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494 lines
26 KiB
XML
494 lines
26 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd-stub" conditional='ENABLE_BOOTLOADER'
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>systemd-stub</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-stub</refentrytitle>
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<manvolnum>7</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-stub</refname>
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<refname>sd-stub</refname>
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<refname>linuxx64.efi.stub</refname>
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<refname>linuxia32.efi.stub</refname>
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<refname>linuxaa64.efi.stub</refname>
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<refpurpose>A simple UEFI kernel boot stub</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>/usr/lib/systemd/boot/efi/linuxx64.efi.stub</filename></para>
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<para><filename>/usr/lib/systemd/boot/efi/linuxia32.efi.stub</filename></para>
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<para><filename>/usr/lib/systemd/boot/efi/linuxaa64.efi.stub</filename></para>
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<para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename></para>
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<para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.cred</filename></para>
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<para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.raw</filename></para>
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<para><filename><replaceable>ESP</replaceable>/loader/addons/*.addon.efi</filename></para>
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<para><filename><replaceable>ESP</replaceable>/loader/credentials/*.cred</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><command>systemd-stub</command> (stored in per-architecture files
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<filename>linuxx64.efi.stub</filename>, <filename>linuxia32.efi.stub</filename>,
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<filename>linuxaa64.efi.stub</filename> on disk) is a simple UEFI boot stub. An UEFI boot stub is
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attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware
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environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux
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kernel is executable as regular UEFI binary, and is able to do various preparations before switching the
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system into the Linux world.</para>
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<para>The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary
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itself. This allows combining various resources inside a single PE binary image (usually called "Unified
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Kernel Image", or "UKI" for short), which may then be signed via UEFI SecureBoot as a whole, covering all
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individual resources at once. Specifically it may include:</para>
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<itemizedlist>
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<!-- Let's keep this in the canonical order we also measure the sections by, i.e. as in
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src/fundamental/uki.h's UnifiedSection enum -->
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<listitem><para>The ELF Linux kernel images will be looked for in the <literal>.linux</literal> PE
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section of the executed image.</para></listitem>
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<listitem><para>OS release information, i.e. the
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<citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file of
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the OS the kernel belongs to, in the <literal>.osrel</literal> PE section.</para></listitem>
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<listitem><para>The kernel command line to pass to the invoked kernel will be looked for in the
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<literal>.cmdline</literal> PE section.</para></listitem>
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<listitem><para>The initrd will be loaded from the <literal>.initrd</literal> PE
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section.</para></listitem>
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<listitem><para>A boot splash (in Windows <filename>.BMP</filename> format) to show on screen before
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invoking the kernel will be looked for in the <literal>.splash</literal> PE section.</para></listitem>
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<listitem><para>A compiled binary DeviceTree will be looked for in the <literal>.dtb</literal> PE
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section.</para></listitem>
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<listitem><para>Kernel version information, i.e. the output of <command>uname -r</command> for the
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kernel included in the UKI, in the <literal>.uname</literal> PE section.</para></listitem>
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<listitem><para><ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT</ulink> revocation
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metadata, in the <literal>.sbat</literal> PE section.</para></listitem>
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<listitem><para>A set of cryptographic signatures for expected TPM2 PCR values when this kernel is
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booted, in JSON format, in the <literal>.pcrsig</literal> section. This is useful for implementing TPM2
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policies that bind disk encryption and similar to kernels that are signed by a specific
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key.</para></listitem>
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<listitem><para>A public key in PEM format matching this TPM2 PCR signature data in the
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<literal>.pcrpkey</literal> section.</para></listitem>
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</itemizedlist>
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<para>If UEFI SecureBoot is enabled and the <literal>.cmdline</literal> section is present in the executed
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image, any attempts to override the kernel command line by passing one as invocation parameters to the
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EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
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SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
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is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is
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present).</para>
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<para>If a DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing
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DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
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<literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para>
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<para>The contents of seven of these eight PE sections are measured into TPM PCR 11, that is otherwise
