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<?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 -->
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<refentry id= "systemd-stub" conditional= 'HAVE_GNU_EFI'
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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>
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<refname > sd-stub</refname>
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<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>
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<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/credentials/*.cred</filename> </para>
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</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
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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>
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<itemizedlist >
<listitem > <para > The ELF Linux kernel images will be looked for in the <literal > .linux</literal> PE
section of the executed image.</para> </listitem>
efi: from the stub measure the ELF kernel + built-in initrd and so on into PCR 11
Here we grab a new – on Linux so far unused (by my Googling skills, that
is) – and measure all static components of the PE kernel image into.
This is useful since for the first time we'll have a PCR that contains
only a PCR of the booted kernel, nothing else. That allows putting
together TPM policies that bind to a specific kernel (+ builtin initrd),
without having to have booted that kernel first. PCRs can be
pre-calculated. Yay!
You might wonder, why we measure just the discovered PE sections we are
about to use, instead of the whole PE image. That's because of the next
step I have in mind: PE images should also be able to carry an
additional section that contains a signature for its own expected,
pre-calculated PCR values. This signature data should then be passed
into the booted kernel and can be used there in TPM policies. Benefit:
TPM policies can now be bound to *signatures* of PCRs, instead of the
raw hash values themselves. This makes update management a *lot* easier,
as policies don't need to be updated whenever a kernel is updated, as
long as the signature is available. Now, if the PCR signature is
embedded in the kernel PE image it cannot be of a PCR hash of the kernel
PE image itself, because that would be a chicken-and-egg problem. Hence,
by only measuring the relavent payload sections (and that means
excluding the future section that will contain the PCR hash signature)
we avoid this problem, naturally.
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<listitem > <para > OS release information, i.e. the
<citerefentry > <refentrytitle > os-release</refentrytitle> <manvolnum > 5</manvolnum> </citerefentry> file of
the OS the kernel belongs to, in the <literal > .osrel</literal> PE section.</para> </listitem>
man: "the initial RAM disk" → "the initrd"
In many places we spelled out the phrase behind "initrd" in full, but this
isn't terribly useful. In fact, no "RAM disk" is used, so emphasizing this
is just confusing to the reader. Let's just say "initrd" everywhere, people
understand what this refers to, and that it's in fact an initramfs image.
Also, s/i.e./e.g./ where appropriate.
Also, don't say "in RAM", when in fact it's virtual memory, whose pages
may or may not be loaded in page frames in RAM, and we have no control over
this.
Also, add <filename></filename> and other minor cleanups.
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<listitem > <para > The initrd will be loaded from the <literal > .initrd</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
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
<literal > .cmdline</literal> PE section.</para> </listitem>
<listitem > <para > A boot splash (in Windows <filename > .BMP</filename> format) to show on screen before
invoking the kernel will be looked for in the <literal > .splash</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
booted, in JSON format, in the <literal > .pcrsig</literal> section. 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 public key in PEM format matching this TPM2 PCR signature data in the
<literal > .pcrpkey</literal> section.</para> </listitem>
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</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
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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
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>
efi: from the stub measure the ELF kernel + built-in initrd and so on into PCR 11
Here we grab a new – on Linux so far unused (by my Googling skills, that
is) – and measure all static components of the PE kernel image into.
This is useful since for the first time we'll have a PCR that contains
only a PCR of the booted kernel, nothing else. That allows putting
together TPM policies that bind to a specific kernel (+ builtin initrd),
without having to have booted that kernel first. PCRs can be
pre-calculated. Yay!
