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a632d8dd9f
This adds a ability to add alternative sections of a specific type in
the same UKI. The primary usecase is for supporting multiple different
kernel cmdlines that are baked into a UKI.
The mechanism is relatively simple (I think), in order to make it robust.
1. A new PE section ".profile" is introduced, that is a lot like
".osrel", but contains information about a specific "profile" to
boot. The ".profile" section can appear multiple times in the same
PE, and acts as delimiter indicating where a new profile starts.
Everything before the first ".profile" is called the "base profile",
and is shared among all other profiles, which can then override or
add addition PE sections on top.
2. An UKI's command line can be prefixed with an argument such as "@0" or
"@1" or "@2" which indicates the "profile" to boot. If no argument is
specified the default is profile 0. Also, a UKI that lacks any
.profile section is treated like one with only a profile 0, but with
no data in that profile section.
3. The stub will first search for its usual set of PE sections
(hereafter called "base sections"), and stop at the first .profile PE
section if any. It will then find the .profile matching the selected
profile by its index, and any sections found as part of that profile
on top of the base sections.
And that's already it.
Example: let's say a distro wants to provide a single UKI that can be
invoked in one of three ways:
1. The regular profile that just boots the system
2. A profile that boots into storagetm
3. A profile that initiates factory reset and reboots.
For this it would define a classic UKI with sections .linux, .initrd,
.cmdline, and whatever else it needs. The .cmdline section would contain
the kernel command line for the regular profile.
It would then insert one ".profile" section, with a contents like the
following:
ID=regular
This is the profile for profile 0. It would immediately afterwards add
another ".profile" section:
ID=storagetm
TITLE=Boot into Storage Target Mode
This would then followed with a .cmdline section that is just like the
basic one, but with "rd.systemd.unit=storage-target-mode.target"
suffixed. Then, another .profile section would be added:
ID=factory-reset
TITLE=Factory Reset
Which is then followed by one last PE section: a .cmdline one with
"systemd.unit=factory-reset.target" suffixed to te regular command line.
i.e. expressed in tabular form the above would be:
The base profile:
.linux
.initrd
.cmdline
.osrel
The regular boot profile:
.profile
The storagetm profile:
.profile
.cmdline
The factory reset profile:
.profile
.cmdline
You might wonder why the first .cmdline in the list above is placed in
the base profile rather than in the regular boot profile, given that it
is overriden in all other profiles anyway. And you are right. The only
reason I'd place it in the base profile is that it makes the UKI more
nicely extensible if later profiles are added that want to replace
something else instead of the .cmdline, for example .ucode or so. But it
really doesn't matter much.
While the primary usecase is of course multiple alternative command
lines, the concept is more powerful than that: for various usecases it
might be valuable to offer multiple choices of devicetree, ucode or
initrds.
The .profile contents is also passed to the invoked kernel as a file in
/.extra/profile (via a synthetic initrd). Thus, this functionality can
even be useful without overriding any section at all, simply by means of
reading that file from userspace.
Design choices:
1. On purposes I used a special command line marker (i.e. the "@" thing,
which maybe we should call the "profile selector"), that doesn't look
like a regular kernel command line option. This is because this is
really not a regular kernel command line option – we process it in
the stub, then remove it as prefix, and measure the unprefixed
command line only after that. The kernel will not see the profile
selector either. I think these special semantics are best
communicated by making it look substantially different from regular
options.
2. This moves around measurements a bit. Previously we measured our UKI
sections right after finding them. Now we first parse the profile
number from the command line, then search for the profile's sections,
and only then measure the sections we actually end up using for this
profile. I think that this logic makes most sense: measure what we
are using, not what we are overriding. Or in other words, if you boot
profile @3, then we'll measure .cmdline (assuming it exists) of
profile 3, and *not* measure .cmdline of the base profile. Also note
that if the user passes in a custom kernel command line via command
line arguments we'll strip off the profile selector (i.e. the initial
"@X" thing) before we pass it on.