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not used. Thus, it can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section
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is not included in this PCR measurement, since it's supposed to contain signatures for the expected
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results for these measurements, i.e. of the outputs of the measurement operation, and thus cannot also be
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input to it.</para>
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<para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> are present in a unified kernel
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image their contents are passed to the booted kernel in an synthetic initrd cpio archive that places them in the
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<filename>/.extra/tpm2-pcr-signature.json</filename> and
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<filename>/.extra/tpm2-pcr-public-key.pem</filename> files. Typically, a
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<citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
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ensures they are copied into <filename>/run/systemd/tpm2-pcr-signature.json</filename> and
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<filename>/run/systemd/tpm2-pcr-public-key.pem</filename> where they remain accessible even after the
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system transitions out of the initrd environment into the host file system. Tools such
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<citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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and <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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will automatically use files present under these paths to unlock protected resources (encrypted storage
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or credentials) or bind encryption to booted kernels.</para>
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<para>For further details about the UKI concept, see the <ulink
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url="https://uapi-group.org/specifications/specs/unified_kernel_image/">UKI specification</ulink>.</para>
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</refsect1>
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<refsect1>
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<title>Companion Files</title>
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<para>The <command>systemd-stub</command> UEFI boot stub automatically collects three types of auxiliary
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companion files optionally placed in drop-in directories on the same partition as the EFI binary,
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dynamically generates <command>cpio</command> initrd archives from them, and passes them to the kernel.
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Specifically:</para>
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<itemizedlist>
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<listitem><para>For a kernel binary called <filename><replaceable>foo</replaceable>.efi</filename>, it
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will look for files with the <filename>.cred</filename> suffix in a directory named
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<filename><replaceable>foo</replaceable>.efi.extra.d/</filename> next to it. A <command>cpio</command>
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archive is generated from all files found that way, placing them in the
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<filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may
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then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
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authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System
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Partition. See
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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and
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<citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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for
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details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM
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PCR 12 (if a TPM is present).</para></listitem>
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<listitem><para>Similarly, files <filename><replaceable>foo</replaceable>.efi.extra.d/*.raw</filename>
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are packed up in a <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename>
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directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension
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images to the initrd. See
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<citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
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details on system extension images. The generated <command>cpio</command> archive containing these
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system extension images is measured into TPM PCR 13 (if a TPM is present).</para></listitem>
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<listitem><para>Similarly, files
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<filename><replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename>
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are loaded and verified as PE binaries, and a <literal>.cmdline</literal> section is parsed from them.
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In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
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Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a
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a <literal>.uname</literal> section, the addon will be rejected if they do not exactly match. It is
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recommended to always add a <literal>.sbat</literal> section to all signed addons, so that they may be
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revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
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<citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will
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add a SBAT policy by default if none is passed when building addons. For more information on SBAT see
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<ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">Shim's documentation</ulink>.
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Addons are supposed to be used to pass additional kernel command line parameters or Devicetree blobs,
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regardless of the kernel image being booted, for example to allow platform vendors to ship
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platform-specific configuration. The loaded command line addon files are sorted, loaded, and measured
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into TPM PCR 12 (if a TPM is present) and appended to the kernel command line. UKI command line options
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are listed first, then options from addons in <filename>/loader/addons/*.addon.efi</filename>, and
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finally UKI-specific addons. Device tree blobs are loaded and measured following the same algorithm.