You might wonder, why we measure just the discovered PE sections we are
about to use, instead of the whole PE image. That's because of the next
step I have in mind: PE images should also be able to carry an
additional section that contains a signature for its own expected,
pre-calculated PCR values. This signature data should then be passed
into the booted kernel and can be used there in TPM policies. Benefit:
TPM policies can now be bound to *signatures* of PCRs, instead of the
raw hash values themselves. This makes update management a *lot* easier,
as policies don't need to be updated whenever a kernel is updated, as
long as the signature is available. Now, if the PCR signature is
embedded in the kernel PE image it cannot be of a PCR hash of the kernel
PE image itself, because that would be a chicken-and-egg problem. Hence,
by only measuring the relavent payload sections (and that means
excluding the future section that will contain the PCR hash signature)
we avoid this problem, naturally.
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<para > The contents of seven of these eight PE sections are measured into TPM PCR 11, that is otherwise
not used. Thus, it can be pre-calculated without too much effort. The <literal > .pcrsig</literal> section
is not included in this PCR measurement, since it's supposed to contain signatures for the expected
results for these measurements, i.e. of the outputs of the measurement operation, and thus cannot also be
input to it.</para>
<para > When <literal > .pcrsig</literal> and/or <literal > .pcrpkey</literal> 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>
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</refsect1>
<refsect1 >
<title > Companion Files</title>
<para > The <command > systemd-stub</command> UEFI boot stub automatically collects two types of auxiliary
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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>
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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
<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
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<citerefentry > <refentrytitle > systemd.exec</refentrytitle> <manvolnum > 5</manvolnum> </citerefentry>
and
<citerefentry > <refentrytitle > systemd-creds</refentrytitle> <manvolnum > 1</manvolnum> </citerefentry>
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>
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
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
sd-stub: measure sysext images picked up by sd-stub into PCR 13
Let's grab another so far unused PCR, and measure all sysext images into
it that we load from the ESP. Note that this is possibly partly redundant,
since sysext images should have dm-verity enabled, and that is hooked up
to IMA. However, measuring this explicitly has the benefit that we can
measure filenames too, easily, and that all without need for IMA or
anything like that.
This means: when booting a unified sd-stub kernel through sd-boot we'll
now have:
1. PCR 11: unified kernel image payload (i.e. kernel, initrd, boot
splash, dtb, osrelease)
2. PCR 12: kernel command line (i.e. the one embedded in the image, plus
optionally an overriden one) + any credential files picked up by
sd-stub
3. PCR 13: sysext images picked up by sd-stub
And each of these three PCRs should carry just the above, and start from
zero, thus be pre-calculatable.
Thus, all components and parameters of the OS boot process (i.e.
everything after the boot loader) is now nicely pre-calculable.
NOTE: this actually replaces previous measuring of the syext images into
PCR 4. I added this back in 845707aae23b3129db635604edb95c4048a5922a,
following the train of thought, that sysext images for the initrd should
be measured like the initrd itself they are for, and according to my
thinking that would be a unified kernel which is measured by firmware
into PCR 4 like any other UEFI executables.
However, I think we should depart from that idea. First and foremost
that makes it harder to pre-calculate PCR 4 (since we actually measured
quite incompatible records to the TPM event log), but also I think
there's great value in being able to write policies that bind to the
used sysexts independently of the earlier boot chain (i.e. shim, boot
loader, unified kernel), hence a separate PCR makes more sense.
Strictly speaking, this is a compatibility break, but I think one we can
get away with, simply because the initrd sysext images are currently not
picked up by systemd-sysext yet in the initrd, and because of that we
can be reasonably sure noone uses this yet, and hence relies on the PCR
register used. Hence, let's clean this up before people actually do
start relying on this.
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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 > 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
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measured into TPM PCR 12 (if a TPM is present)</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
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>
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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
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
efi: from the stub measure the ELF kernel + built-in initrd and so on into PCR 11
Here we grab a new – on Linux so far unused (by my Googling skills, that
is) – and measure all static components of the PE kernel image into.
This is useful since for the first time we'll have a PCR that contains
only a PCR of the booted kernel, nothing else. That allows putting
together TPM policies that bind to a specific kernel (+ builtin initrd),
without having to have booted that kernel first. PCRs can be
pre-calculated. Yay!
