Enabled with:
\# **crossystem dev\_boot\_legacy=1**
- Disabled with:
\# **crossystem dev\_boot\_legacy=0**
- Default boot medium:
- Internal storage:
\# **crossystem dev\_default\_boot=disk**
- External media:
\# **crossystem dev\_default\_boot=usb**
- Legacy payload:
\# **crossystem dev\_default\_boot=legacy**
Copyright © 2015 Paul Kocialkowski <contact@paulk.fr>\
Permission is granted to copy, distribute and/or modify this document
under the terms of the Creative Commons Attribution-ShareAlike 4.0
International license or any later version published by Creative
Commons; A copy of the license can be found at
[../cc-by-sa-4.0.txt](../cc-by-sa-4.0.txt)
Updated versions of the license (when available) can be found at
<https://creativecommons.org/licenses/by-sa/4.0/legalcode>
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|