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Addons are always loaded in the same order based on the filename, so that, given the same set of
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addons, the same set of measurements can be expected in PCR12. However, note that the filename is not
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protected by the PE signature, and as such an attacker with write access to the ESP could potentially
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rename these files to change the order in which they are loaded, in a way that could alter the
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functionality of the kernel, as some options might be order dependent. If you sign such addons, you
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should pay attention to the PCR12 values and make use of an attestation service so that improper use
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of your signed addons can be detected and dealt with using one of the aforementioned revocation
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mechanisms.</para></listitem>
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<listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
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<command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
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directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
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the initrd, regardless of the kernel being booted. The generated <command>cpio</command> archive is
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measured into TPM PCR 12 (if a TPM is present).</para></listitem>
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<listitem><para>Additionally, files <filename>/loader/addons/*.addon.efi</filename> are loaded and
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verified as PE binaries, and <literal>.cmdline</literal> and/or <literal>.dtb</literal> sections are
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parsed from them. This is supposed to be used to pass additional command line parameters or Devicetree
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blobs to the kernel, regardless of the kernel being booted.</para></listitem>
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</itemizedlist>
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<para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
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images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
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further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
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by <command>systemd-creds encrypt -T</command> (see
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<citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
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details); in case of the system extension images by using signed Verity images.</para>
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</refsect1>
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<refsect1>
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<title>TPM PCR Notes</title>
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<para>Note that when a unified kernel using <command>systemd-stub</command> is invoked the firmware will
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measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the
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core kernel, the embedded initrd and kernel command line (see above for a full list).</para>
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<para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
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every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
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measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials will be measured to both PCR
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9 and PCR 12; the initrd synthesized from system extensions will be measured to both PCR 4 and PCR
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9. Let's summarize the OS resources and the PCRs they are measured to:</para>
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<table>
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<title>OS Resource PCR Summary</title>
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<tgroup cols='2' align='left' colsep='1' rowsep='1'>
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<colspec colname="pcr" />
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<colspec colname="definition" />
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<thead>
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<row>
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<entry>OS Resource</entry>
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<entry>Measurement PCR</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><command>systemd-stub</command> code (the entry point of the unified PE binary)</entry>
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<entry>4</entry>
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</row>
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<row>
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<entry>Core kernel code (embedded in unified PE binary)</entry>
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<entry>4 + 11</entry>
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</row>
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<row>
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<entry>OS release information (embedded in the unified PE binary)</entry>
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<entry>4 + 11</entry>
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</row>
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<row>
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<entry>Main initrd (embedded in unified PE binary)</entry>
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<entry>4 + 9 + 11</entry>
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</row>
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<row>
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<entry>Default kernel command line (embedded in unified PE binary)</entry>
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<entry>4 + 11</entry>
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</row>
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<row>
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<entry>Overridden kernel command line</entry>
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<entry>12</entry>
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</row>
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<row>
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<entry>Boot splash (embedded in the unified PE binary)</entry>
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<entry>4 + 11</entry>
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</row>
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<row>
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<entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry>
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<entry>4 + 9</entry>
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</row>
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<row>
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<entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry>
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<entry>4 + 9 + 11</entry>
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</row>
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<row>
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<entry>Credentials (synthesized initrd from companion files)</entry>
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<entry>9 + 12</entry>
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</row>
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<row>
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<entry>System Extensions (synthesized initrd from companion files)</entry>
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<entry>9 + 13</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</refsect1>
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<refsect1>
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<title>EFI Variables</title>
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<para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
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vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
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stub and the OS:</para>
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<variablelist class='efi-variables'>
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<varlistentry>
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<term><varname>LoaderDevicePartUUID</varname></term>
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<listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