3. The .profile stuff is supposed to be generic and extensible. For
example we could use it in future to mark "dangerous" options such as
factory reset, so that boot menus can ask for confirmation before
booting into it. Or we could introduce match expressions against
SMBIOS or other system identifiers, to filter out profiles on
specific hw.
Note btw, that PE allows defining multiple sections that point to the
same offsets in the file. This allows sharing payload under different
names. For example, if profile @4 and @7 shall carry the same .ucode
section, they can define .ucode in each profile and then make it point to
the same offset.
Also note that that one can even "mask" a base section in a profile, by
inserting an empty section. For example, if the base .dtb section should
not be used for profile @4, then add a section .dtb right after the
fourth .profile with a zero size to the UKI, and you will get your wish
fulfilled.
This code only contains changes to sd-stub. A follow-up commit will
teach sd-boot to also find this profile PE sections to synthesize
additional menu entries from a single UKI.
A later commit will add support for gnerating this via ukify.
Fixes: #24539
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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/4.5/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><simplelist>
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<member><filename>/usr/lib/systemd/boot/efi/linuxx64.efi.stub</filename></member>
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<member><filename>/usr/lib/systemd/boot/efi/linuxia32.efi.stub</filename></member>
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<member><filename>/usr/lib/systemd/boot/efi/linuxaa64.efi.stub</filename></member>
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<member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.addon.efi</filename></member>
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<member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.cred</filename></member>
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<member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.raw</filename></member>
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<member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.sysext.raw</filename></member>
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<member><filename><replaceable>ESP</replaceable>/.../<replaceable>foo</replaceable>.efi.extra.d/*.confext.raw</filename></member>
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<member><filename><replaceable>ESP</replaceable>/loader/addons/*.addon.efi</filename></member>
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<member><filename><replaceable>ESP</replaceable>/loader/credentials/*.cred</filename></member>
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</simplelist></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 the following PE sections:</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>A <literal>.linux</literal> section with the ELF Linux kernel
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image. (Required)</para></listitem>
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<listitem><para>An <literal>.osrel</literal> section with OS release information, i.e. the contents of
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the <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
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of the OS the kernel belongs to.</para></listitem>
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<listitem><para>A <literal>.cmdline</literal> section with the kernel command line to pass to the
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invoked kernel.</para></listitem>
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<listitem><para>An <literal>.initrd</literal> section with the initrd.</para></listitem>
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<listitem><para>A <literal>.ucode</literal> section with an initrd containing microcode, to be handed
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to the kernel before any other initrd. This initrd must not be compressed.</para></listitem>
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<listitem><para>A <literal>.splash</literal> section with an image (in the Windows
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<filename>.BMP</filename> format) to show on screen before invoking the kernel.</para></listitem>
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<listitem><para>A <literal>.dtb</literal> section with a compiled binary DeviceTree.</para></listitem>
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<listitem><para>A <literal>.uname</literal> section with the kernel version information, i.e. the
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output of <command>uname -r</command> for the kernel included in the <literal>.linux</literal>
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section.</para></listitem>
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<listitem><para>An <literal>.sbat</literal> section with
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<ulink url="https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT</ulink> revocation
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metadata.</para></listitem>
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<listitem><para>A <literal>.pcrsig</literal> section with a set of cryptographic signatures for the
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expected TPM2 PCR values after the kernel has been booted, in JSON format. This is useful for
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implementing TPM2 policies that bind disk encryption and similar to kernels that are signed by a
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specific key.</para></listitem>
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<listitem><para>A <literal>.pcrpkey</literal> section with a public key in the PEM format matching the
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signature data in the <literal>.pcrsig</literal> section.</para></listitem>
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</itemizedlist>