You might wonder, why we measure just the discovered PE sections we are
about to use, instead of the whole PE image. That's because of the next
step I have in mind: PE images should also be able to carry an
additional section that contains a signature for its own expected,
pre-calculated PCR values. This signature data should then be passed
into the booted kernel and can be used there in TPM policies. Benefit:
TPM policies can now be bound to *signatures* of PCRs, instead of the
raw hash values themselves. This makes update management a *lot* easier,
as policies don't need to be updated whenever a kernel is updated, as
long as the signature is available. Now, if the PCR signature is
embedded in the kernel PE image it cannot be of a PCR hash of the kernel
PE image itself, because that would be a chicken-and-egg problem. Hence,
by only measuring the relavent payload sections (and that means
excluding the future section that will contain the PCR hash signature)
we avoid this problem, naturally.
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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>
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<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 >
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<entry > Core kernel code (embedded in unified PE binary)</entry>
efi: from the stub measure the ELF kernel + built-in initrd and so on into PCR 11
Here we grab a new – on Linux so far unused (by my Googling skills, that
is) – and measure all static components of the PE kernel image into.
This is useful since for the first time we'll have a PCR that contains
only a PCR of the booted kernel, nothing else. That allows putting
together TPM policies that bind to a specific kernel (+ builtin initrd),
without having to have booted that kernel first. PCRs can be
pre-calculated. Yay!
You might wonder, why we measure just the discovered PE sections we are
about to use, instead of the whole PE image. That's because of the next
step I have in mind: PE images should also be able to carry an
additional section that contains a signature for its own expected,
pre-calculated PCR values. This signature data should then be passed
into the booted kernel and can be used there in TPM policies. Benefit:
TPM policies can now be bound to *signatures* of PCRs, instead of the
raw hash values themselves. This makes update management a *lot* easier,
as policies don't need to be updated whenever a kernel is updated, as
long as the signature is available. Now, if the PCR signature is
embedded in the kernel PE image it cannot be of a PCR hash of the kernel
PE image itself, because that would be a chicken-and-egg problem. Hence,
by only measuring the relavent payload sections (and that means
excluding the future section that will contain the PCR hash signature)
we avoid this problem, naturally.
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<entry > 4 + 11</entry>
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</row>
<row >
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<entry > OS release information (embedded in the unified PE binary)</entry>
efi: from the stub measure the ELF kernel + built-in initrd and so on into PCR 11
Here we grab a new – on Linux so far unused (by my Googling skills, that
is) – and measure all static components of the PE kernel image into.
This is useful since for the first time we'll have a PCR that contains
only a PCR of the booted kernel, nothing else. That allows putting
together TPM policies that bind to a specific kernel (+ builtin initrd),
without having to have booted that kernel first. PCRs can be
pre-calculated. Yay!
You might wonder, why we measure just the discovered PE sections we are
about to use, instead of the whole PE image. That's because of the next
step I have in mind: PE images should also be able to carry an
additional section that contains a signature for its own expected,
pre-calculated PCR values. This signature data should then be passed
into the booted kernel and can be used there in TPM policies. Benefit:
TPM policies can now be bound to *signatures* of PCRs, instead of the
raw hash values themselves. This makes update management a *lot* easier,
as policies don't need to be updated whenever a kernel is updated, as
long as the signature is available. Now, if the PCR signature is
embedded in the kernel PE image it cannot be of a PCR hash of the kernel
PE image itself, because that would be a chicken-and-egg problem. Hence,
by only measuring the relavent payload sections (and that means
excluding the future section that will contain the PCR hash signature)
we avoid this problem, naturally.
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<entry > 4 + 11</entry>
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</row>
<row >
<entry > Main initrd (embedded in unified PE binary)</entry>
efi: from the stub measure the ELF kernel + built-in initrd and so on into PCR 11
Here we grab a new – on Linux so far unused (by my Googling skills, that
is) – and measure all static components of the PE kernel image into.
