185 lines
6.7 KiB
Markdown
185 lines
6.7 KiB
Markdown
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---
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title: Depthcharge payload
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x-toc-enable: true
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...
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TODO: NUKE. depthcharge was dropped in recent releases.
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TODO: replace with uboot information. ARM has been dropped for now
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NOTE: this only applies to the older Libreboot 20160907 release
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TODO: NUKE! (this info is in the documentation provided by Libreboot 20160907)
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This section relates to the depthcharge payload used in libreboot.
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CrOS security model
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===================
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CrOS (Chromium OS/Chrome OS) devices such as Chromebooks implement a strict
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security model to ensure that these devices do not become compromised, that is
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implemented as the verified boot (vboot) reference, most of which is executed
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within depthcharge. A detailed overview of the CrOS security model is available
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on the dedicated page.
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In spite of the CrOS security model, depthcharge won't allow booting kernels
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without verifying their signature and booting from external media or legacy
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payload unless explicitly allowed: see [configuring verified boot
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parameters](#configuring_verified_boot_parameters).
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Developer mode screen
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=====================
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The developer mode screen can be accessed in depthcharge when developer mode is
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enabled. Developer mode can be enabled from the recovery mode screen.
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It allows booting normally, booting from internal storage, booting from
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external media (when enabled), booting from legacy payload (when enabled),
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showing information about the device and disabling developer mode.
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Holding the developer mode screen
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---------------------------------
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As instructed on the developer mode screen, the screen can be held by pressing
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*Ctrl + H* in the first 3 seconds after the screen is shown. After that delay,
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depthcharge will resume booting normally.
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Booting normally
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----------------
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As instructed on the developer mode screen, a regular boot will happen after *3
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seconds* (if developer mode screen is not held).
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The default boot medium (internal storage, external media, legacy payload) is
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shown on screen.
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Booting from different mediums
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------------------------------
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Depthcharge allows booting from different mediums, when they are allowed (see
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[configuring verified boot parameters](#configuring_verified_boot_parameters)
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to enable or disable boot mediums).
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As instructed on the developer mode screen, booting from various mediums can be
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triggered by pressing various key combinations:
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- Internal storage: *Ctrl + D*
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- External media: *Ctrl + U* (when enabled)
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- Legacy payload: *Ctrl + L* (when enabled)
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Showing device information
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--------------------------
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As instructed on the developer mode screen, showing device information can be
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triggered by pressing *Ctrl + I* or *Tab*. Various information is shown,
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including vboot non-volatile data, TPM status, GBB flags and key hashes.
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Warnings
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--------
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The developer mode screen will show warnings when:
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- Booting kernels without verifying their signature is enabled
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- Booting from external media is enabled
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- Booting legacy payloads is enabled
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Recovery mode screen
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====================
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The recovery mode screen can be accessed in depthcharge, by pressing *Escape +
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Refresh + Power* when the device is off.
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It allows recovering the device from a bad state by booting from a trusted
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recovery media. When accessed with the device in a good state, it also allows
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enabling developer mode.
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Recovering from a bad state
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---------------------------
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When the device fails to verify the signature of a piece of the boot software
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or when an error occurs, it is considered to be in a bad state and will
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instruct the user to reboot to recovery mode.
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Recovery mode boots using only software located in write-protected memory, that
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is considered to be trusted and safe.
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Recovery mode then allows recovering the device by booting from a trusted
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recovery media, that is automatically detected when recovery mode starts. When
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no external media is found or when the recovery media is invalid, instructions
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are shown on screen.
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Trusted recovery media are external media (USB drives, SD cards, etc) that hold
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a kernel signed with the recovery key.
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Google provides images of such recovery media for Chrome OS (which are not
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advised to users as they contain proprietary software).
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They are signed with Google's recovery keys, that are pre-installed on the
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device when it ships.
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When replacing the full flash of the device, the pre-installed keys are
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replaced. When the recovery private key is available (e.g. when using
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self-generated keys), it can be used to sign a kernel for recovery purposes.
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Enabling developer mode
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-----------------------
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As instructed on the recovery mode screen, developer mode can be enabled by
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pressing *Ctrl + D*. Instructions to confirm enabling developer mode are then
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shown on screen.
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Configuring verified boot parameters
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====================================
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Depthcharge's behavior relies on the verified boot (vboot) reference
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implementation, that can be configured with parameters stored in the verified
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boot non-volatile storage.
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These parameters can be modified with the `crossystem` tool, that requires
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sufficient privileges to access the verified boot non-volatile storage.
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`crossystem` relies on `mosys`, that is used to access the verified boot
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non-volatile storage on some devices. `crossystem` and `mosys` are both free
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software and their source code is made available by Google:
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[crossystem](https://chromium.googlesource.com/chromiumos/platform/vboot_reference/).
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[mosys](https://chromium.googlesource.com/chromiumos/platform/mosys/).
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These tools are not distributed along with Libreboot yet. However, they are
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preinstalled on the device, with ChromeOS.
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Some of these parameters have the potential of *weakening the security of the
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device*. In particular, disabling kernels signature verification, external
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media boot and legacy payload boot can weaken the security of the device.
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The following parameters can be configured:
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Kernels signature verification:
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crossystem dev_boot_signed_only=1 # enable
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crossystem dev_boot_signed_only=0 # disable
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External media boot:
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crossystem dev_boot_usb=1 # enable
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crossystem dev_boot_usb=0 # disable
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Legacy payload boot:
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crossystem dev_boot_legacy=1 # enable
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crossystem dev_boot_legacy=0 # disable
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Default boot medium:
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crossystem dev_default_boot=disk # internal storage
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crossystem dev_default_boot=usb # external media
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crossystem dev_default_boot=legacy # legacy payload
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Copyright © 2015 Paul Kocialkowski <contact@paulk.fr>\
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License Version 1.3 or any later
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version published by the Free Software Foundation
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with no Invariant Sections, no Front Cover Texts, and no Back Cover Texts.
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A copy of this license is found in [../fdl-1.3.md](../fdl-1.3.md)
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