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<para>Generally, the sections above should appear at most once in a UKI. That said, a concept of
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"profiles" is defined, that allows multiple sets of these sections to exist in a single UKI file, of
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which one can be selected at boot. For this an additional PE section <literal>.profile</literal> is
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defined which can be used as separator between multiple sets of these settings. The
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<literal>.profile</literal> section itself may contain meta-information about the section, and follows a
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similar structure as the contents of the <literal>.osrel</literal> section. For further details about
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multi-profile UKIs, see below.</para> <para>If UEFI SecureBoot is enabled and the
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<literal>.cmdline</literal> section is present in the executed image, any attempts to override the kernel
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command line by passing one as invocation parameters to the EFI binary are ignored. Thus, in order to
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allow overriding the kernel command line, either disable UEFI SecureBoot, or don't include a kernel
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command line PE section in the kernel image file. If a command line is accepted via EFI invocation
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parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is present).</para> <para>If a
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DeviceTree is embedded in the <literal>.dtb</literal> section, it replaces an existing DeviceTree in the
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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> <para>The
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contents of 11 of these 12 sections are measured into TPM PCR 11. It is otherwise not used and thus the
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result can be pre-calculated without too much effort. The <literal>.pcrsig</literal> section is not
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included in this PCR measurement, since it is supposed to contain signatures for the output of the
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measurement operation, and thus cannot also be input to it. If an UKI contains multiple profiles, only
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the PE sections of the selected profile (and those of the base profile, except if overriden) are
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measured.</para>
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<para>If non-zero, the selected numeric profile is measured into PCR 12.</para>
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<para>When <literal>.pcrsig</literal> and/or <literal>.pcrpkey</literal> sections are present in a
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unified kernel image their contents are passed to the booted kernel in an synthetic initrd cpio archive
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that places them in the <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. If the kernel binary
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uses a counter for the purpose of
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<ulink url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot Assessment</ulink>, this
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counter will be ignored. For example, <filename><replaceable>foo</replaceable>+3-0.efi</filename>
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will look in directory <filename><replaceable>foo</replaceable>.efi.extra.d/</filename>.
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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
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<filename><replaceable>foo</replaceable>.efi.extra.d/*.sysext.raw</filename> are packed up in a
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<command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename> directory in the
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initrd file hierarchy. This is supposed to be used to pass additional system extension images to the
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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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<!-- Note: the actual suffix we look for for sysexts is just *.raw (not *.sysext.raw), for
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compatibility reasons with old versions. But we want people to name their system extensions
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properly, hence we document the *.sysext.raw suffix only. -->
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<listitem><para>Similarly, files
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<filename><replaceable>foo</replaceable>.efi.extra.d/*.confext.raw</filename> are packed up in a
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<command>cpio</command> archive and placed in the <filename>/.extra/confext/</filename> directory in
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the initrd file hierarchy. This is supposed to be used to pass additional configuration extension
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images to the initrd. See
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<citerefentry><refentrytitle>systemd-confext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
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details on configuration extension images. The generated <command>cpio</command> archive containing
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these system extension images is measured into TPM PCR 12 (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> are loaded and verified as
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PE binaries, and a <literal>.cmdline</literal> or <literal>.ucode</literal> section is parsed from them.
