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diff --git a/man/systemd-stub.xml b/man/systemd-stub.xml new file mode 100644 index 0000000..6e85333 --- /dev/null +++ b/man/systemd-stub.xml @@ -0,0 +1,498 @@ +<?xml version='1.0'?> <!--*-nxml-*--> +<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" + "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> +<!-- SPDX-License-Identifier: LGPL-2.1-or-later --> + +<refentry id="systemd-stub" conditional='ENABLE_BOOTLOADER' + xmlns:xi="http://www.w3.org/2001/XInclude"> + <refentryinfo> + <title>systemd-stub</title> + <productname>systemd</productname> + </refentryinfo> + + <refmeta> + <refentrytitle>systemd-stub</refentrytitle> + <manvolnum>7</manvolnum> + </refmeta> + + <refnamediv> + <refname>systemd-stub</refname> + <refname>sd-stub</refname> + <refname>linuxx64.efi.stub</refname> + <refname>linuxia32.efi.stub</refname> + <refname>linuxaa64.efi.stub</refname> + <refpurpose>A simple UEFI kernel boot stub</refpurpose> + </refnamediv> + + <refsynopsisdiv> + <para><filename>/usr/lib/systemd/boot/efi/linuxx64.efi.stub</filename></para> + <para><filename>/usr/lib/systemd/boot/efi/linuxia32.efi.stub</filename></para> + <para><filename>/usr/lib/systemd/boot/efi/linuxaa64.efi.stub</filename></para> + <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename></para> + <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.cred</filename></para> + <para><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.raw</filename></para> + <para><filename><replaceable>ESP</replaceable>/loader/addons/*.addon.efi</filename></para> + <para><filename><replaceable>ESP</replaceable>/loader/credentials/*.cred</filename></para> + </refsynopsisdiv> + + <refsect1> + <title>Description</title> + + <para><command>systemd-stub</command> (stored in per-architecture files + <filename>linuxx64.efi.stub</filename>, <filename>linuxia32.efi.stub</filename>, + <filename>linuxaa64.efi.stub</filename> on disk) is a simple UEFI boot stub. An UEFI boot stub is + attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware + environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux + kernel is executable as regular UEFI binary, and is able to do various preparations before switching the + system into the Linux world.</para> + + <para>The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary + itself. This allows combining various resources inside a single PE binary image (usually called "Unified + Kernel Image", or "UKI" for short), which may then be signed via UEFI SecureBoot as a whole, covering all + individual resources at once. Specifically it may include:</para> + + <itemizedlist> + <!-- Let's keep this in the canonical order we also measure the sections by, i.e. as in + src/fundamental/uki.h's UnifiedSection enum --> + + <listitem><para>A <literal>.linux</literal> section with the ELF Linux kernel image.</para></listitem> + + <listitem><para>An <literal>.osrel</literal> section with OS release information, i.e. the contents of + the <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file + of the OS the kernel belongs to.</para></listitem> + + <listitem><para>A <literal>.cmdline</literal> section with the kernel command line to pass to the + invoked kernel.</para></listitem> + + <listitem><para>An <literal>.initrd</literal> section with the initrd.</para></listitem> + + <listitem><para>A <literal>.splash</literal> section with an image (in the Windows + <filename>.BMP</filename> format) to show on screen before invoking the kernel.</para></listitem> + + <listitem><para>A <literal>.dtb</literal> section with a compiled binary DeviceTree.</para></listitem> + + <listitem><para>A <literal>.uname</literal> section with the kernel version information, i.e. the + output of <command>uname -r</command> for the kernel included in the <literal>.linux</literal> + section.</para></listitem> + + <listitem><para>An <literal>.sbat</literal> section with + <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT</ulink> revocation + metadata.</para></listitem> + + <listitem><para>A <literal>.pcrsig</literal> section with a set of cryptographic signatures for the + expected TPM2 PCR values after the kernel has been booted, in JSON format. This is useful for + implementing TPM2 policies that bind disk encryption and similar to kernels that are signed by a + specific key.</para></listitem> + + <listitem><para>A <literal>.pcrpkey</literal> section with a public key in the PEM format matching the + signature data in the the <literal>.pcrsig</literal> section.</para></listitem> + </itemizedlist> + + <para>If UEFI SecureBoot is enabled and the <literal>.cmdline</literal> section is present in the executed + image, any attempts to override the kernel command line by passing one as invocation parameters to the + EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI + SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line + is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is + present).</para> + + <para>If a DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing + DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the + <literal>EFI_DT_FIXUP_PROTOCOL</literal> for hardware specific fixups to the DeviceTree.</para> + + <para>The contents of eight of these nine sections are measured into TPM PCR 11. It is otherwise not used + and thus the result can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section + is not included in this PCR measurement, since it is supposed to contain signatures for the output of the + measurement operation, and thus cannot also be input to it.