This is useful since for the first time we'll have a PCR that contains
only a PCR of the booted kernel, nothing else. That allows putting
together TPM policies that bind to a specific kernel (+ builtin initrd),
without having to have booted that kernel first. PCRs can be
pre-calculated. Yay!
You might wonder, why we measure just the discovered PE sections we are
about to use, instead of the whole PE image. That's because of the next
step I have in mind: PE images should also be able to carry an
additional section that contains a signature for its own expected,
pre-calculated PCR values. This signature data should then be passed
into the booted kernel and can be used there in TPM policies. Benefit:
TPM policies can now be bound to *signatures* of PCRs, instead of the
raw hash values themselves. This makes update management a *lot* easier,
as policies don't need to be updated whenever a kernel is updated, as
long as the signature is available. Now, if the PCR signature is
embedded in the kernel PE image it cannot be of a PCR hash of the kernel
PE image itself, because that would be a chicken-and-egg problem. Hence,
by only measuring the relavent payload sections (and that means
excluding the future section that will contain the PCR hash signature)
we avoid this problem, naturally.
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<entry > 4 + 9 + 11</entry>
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</row>
<row >
<entry > Default kernel command line (embedded in unified PE binary)</entry>
efi: from the stub measure the ELF kernel + built-in initrd and so on into PCR 11
Here we grab a new – on Linux so far unused (by my Googling skills, that
is) – and measure all static components of the PE kernel image into.
This is useful since for the first time we'll have a PCR that contains
only a PCR of the booted kernel, nothing else. That allows putting
together TPM policies that bind to a specific kernel (+ builtin initrd),
without having to have booted that kernel first. PCRs can be
pre-calculated. Yay!
You might wonder, why we measure just the discovered PE sections we are
about to use, instead of the whole PE image. That's because of the next
step I have in mind: PE images should also be able to carry an
additional section that contains a signature for its own expected,
pre-calculated PCR values. This signature data should then be passed
into the booted kernel and can be used there in TPM policies. Benefit:
TPM policies can now be bound to *signatures* of PCRs, instead of the
raw hash values themselves. This makes update management a *lot* easier,
as policies don't need to be updated whenever a kernel is updated, as
long as the signature is available. Now, if the PCR signature is
embedded in the kernel PE image it cannot be of a PCR hash of the kernel
PE image itself, because that would be a chicken-and-egg problem. Hence,
by only measuring the relavent payload sections (and that means
excluding the future section that will contain the PCR hash signature)
we avoid this problem, naturally.
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<entry > 4 + 11</entry>
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</row>
<row >
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<entry > Overridden kernel command line</entry>
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<entry > 12</entry>
</row>
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<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>
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<row >
<entry > Credentials (synthesized initrd from companion files)</entry>
2022-07-26 19:32:21 +03:00
<entry > 9 + 12</entry>
2022-04-14 15:38:52 +03:00
</row>
<row >
<entry > System Extensions (synthesized initrd from companion files)</entry>
sd-stub: measure sysext images picked up by sd-stub into PCR 13
Let's grab another so far unused PCR, and measure all sysext images into
it that we load from the ESP. Note that this is possibly partly redundant,
since sysext images should have dm-verity enabled, and that is hooked up
to IMA. However, measuring this explicitly has the benefit that we can
measure filenames too, easily, and that all without need for IMA or
anything like that.
This means: when booting a unified sd-stub kernel through sd-boot we'll
now have:
1. PCR 11: unified kernel image payload (i.e. kernel, initrd, boot
splash, dtb, osrelease)
2. PCR 12: kernel command line (i.e. the one embedded in the image, plus
optionally an overriden one) + any credential files picked up by
sd-stub
3. PCR 13: sysext images picked up by sd-stub
And each of these three PCRs should carry just the above, and start from
zero, thus be pre-calculatable.
Thus, all components and parameters of the OS boot process (i.e.
everything after the boot loader) is now nicely pre-calculable.