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Addons are supposed to be used to pass additional kernel command line parameters, or Devicetree blobs,
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and microcode updates, regardless of the kernel image being booted, for example to allow platform vendors
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to ship platform-specific configuration.</para>
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<para>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 both the addon and the UKI contain a
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<literal>.uname</literal> section, the addon will be rejected if they do not match exactly. 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 add
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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 documentation</ulink>.</para>
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<para>Addon files are sorted, loaded, and measured into TPM PCR 12 (if a TPM is present) and appended
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to the kernel command line. UKI command line options are listed first, then options from addons in
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<filename>/loader/addons/*.addon.efi</filename>, and finally UKI-specific addons. Device tree blobs are
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loaded and measured following the same algorithm. Microcode addons are passed to the kernel in inverse
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order (UKI specific addons, global addons, UKI embedded section). This is because the microcode update
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driver stops on the first matching filename. Addons are always loaded in the same order based on
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the filename, so that, given the same set of addons, the same set of measurements can be expected in
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PCR12. However, note that the filename is not protected by the PE signature, and as such an attacker
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with write access to the ESP could potentially rename these files to change the order in which they are
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loaded, in a way that could alter the functionality of the kernel, as some options might be
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order-dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of
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an attestation service so that improper use of your signed addons can be detected and dealt with using
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one of the aforementioned revocation 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>, <literal>.dtb</literal> and/or
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<literal>.ucode</literal>sections are parsed from them. This is supposed to be used to pass additional
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command line parameters, Devicetree blobs and microcode updates to the kernel, regardless of the
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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>Multi-Profile UKIs</title>
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<para>In many contexts it is useful to allow invocation of a single UKI in multiple different modes (or
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"profiles") without compromising the cryptographic integrity, measurements and so on of the boot
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process. For example, a single UKI might provide three distinct profiles: a regular boot one, one that
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invokes a "factory reset" operation, and one that boots into a storage target mode (as in
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<citerefentry><refentrytitle>systemd-storagetm.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>). Each
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profile would then use the same <literal>.linux</literal> and <literal>.initrd</literal> sections, but would
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have a separate <literal>.cmdline</literal> section. For example the latter two profiles would extend the
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regular kernel command line with <literal>systemd.unit=factory-reset.target</literal> or
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<literal>rd.systemd.unit=storagetm.target</literal>.</para>
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<para>A single UKI may support multiple profiles by means of the special <literal>.profile</literal> PE
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section. This section acts as separator between the PE sections of the individual
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profiles. <literal>.profile</literal> PE sections hence may appear multiple times in a single UKI, and
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the other PE sections listed above may appear multiple times too, if <literal>.profile</literal> are
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used, but only once before the first <literal>.profile</literal> section, once between each subsequent
|
|
pair, and once after the last appearance of <literal>.profile</literal>. The sections listed before the
|
|
first <literal>.profile</literal> are considered the "base" profile of the UKI. Each
|
|
<literal>.profile</literal> section then introduces a new profile, which are numbered starting from
|
|
zero. The PE sections following each <literal>.profile</literal> are specific to that profile. When
|
|
booting into a specific profile the base section's profiles are used in combination with the specific
|
|
profile's sections: if the same section is defined in both, the per-profile section overrides the base
|
|
profile's version, otherwise the per-profile sections is used together with the base profile
|
|
sections.</para> <para>A UKI that contains no <literal>.profile</literal> is consider equivalent to one
|
|
that just contains a single <literal>.profile</literal>, as having only a single profile @0.</para>
|
|
|
|
<para>Here's a simple example for a multi-profile UKI's sections, inspired by the setup suggested above:</para>
|
|
|
|
<table>
|
|
<title>Multi-Profile UKI Example</title>