</para> + + <para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> sections are present in a + unified kernel image their contents are passed to the booted kernel in an synthetic initrd cpio archive + that places them in the <filename>/.extra/tpm2-pcr-signature.json</filename> and + <filename>/.extra/tpm2-pcr-public-key.pem</filename> files. Typically, a + <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then + ensures they are copied into <filename>/run/systemd/tpm2-pcr-signature.json</filename> and + <filename>/run/systemd/tpm2-pcr-public-key.pem</filename> where they remain accessible even after the + system transitions out of the initrd environment into the host file system. Tools such + <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>, + <citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry> + and <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> + will automatically use files present under these paths to unlock protected resources (encrypted storage + or credentials) or bind encryption to booted kernels.</para> + + <para>For further details about the UKI concept, see the <ulink + url="https://uapi-group.org/specifications/specs/unified_kernel_image/">UKI specification</ulink>.</para> + </refsect1> + + <refsect1> + <title>Companion Files</title> + + <para>The <command>systemd-stub</command> UEFI boot stub automatically collects three types of auxiliary + companion files optionally placed in drop-in directories on the same partition as the EFI binary, + dynamically generates <command>cpio</command> initrd archives from them, and passes them to the kernel. + Specifically:</para> + + <itemizedlist> + <listitem><para>For a kernel binary called <filename><replaceable>foo</replaceable>.efi</filename>, it + will look for files with the <filename>.cred</filename> suffix in a directory named + <filename><replaceable>foo</replaceable>.efi.extra.d/</filename> next to it. If the kernel binary + uses a counter for the purpose of + <ulink url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot Assessment</ulink>, this + counter will be ignored. For example, <filename><replaceable>foo</replaceable>+3-0.efi</filename> + will look in directory <filename><replaceable>foo</replaceable>.efi.extra.d/</filename>. + A <command>cpio</command> + archive is generated from all files found that way, placing them in the + <filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may + then access them in this directory. This is supposed to be used to store auxiliary, encrypted, + authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System + Partition. See + <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> + and + <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> + for + details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM + PCR 12 (if a TPM is present).</para></listitem> + + <listitem><para>Similarly, files <filename><replaceable>foo</replaceable>.efi.extra.d/*.raw</filename> + are packed up in a <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename> + directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension + images to the initrd. See + <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for + details on system extension images. The generated <command>cpio</command> archive containing these + system extension images is measured into TPM PCR 13 (if a TPM is present).</para></listitem> + + <listitem><para>Similarly, files + <filename><replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename> are loaded and verified as + PE binaries, and a <literal>.cmdline</literal> section is parsed from them. Addons are supposed to be + used to pass additional kernel command line parameters or Devicetree blobs, regardless of the kernel + image being booted, for example to allow platform vendors to ship platform-specific + configuration.</para> + + <para>In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or + Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a a + <literal>.uname</literal> section, the addon will be rejected if they do not match exactly. It is + recommended to always add a <literal>.sbat</literal> section to all signed addons, so that they may be + revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The + <citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will add + a SBAT policy by default if none is passed when building addons. For more information on SBAT see + <ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">Shim documentation</ulink>.</para> + + <para>Addon files are sorted, loaded, and measured into TPM PCR 12 (if a TPM is present) and appended + to the kernel command line. UKI command line options are listed first, then options from addons in + <filename>/loader/addons/*.addon.efi</filename>, and finally UKI-specific addons. Device tree blobs are + loaded and measured following the same algorithm. Addons are always loaded in the same order based on + the filename, so that, given the same set of addons, the same set of measurements can be expected in + PCR12. However, note that the filename is not protected by the PE signature, and as such an attacker + with write access to the ESP could potentially rename these files to change the order in which they are + loaded, in a way that could alter the functionality of the kernel, as some options might be + order-dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of + an attestation service so that improper use of your signed addons can be detected and dealt with using + one of the aforementioned revocation mechanisms.</para></listitem> + + <listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a + <command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename> + directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to + the initrd, regardless of the kernel being booted. The generated <command>cpio</command> archive is + measured into TPM PCR 12 (if a TPM is present).</para></listitem> + + <listitem><para>Additionally, files <filename>/loader/addons/*.addon.efi</filename> are loaded and + verified as PE binaries, and <literal>.cmdline</literal> and/or <literal>.dtb</literal> sections are + parsed from them. This is supposed to be used to pass additional command line parameters or Devicetree + blobs to the kernel, regardless of the kernel being booted.