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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<style type="text/css">
@import url('../css/main.css');
</style>
<title>Depthcharge payload</title>
</head>
<body>
<div class="section">
<h1 id="pagetop">Depthcharge payload</h1>
<p>
This section relates to the depthcharge payload used in libreboot.
</p>
<p>
Or <a href="../">Back to main index</a>.
</p>
<ul>
<li><a href="#cros_security_model">CrOS security model</a></li>
<li><a href="#developer_mode_screen">Developer mode screen</a>
<ul>
<li><a href="#holding_developer_mode_screen">Holding the developer mode screen</li>
<li><a href="#booting_normally">Booting normally</li>
<li><a href="#booting_different_mediums">Booting from different mediums</li>
<li><a href="#showing_device_information">Showing device information</li>
<li><a href="#warnings">Warnings</li>
</ul>
</li>
<li><a href="#recovery_mode_screen">Recovery mode screen</a>
<ul>
<li><a href="#recovering_bad_state">Recovering from a bad state</a></li>
<li><a href="#enabling_developer_mode">Enabling developer mode</a></li>
</ul>
</li>
<li><a href="#configuring_verified_boot_parameters">Configuring verified boot parameters</a></li>
</ul>
</div>
<div class="section">
<h1 id="cros_security_model">CrOS security model</h1>
<p>
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.
</p>
<div class="subsection">
<p>
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 <a href="#configuring_verified_boot_parameters">configuring verified boot parameters</a>.
</p>
</div>
</div>
<div class="section">
<h1 id="developer_mode_screen">Developer mode screen</h1>
<p>
The developer mode screen can be accessed in depthcharge when developer mode is enabled.<br />
Developer mode can be enabled from the <a href="#recovery_mode_screen">recovery mode screen</a>.
</p>
<p>
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.
</p>
<div class="subsection">
<h2 id="holding_developer_mode_screen">Holding the developer mode screen</h2>
<p>
As instructed on the developer mode screen, the screen can be held by pressing <b>Ctrl + H</b> in the first 3 seconds after the screen is shown.
After that delay, depthcharge will resume booting normally.
</p>
</div>
<div class="subsection">
<h2 id="booting_normally">Booting normally</h2>
<p>
As instructed on the developer mode screen, a regular boot will happen after <b>3 seconds</b> (if developer mode screen is not held).<br />
The default boot medium (internal storage, external media, legacy payload) is shown on screen.
</p>
</div>
<div class="subsection">
<h2 id="booting_different_mediums">Booting from different mediums</h2>
<p>
Depthcharge allows booting from different mediums, when they are allowed (see <a href="#configuring_verified_boot_parameters">configuring verified boot parameters</a> to enable or disable boot mediums).<br />
As instructed on the developer mode screen, booting from various mediums can be triggered by pressing various key combinations:
</p>
<ul>
<li>Internal storage: <b>Ctrl + D</b></li>
<li>External media: <b>Ctrl + U</b> (when enabled)</li>
<li>Legacy payload: <b>Ctrl + L</b> (when enabled)</li>
</ul>
</div>
<div class="subsection">
<h2 id="showing_device_information">Showing device information</h2>
<p>
As instructed on the developer mode screen, showing device information can be triggered by pressing <b>Ctrl + I</b> or <b>Tab</b>.<br />
Various information is shown, including vboot non-volatile data, TPM status, GBB flags and key hashes.<br />
</p>
</div>
<div class="subsection">
<h2 id="warnings">Warnings</h2>
<p>
The developer mode screen will show warnings when:
<ul>
<li>Booting kernels without verifying their signature is enabled</li>
<li>Booting from external media is enabled</li>
<li>Booting legacy payloads is enabled</li>
</ul>
</p>
</div>
</div>
<div class="section">
<h1 id="recovery_mode_screen">Recovery mode screen</h1>
<p>
The recovery mode screen can be accessed in depthcharge, by pressing <b>Escape + Refresh + Power</b> when the device is off.