|
|
|
|
<tgroup cols='2' align='left' colsep='1' rowsep='1'>
|
|
<colspec colname="section" />
|
|
<colspec colname="profile" />
|
|
|
|
<thead>
|
|
<row>
|
|
<entry>Section</entry>
|
|
<entry>Profile</entry>
|
|
</row>
|
|
</thead>
|
|
|
|
<tbody>
|
|
<row>
|
|
<entry><literal>.linux</literal></entry>
|
|
<entry morerows="3" valign="middle">Base profile</entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>.osrel</literal></entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>.cmdline</literal></entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>.initrd</literal></entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>.profile</literal></entry>
|
|
<entry>Profile @0</entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>.profile</literal></entry>
|
|
<entry morerows="1" valign="middle">Profile @1</entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>.cmdline</literal></entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>.profile</literal></entry>
|
|
<entry morerows="1" valign="middle">Profile @2</entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>.cmdline</literal></entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
<para>The section list above would define three profiles. The first four sections make up the base
|
|
profile. A <literal>.profile</literal> section then introduces profile @0. It doesn't override any
|
|
sections (or add any) from the base section, hence it is immediately followed by another
|
|
<literal>.profile</literal> section that then introduces section @1. This profile overrides the kernel
|
|
command line. Finally, the last two sections define section @2, again overriding the command line. (Note
|
|
that in this example the first <literal>.cmdline</literal> could also moved behind the first
|
|
<literal>.profile</literal> with equivalent effect. To keep things nicely extensible, it's probably a
|
|
good idea to keep the generic command line in the base section instead of profile 0, in case later added
|
|
profiles might want to reuse it.)</para>
|
|
|
|
<para>The profile to boot may be controlled via the UKI's own command line: if the first argument starts
|
|
with <literal>@</literal>, followed by a positive integer number in decimal, it selects the profile to
|
|
boot into. If the first argument is not specified like that, the UKI will automatically boot into profile
|
|
0.</para>
|
|
|
|
<para>A <literal>.profile</literal> section may contain meta-information about the profile. It follows a
|
|
similar format as <literal>.osrel</literal> (i.e. an environment-variable-assignment-block-like list of
|
|
newline separated strings). Currently two fields are defined: <literal>ID=</literal> is supposed to carry
|
|
a short identifying string that identifies the profile
|
|
(e.g. <literal>ID=factory-reset</literal>). <literal>TITLE=</literal> should contain a human readable
|
|
string that may appear in the boot menu entry for this profile (e.g. <literal>TITLE='Factory Reset this
|
|
Device'</literal>).</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), including all UKI
|
|
profiles.</para>
|
|
|
|
<para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR 9. This means
|
|
every type of initrd (of the selected UKI profile) will possibly be measured two or three times: the
|
|
initrds embedded in the kernel image will be measured to PCR 4, PCR 9 and PCR 11; the initrd synthesized
|
|
from credentials (and the one synthesized from configuration extensions) 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>Microcode 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>
|
|
|
|
<row>
|
|
<entry>Configuration Extensions (synthesized initrd from companion files)</entry>
|
|
<entry>9 + 12</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry>Selected profile unless zero</entry>
|
|
<entry>12</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 system 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>
|
|
|
|
<varlistentry>
|
|
<term><varname>StubPcrInitRDConfExts</varname></term>
|
|
|
|
<listitem><para>The PCR register index the configuration 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>12</literal>). This variable is set if a measurement was successfully completed, and remains
|
|
unset otherwise.</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v255"/></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>StubProfile</varname></term>
|
|
|
|
<listitem><para>The numeric index of the selected profile, without the <literal>@</literal>,
|
|
formatted as decimal string. Set both on single-profile and multi-profile UKIs. (In the former case
|
|
this variable will be set to <literal>0</literal> unconditionally.)</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v257"/></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/*.sysext.raw</filename></term>
|
|
<listitem><para>System extension image files (suffix <literal>.sysext.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/confext/*.confext.raw</filename></term>
|
|
<listitem><para>Configuration extension image files (suffix <literal>.confext.raw</literal>) that are
|
|
placed next to the unified kernel image (as described above) are copied into the
|
|
<filename>/.extra/confext/</filename> directory in the initrd execution environment.</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v255"/></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>
|
|
|
|
<varlistentry>
|
|
<term><filename>/.extra/profile</filename></term>
|
|
<term><filename>/.extra/os-release</filename></term>
|
|
<listitem><para>The contents of the <literal>.profile</literal> and <literal>.osrel</literal>
|
|
sections of the selected profile, if any.</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v257"/></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. Unless
|
|
<command>systemd-stub</command> detects it is running inside a confidential computing environment,
|
|
<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><simplelist type="inline">
|
|
<member><citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
|
|
<member><ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink></member>
|
|
<member><ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink></member>
|
|
<member><citerefentry><refentrytitle>ukify</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>systemd-measure</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
|
|
<member><ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink></member>
|
|
</simplelist></para>
|
|
</refsect1>
|
|
</refentry>
|