NOTE: this actually replaces previous measuring of the syext images into
PCR 4. I added this back in 845707aae23b3129db635604edb95c4048a5922a,
following the train of thought, that sysext images for the initrd should
be measured like the initrd itself they are for, and according to my
thinking that would be a unified kernel which is measured by firmware
into PCR 4 like any other UEFI executables.
However, I think we should depart from that idea. First and foremost
that makes it harder to pre-calculate PCR 4 (since we actually measured
quite incompatible records to the TPM event log), but also I think
there's great value in being able to write policies that bind to the
used sysexts independently of the earlier boot chain (i.e. shim, boot
loader, unified kernel), hence a separate PCR makes more sense.
Strictly speaking, this is a compatibility break, but I think one we can
get away with, simply because the initrd sysext images are currently not
picked up by systemd-sysext yet in the initrd, and because of that we
can be reasonably sure noone uses this yet, and hence relies on the PCR
register used. Hence, let's clean this up before people actually do
start relying on this.
2022-07-26 12:35:57 +03:00
<entry > 9 + 13</entry>
2022-04-14 15:38:52 +03:00
</row>
</tbody>
</tgroup>
</table>
</refsect1>
2021-09-21 17:52:24 +03:00
<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> </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> </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> </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> </listitem>
</varlistentry>
2022-07-26 19:32:21 +03:00
efi: from the stub measure the ELF kernel + built-in initrd and so on into PCR 11
Here we grab a new – on Linux so far unused (by my Googling skills, that
is) – and measure all static components of the PE kernel image into.
This is useful since for the first time we'll have a PCR that contains
only a PCR of the booted kernel, nothing else. That allows putting
together TPM policies that bind to a specific kernel (+ builtin initrd),
without having to have booted that kernel first. PCRs can be
pre-calculated. Yay!
You might wonder, why we measure just the discovered PE sections we are
about to use, instead of the whole PE image. That's because of the next
step I have in mind: PE images should also be able to carry an
additional section that contains a signature for its own expected,
pre-calculated PCR values. This signature data should then be passed
into the booted kernel and can be used there in TPM policies. Benefit:
TPM policies can now be bound to *signatures* of PCRs, instead of the
raw hash values themselves. This makes update management a *lot* easier,
as policies don't need to be updated whenever a kernel is updated, as
long as the signature is available. Now, if the PCR signature is
embedded in the kernel PE image it cannot be of a PCR hash of the kernel
PE image itself, because that would be a chicken-and-egg problem. Hence,
by only measuring the relavent payload sections (and that means
excluding the future section that will contain the PCR hash signature)
we avoid this problem, naturally.
2022-07-25 18:44:24 +03:00
<varlistentry >
<term > <varname > StubPcrKernelImage</varname> </term>
man: "the initial RAM disk" → "the initrd"
In many places we spelled out the phrase behind "initrd" in full, but this
isn't terribly useful. In fact, no "RAM disk" is used, so emphasizing this
is just confusing to the reader. Let's just say "initrd" everywhere, people
understand what this refers to, and that it's in fact an initramfs image.
Also, s/i.e./e.g./ where appropriate.
Also, don't say "in RAM", when in fact it's virtual memory, whose pages
may or may not be loaded in page frames in RAM, and we have no control over
this.
Also, add <filename></filename> and other minor cleanups.
2022-09-15 15:43:59 +03:00
<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> </listitem>
efi: from the stub measure the ELF kernel + built-in initrd and so on into PCR 11
Here we grab a new – on Linux so far unused (by my Googling skills, that
is) – and measure all static components of the PE kernel image into.
This is useful since for the first time we'll have a PCR that contains
only a PCR of the booted kernel, nothing else. That allows putting
together TPM policies that bind to a specific kernel (+ builtin initrd),
without having to have booted that kernel first. PCRs can be
pre-calculated. Yay!