</p>
<p>
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.
</p>
<div class="subsection">
<h2 id="recovering_bad_state">Recovering from a bad state</h2>
<p>
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.<br />
Recovery mode boots using only software located in write-protected memory, that is considered to be trusted and safe.
</p>
<p>
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. <br />
Trusted recovery media are external media (USB drives, SD cards, etc) that hold a kernel signed with the recovery key.
</p>
<p>
Google provides images of such recovery media for Chrome OS (which are not advised to users as they contain proprietary software). <br />
They are signed with Google's recovery keys, that are pre-installed on the device when it ships.
</p>
<p>
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.
</p>
</div>
<div class="subsection">
<h2 id="enabling_developer_mode">Enabling developer mode</h2>
<p>
As instructed on the recovery mode screen, developer mode can be enabled by pressing <b>Ctrl + D</b>.<br />
Instructions to confirm enabling developer mode are then shown on screen.
</p>
</div>
</div>
<div class="section">
<h1 id="configuring_verified_boot_parameters">Configuring verified boot parameters</h1>
<p>
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.<br />
These parameters can be modified with the <b>crossystem</b> tool, that requires sufficient privileges to access the verified boot non-volatile storage.
</p>
<p>
<b>crossystem</b> relies on <b>mosys</b>, that is used to access the verified boot non-volatile storage on some devices.
<b>crossystem</b> and <b>mosys</b> are both free software and their source code is made available by Google: <a href="https://chromium.googlesource.com/chromiumos/platform/vboot_reference/">crossystem</a>. <a href="https://chromium.googlesource.com/chromiumos/platform/mosys/">mosys</a>.<br />
These tools are not distributed along with Libreboot yet. However, they are preinstalled on the device, with ChromeOS.
</p>
<p>
Some of these parameters have the potential of <b>weakening the security of the device</b>.
In particular, disabling kernels signature verification, external media boot and legacy payload boot can weaken the security of the device.
</p>
<div class="subsection">
<p>
The following parameters can be configured:
</p>
<ul>
<li>
Kernels signature verification:
<ul>
<li>
Enabled with:<br />
# <b>crossystem dev_boot_signed_only=1</b>
</li>
<li>
Disabled with:<br />
# <b>crossystem dev_boot_signed_only=0</b>
</li>
</ul>
</li>
<li>
External media boot:
<ul>
<li>
Enabled with:<br />
# <b>crossystem dev_boot_usb=1</b>
</li>
<li>
Disabled with:<br />
# <b>crossystem dev_boot_usb=0</b>
</li>
</ul>
</li>
<li>
Legacy payload boot:
<ul>
<li>
Enabled with:<br />
# <b>crossystem dev_boot_legacy=1</b>
</li>
<li>
Disabled with:<br />
# <b>crossystem dev_boot_legacy=0</b>
</li>
</ul>
</li>
<li>
Default boot medium:
<ul>
<li>
Internal storage:<br />
# <b>crossystem dev_default_boot=disk</b>
</li>
<li>
External media:<br />
# <b>crossystem dev_default_boot=usb</b>
</li>
<li>
Legacy payload:<br />
# <b>crossystem dev_default_boot=legacy</b>
</li>
</ul>
</ul>
</div>
</div>
<div class="section">
<p>
Copyright © 2015 Paul Kocialkowski <contact@paulk.fr><br/>
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 <a href="../cc-by-sa-4.0.txt">../cc-by-sa-4.0.txt</a>
</p>
<p>
Updated versions of the license (when available) can be found at
<a href="https://creativecommons.org/licenses/by-sa/4.0/legalcode">https://creativecommons.org/licenses/by-sa/4.0/legalcode</a>
</p>
<p>
UNLESS OTHERWISE SEPARATELY UNDERTAKEN BY THE LICENSOR, TO THE
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ACCURACY, OR THE PRESENCE OR ABSENCE OF ERRORS, WHETHER OR NOT
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<p>
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</p>
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