emily
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libreboot


			
				
					
						
						
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	<title>Depthcharge payload</title>
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<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="../index.html">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 &copy;  2015 Paul Kocialkowski &lt;contact@paulk.fr&gt;<br/>
			Permission is granted to copy, distribute and/or modify this document
			under the terms of the GNU Free Documentation License, Version 1.3
			or any later version published by the Free Software Foundation;
			with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
			A copy of the license can be found at <a href="../gfdl-1.3.txt">../gfdl-1.3.txt</a>
		</p>

		<p>
			Updated versions of the license (when available) can be found at
			<a href="https://www.gnu.org/licenses/licenses.html">https://www.gnu.org/licenses/licenses.html</a>
		</p>

		<p>
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