You might wonder, why we measure just the discovered PE sections we are
about to use, instead of the whole PE image. That's because of the next
step I have in mind: PE images should also be able to carry an
additional section that contains a signature for its own expected,
pre-calculated PCR values. This signature data should then be passed
into the booted kernel and can be used there in TPM policies. Benefit:
TPM policies can now be bound to *signatures* of PCRs, instead of the
raw hash values themselves. This makes update management a *lot* easier,
as policies don't need to be updated whenever a kernel is updated, as
long as the signature is available. Now, if the PCR signature is
embedded in the kernel PE image it cannot be of a PCR hash of the kernel
PE image itself, because that would be a chicken-and-egg problem. Hence,
by only measuring the relavent payload sections (and that means
excluding the future section that will contain the PCR hash signature)
we avoid this problem, naturally.
2022-07-25 18:44:24 +03:00
</varlistentry>
2022-07-26 19:32:21 +03:00
<varlistentry >
<term > <varname > StubPcrKernelParameters</varname> </term>
<listitem > <para > The PCR register index the kernel command line and credentials are measured into,
man: "the initial RAM disk" → "the initrd"
In many places we spelled out the phrase behind "initrd" in full, but this
isn't terribly useful. In fact, no "RAM disk" is used, so emphasizing this
is just confusing to the reader. Let's just say "initrd" everywhere, people
understand what this refers to, and that it's in fact an initramfs image.
Also, s/i.e./e.g./ where appropriate.
Also, don't say "in RAM", when in fact it's virtual memory, whose pages
may or may not be loaded in page frames in RAM, and we have no control over
this.
Also, add <filename></filename> and other minor cleanups.
2022-09-15 15:43:59 +03:00
formatted as decimal ASCII string (e.g. <literal > 12</literal> ). This variable is set if a measurement
2022-07-26 19:32:21 +03:00
was successfully completed, and remains unset otherwise.</para> </listitem>
</varlistentry>
sd-stub: measure sysext images picked up by sd-stub into PCR 13
Let's grab another so far unused PCR, and measure all sysext images into
it that we load from the ESP. Note that this is possibly partly redundant,
since sysext images should have dm-verity enabled, and that is hooked up
to IMA. However, measuring this explicitly has the benefit that we can
measure filenames too, easily, and that all without need for IMA or
anything like that.
This means: when booting a unified sd-stub kernel through sd-boot we'll
now have:
1. PCR 11: unified kernel image payload (i.e. kernel, initrd, boot
splash, dtb, osrelease)
2. PCR 12: kernel command line (i.e. the one embedded in the image, plus
optionally an overriden one) + any credential files picked up by
sd-stub
3. PCR 13: sysext images picked up by sd-stub
And each of these three PCRs should carry just the above, and start from
zero, thus be pre-calculatable.
Thus, all components and parameters of the OS boot process (i.e.
everything after the boot loader) is now nicely pre-calculable.
NOTE: this actually replaces previous measuring of the syext images into
PCR 4. I added this back in 845707aae23b3129db635604edb95c4048a5922a,
following the train of thought, that sysext images for the initrd should
be measured like the initrd itself they are for, and according to my
thinking that would be a unified kernel which is measured by firmware
into PCR 4 like any other UEFI executables.
However, I think we should depart from that idea. First and foremost
that makes it harder to pre-calculate PCR 4 (since we actually measured
quite incompatible records to the TPM event log), but also I think
there's great value in being able to write policies that bind to the
used sysexts independently of the earlier boot chain (i.e. shim, boot
loader, unified kernel), hence a separate PCR makes more sense.
Strictly speaking, this is a compatibility break, but I think one we can
get away with, simply because the initrd sysext images are currently not
picked up by systemd-sysext yet in the initrd, and because of that we
can be reasonably sure noone uses this yet, and hence relies on the PCR
register used. Hence, let's clean this up before people actually do
start relying on this.