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from. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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uses this information to automatically find the disk booted from, in order to discover various other
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partitions on the same disk automatically.</para>
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<xi:include href="version-info.xml" xpointer="v250"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>LoaderFirmwareInfo</varname></term>
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<term><varname>LoaderFirmwareType</varname></term>
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<listitem><para>Brief firmware information. Use
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<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
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data.</para>
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<xi:include href="version-info.xml" xpointer="v250"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>LoaderImageIdentifier</varname></term>
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<listitem><para>The path of EFI executable, relative to the EFI System Partition's root
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directory. Use
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<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
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this data.</para>
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<xi:include href="version-info.xml" xpointer="v250"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>StubInfo</varname></term>
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<listitem><para>Brief stub information. Use
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<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
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this data.</para>
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<xi:include href="version-info.xml" xpointer="v250"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>StubPcrKernelImage</varname></term>
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<listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
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database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
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<literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
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unset otherwise.</para>
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<xi:include href="version-info.xml" xpointer="v252"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>StubPcrKernelParameters</varname></term>
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<listitem><para>The PCR register index the kernel command line and credentials are measured into,
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formatted as decimal ASCII string (e.g. <literal>12</literal>). This variable is set if a measurement
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was successfully completed, and remains unset otherwise.</para>
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<xi:include href="version-info.xml" xpointer="v252"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>StubPcrInitRDSysExts</varname></term>
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<listitem><para>The PCR register index the systemd extensions for the initrd, which are picked up
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from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
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<literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
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unset otherwise.</para>
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<xi:include href="version-info.xml" xpointer="v252"/></listitem>
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</varlistentry>
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</variablelist>
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<para>Note that some of the variables above may also be set by the boot loader. The stub will only set
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them if they aren't set already. Some of these variables are defined by the <ulink
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url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
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</refsect1>
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<refsect1>
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<title>initrd Resources</title>
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<para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
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the initial file system hierarchy in the initrd execution environment:</para>
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term><filename>/</filename></term>
|
|
|
|
<listitem><para>The main initrd from the <literal>.initrd</literal> PE section of the unified kernel image.</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v252"/></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><filename>/.extra/credentials/*.cred</filename></term>
|
|
<listitem><para>Credential files (suffix <literal>.cred</literal>) that are placed next to the
|
|
unified kernel image (as described above) are copied into the
|
|
<filename>/.extra/credentials/</filename> directory in the initrd execution
|
|
environment.</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v252"/></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><filename>/.extra/global_credentials/*.cred</filename></term>
|
|
<listitem><para>Similar, credential files in the <filename>/loader/credentials/</filename> directory
|
|
in the file system the unified kernel image is placed in are copied into the
|
|
<filename>/.extra/global_credentials/</filename> directory in the initrd execution
|
|
environment.</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v252"/></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><filename>/.extra/sysext/*.raw</filename></term>
|
|
<listitem><para>System extension image files (suffix <literal>.raw</literal>) that are placed next to
|
|
the unified kernel image (as described above) are copied into the
|
|
<filename>/.extra/sysext/</filename> directory in the initrd execution environment.</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v252"/></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><filename>/.extra/tpm2-pcr-signature.json</filename></term>
|
|
<listitem><para>The TPM2 PCR signature JSON object included in the <literal>.pcrsig</literal> PE
|
|
section of the unified kernel image is copied into the
|
|
<filename>/.extra/tpm2-pcr-signature.json</filename> file in the initrd execution
|
|
environment.</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v252"/></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><filename>/.extra/tpm2-pcr-pkey.pem</filename></term>
|
|
<listitem><para>The PEM public key included in the <literal>.pcrpkey</literal> PE section of the
|
|
unified kernel image is copied into the <filename>/.extra/tpm2-pcr-public-key.pem</filename> file in
|
|
the initrd execution environment.</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v252"/></listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
|
|
<para>Note that all these files are located in the <literal>tmpfs</literal> file system the kernel sets
|
|
up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
|
|
environment into the host file system. If these resources shall be kept around over this transition they
|
|
need to be copied to a place that survives the transition first, for example via a suitable
|
|
<citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
|
|
default, this is done for the TPM2 PCR signature and public key files.</para>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>SMBIOS Type 11 Strings</title>
|
|
|
|
<para><command>systemd-stub</command> can be configured using SMBIOS Type 11 strings. Applicable strings
|
|
consist of a name, followed by <literal>=</literal>, followed by the value.
|
|
<command>systemd-stub</command> will search the table for a string with a specific name, and if found,
|
|
use its value. The following strings are read:</para>
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term><varname>io.systemd.stub.kernel-cmdline-extra</varname></term>
|
|
<listitem><para>If set, the value of this string is added to the list of kernel command line
|
|
arguments that are measured in PCR12 and passed to the kernel.</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v254"/></listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>Assembling Kernel Images</title>
|
|
|
|
<para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
|
|
<citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>See Also</title>
|
|
<para>
|
|
<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
|
|
<ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink>,
|
|
<ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
|
|
<citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
|
|
<ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink>
|
|
</para>
|
|
</refsect1>
|
|
</refentry>
|