</para></listitem> + </itemizedlist> + + <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd + images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be + further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed + by <command>systemd-creds encrypt -T</command> (see + <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for + details); in case of the system extension images by using signed Verity images.</para> + </refsect1> + + <refsect1> + <title>TPM PCR Notes</title> + + <para>Note that when a unified kernel using <command>systemd-stub</command> is invoked the firmware will + measure it as a whole to TPM PCR 4, covering all embedded resources, such as the stub code itself, the + core kernel, the embedded initrd and kernel command line (see above for a full list).</para> + + <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means + every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be + measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized from credentials will be measured to both PCR + 9 and PCR 12; the initrd synthesized from system extensions will be measured to both PCR 4 and PCR + 9. Let's summarize the OS resources and the PCRs they are measured to:</para> + + <table> + <title>OS Resource PCR Summary</title> + + <tgroup cols='2' align='left' colsep='1' rowsep='1'> + <colspec colname="pcr" /> + <colspec colname="definition" /> + + <thead> + <row> + <entry>OS Resource</entry> + <entry>Measurement PCR</entry> + </row> + </thead> + + <tbody> + <row> + <entry><command>systemd-stub</command> code (the entry point of the unified PE binary)</entry> + <entry>4</entry> + </row> + + <row> + <entry>Core kernel code (embedded in unified PE binary)</entry> + <entry>4 + 11</entry> + </row> + + <row> + <entry>OS release information (embedded in the unified PE binary)</entry> + <entry>4 + 11</entry> + </row> + + <row> + <entry>Main initrd (embedded in unified PE binary)</entry> + <entry>4 + 9 + 11</entry> + </row> + + <row> + <entry>Default kernel command line (embedded in unified PE binary)</entry> + <entry>4 + 11</entry> + </row> + + <row> + <entry>Overridden kernel command line</entry> + <entry>12</entry> + </row> + + <row> + <entry>Boot splash (embedded in the unified PE binary)</entry> + <entry>4 + 11</entry> + </row> + + <row> + <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)</entry> + <entry>4 + 9</entry> + </row> + + <row> + <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)</entry> + <entry>4 + 9 + 11</entry> + </row> + + <row> + <entry>Credentials (synthesized initrd from companion files)</entry> + <entry>9 + 12</entry> + </row> + + <row> + <entry>System Extensions (synthesized initrd from companion files)</entry> + <entry>9 + 13</entry> + </row> + </tbody> + </tgroup> + </table> + </refsect1> + + <refsect1> + <title>EFI Variables</title> + + <para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the + vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot + stub and the OS:</para> + + <variablelist class='efi-variables'> + <varlistentry> + <term><varname>LoaderDevicePartUUID</varname></term> + + <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run + from. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry> + uses this information to automatically find the disk booted from, in order to discover various other + partitions on the same disk automatically.</para> + + <xi:include href="version-info.xml" xpointer="v250"/></listitem> + </varlistentry> + + <varlistentry> + <term><varname>LoaderFirmwareInfo</varname></term> + <term><varname>LoaderFirmwareType</varname></term> + + <listitem><para>Brief firmware information. Use + <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this + data.</para> + + <xi:include href="version-info.xml" xpointer="v250"/></listitem> + </varlistentry> + + <varlistentry> + <term><varname>LoaderImageIdentifier</varname></term> + + <listitem><para>The path of EFI executable, relative to the EFI System Partition's root + directory. Use + <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view + this data.</para> + + <xi:include href="version-info.xml" xpointer="v250"/></listitem> + </varlistentry> + + <varlistentry> + <term><varname>StubInfo</varname></term> + + <listitem><para>Brief stub information. Use + <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view + this data.</para> + + <xi:include href="version-info.xml" xpointer="v250"/></listitem> + </varlistentry> + + <varlistentry> + <term><varname>StubPcrKernelImage</varname></term> + + <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree + database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g. + <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains + unset otherwise.</para> + + <xi:include href="version-info.xml" xpointer="v252"/></listitem> + </varlistentry> + + <varlistentry> + <term><varname>StubPcrKernelParameters</varname></term> + + <listitem><para>The PCR register index the kernel command line and credentials are measured into, + formatted as decimal ASCII string (e.g. <literal>12</literal>). This variable is set if a measurement + was successfully completed, and remains unset otherwise.</para> + + <xi:include href="version-info.xml" xpointer="v252"/></listitem> + </varlistentry> + + <varlistentry> + <term><varname>StubPcrInitRDSysExts</varname></term> + + <listitem><para>The PCR register index the systemd extensions for the initrd, which are picked up + from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g. + <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains + unset otherwise.</para> + + <xi:include href="version-info.xml" xpointer="v252"/></listitem> + </varlistentry> + </variablelist> + + <para>Note that some of the variables above may also be set by the boot loader. The stub will only set + them if they aren't set already. Some of these variables are defined by the <ulink + url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para> + </refsect1> + + <refsect1> + <title>initrd Resources</title> + + <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up + 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>Similarly, 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> |