2022-07-26 12:35:57 +03:00
<varlistentry >
<term > <varname > StubPcrInitRDSysExts</varname> </term>
man: "the initial RAM disk" → "the initrd"
In many places we spelled out the phrase behind "initrd" in full, but this
isn't terribly useful. In fact, no "RAM disk" is used, so emphasizing this
is just confusing to the reader. Let's just say "initrd" everywhere, people
understand what this refers to, and that it's in fact an initramfs image.
Also, s/i.e./e.g./ where appropriate.
Also, don't say "in RAM", when in fact it's virtual memory, whose pages
may or may not be loaded in page frames in RAM, and we have no control over
this.
Also, add <filename></filename> and other minor cleanups.
2022-09-15 15:43:59 +03:00
<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> </listitem>
sd-stub: measure sysext images picked up by sd-stub into PCR 13
Let's grab another so far unused PCR, and measure all sysext images into
it that we load from the ESP. Note that this is possibly partly redundant,
since sysext images should have dm-verity enabled, and that is hooked up
to IMA. However, measuring this explicitly has the benefit that we can
measure filenames too, easily, and that all without need for IMA or
anything like that.
This means: when booting a unified sd-stub kernel through sd-boot we'll
now have:
1. PCR 11: unified kernel image payload (i.e. kernel, initrd, boot
splash, dtb, osrelease)
2. PCR 12: kernel command line (i.e. the one embedded in the image, plus
optionally an overriden one) + any credential files picked up by
sd-stub
3. PCR 13: sysext images picked up by sd-stub
And each of these three PCRs should carry just the above, and start from
zero, thus be pre-calculatable.
Thus, all components and parameters of the OS boot process (i.e.
everything after the boot loader) is now nicely pre-calculable.
NOTE: this actually replaces previous measuring of the syext images into
PCR 4. I added this back in 845707aae23b3129db635604edb95c4048a5922a,
following the train of thought, that sysext images for the initrd should
be measured like the initrd itself they are for, and according to my
thinking that would be a unified kernel which is measured by firmware
into PCR 4 like any other UEFI executables.
However, I think we should depart from that idea. First and foremost
that makes it harder to pre-calculate PCR 4 (since we actually measured
quite incompatible records to the TPM event log), but also I think
there's great value in being able to write policies that bind to the
used sysexts independently of the earlier boot chain (i.e. shim, boot
loader, unified kernel), hence a separate PCR makes more sense.
Strictly speaking, this is a compatibility break, but I think one we can
get away with, simply because the initrd sysext images are currently not
picked up by systemd-sysext yet in the initrd, and because of that we
can be reasonably sure noone uses this yet, and hence relies on the PCR
register used. Hence, let's clean this up before people actually do
start relying on this.
2022-07-26 12:35:57 +03:00
</varlistentry>
2021-09-21 17:52:24 +03:00
</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>
2022-09-09 12:08:35 +03:00
<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> </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> </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> </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> </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> </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> </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>
2023-01-30 18:26:50 +03:00
<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 passed to the kernel.</para> </listitem>
</varlistentry>
</variablelist>
</refsect1>
2021-09-21 17:52:24 +03:00
<refsect1 >
<title > Assembling Kernel Images</title>
2023-01-10 18:00:49 +03:00
<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>
2021-09-21 17:52:24 +03:00
</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> ,
2022-11-14 11:44:39 +03:00
<ulink url= "https://uapi-group.org/specifications/specs/boot_loader_specification" > Boot Loader Specification</ulink> ,
2021-09-21 17:52:24 +03:00
<ulink url= "https://systemd.io/BOOT_LOADER_INTERFACE" > Boot Loader Interface</ulink> ,
2023-01-10 18:00:49 +03:00
<citerefentry > <refentrytitle > ukify</refentrytitle> <manvolnum > 1</manvolnum> </citerefentry> ,
2022-09-09 12:08:35 +03:00
<citerefentry > <refentrytitle > systemd-measure</refentrytitle> <manvolnum > 1</manvolnum> </citerefentry>
2021-09-21 17:52:24 +03:00
</para>
</refsect1>
</refentry>
