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man: document the systemd-random-seed rework
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Lennart Poettering
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@ -45,15 +45,15 @@
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<varlistentry>
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<term><option>--esp-path=</option></term>
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<listitem><para>Path to the EFI System Partition (ESP). If not specified, <filename>/efi/</filename>,
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<filename>/boot/</filename>, and <filename>/boot/efi</filename> are checked in turn. It is recommended to mount
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the ESP to <filename>/efi/</filename>, if possible.</para></listitem>
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<filename>/boot/</filename>, and <filename>/boot/efi/</filename> are checked in turn. It is
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recommended to mount the ESP to <filename>/efi/</filename>, if possible.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--boot-path=</option></term>
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<listitem><para>Path to the Extended Boot Loader partition, as defined in the <ulink
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url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink>. If not
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specified, <filename>/boot/</filename> are checked. It is recommended to mount the Extended Boot
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specified, <filename>/boot/</filename> is checked. It is recommended to mount the Extended Boot
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Loader partition to <filename>/boot/</filename>, if possible.</para></listitem>
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</varlistentry>
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@ -124,6 +124,19 @@
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and the firmware's boot loader list.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>random-seed</option></term>
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<listitem><para>Generates a random seed and stores it in the EFI System Partition, for use by the
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<command>systemd-boot</command> boot loader. Also, generates a random 'system token' and stores it
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persistently as an EFI variable, if one has not been set before. If the boot loader finds the random
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seed in the ESP and the system token in the EFI variable it will derive a random seed to pass to the
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OS and a new seed to store in the ESP from the combination of both. The random seed passed to the OS
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is credited to the kernel's entropy pool by the system manager during early boot, and permits
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userspace to boot up with an entropy pool fully initialized very early on. Also see
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<citerefentry><refentrytitle>systemd-boot-system-token.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>list</option></term>
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@ -165,7 +178,8 @@
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<para>
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<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<ulink url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink>,
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<ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>
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<ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
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<citerefentry><refentrytitle>systemd-boot-system-token.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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@ -153,6 +153,22 @@
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<listitem><para>Takes a boolean argument. Enable (the default) or disable
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the "Reboot into firmware" entry.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>random-seed-mode</term>
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<listitem><para>Takes one of <literal>off</literal>, <literal>with-system-token</literal> and
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<literal>always</literal>. If <literal>off</literal> no random seed data is read off the ESP, nor
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passed to the OS. If <literal>with-system-token</literal> (the default)
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<command>systemd-boot</command> will read a random seed from the ESP (from the file
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<filename>/loader/random-seed</filename>) only if the <varname>LoaderSystemToken</varname> EFI
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variable is set, and then derive the random seed to pass to the OS from the combination. If
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<literal>always</literal> the boot loader will do so even if <varname>LoaderSystemToken</varname> is
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not set. This mode is useful in environments where protection against OS image reuse is not a
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concern, and the random seed shall be used even with no further setup in place. User <command>bootctl
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random-seed</command> to initialize both the random seed file in the ESP and the system token EFI
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variable.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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@ -655,6 +655,7 @@ manpages = [
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['systemd-bless-boot-generator', '8', [], 'ENABLE_EFI'],
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['systemd-bless-boot.service', '8', [], 'ENABLE_EFI'],
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['systemd-boot-check-no-failures.service', '8', [], ''],
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['systemd-boot-system-token.service', '8', [], 'ENABLE_EFI'],
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['systemd-boot', '7', ['sd-boot'], 'ENABLE_EFI'],
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['systemd-cat', '1', [], ''],
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['systemd-cgls', '1', [], ''],
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76
man/systemd-boot-system-token.service.xml
Normal file
76
man/systemd-boot-system-token.service.xml
Normal file
@ -0,0 +1,76 @@
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<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1+ -->
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<refentry id="systemd-boot-system-token.service" conditional='ENABLE_EFI'
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>systemd-boot-system-token.service</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-boot-system-token.service</refentrytitle>
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<manvolnum>8</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-boot-system-token.service</refname>
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<refpurpose>Generate an initial boot loader system token and random seed</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>systemd-boot-system-token.service</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><filename>systemd-boot-system-token.service</filename> is a system service that automatically
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generates a 'system token' to store in an EFI variable in the system's NVRAM and a random seed to store
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on the EFI System Partition ESP on disk. The boot loader may then combine these two randomized data
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fields by cryptographic hashing, and pass it to the OS it boots as initialization seed for its entropy
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pool. The random seed stored in the ESP is refreshed on each reboot ensuring that multiple subsequent
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boots will boot with different seeds. The 'system token' is generated randomly once, and then
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persistently stored in the system's EFI variable storage.</para>
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<para>The <filename>systemd-boot-system-token.service</filename> unit invokes the <command>bootctl
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random-seed</command> command, which updates the random seed in the ESP, and initializes the 'system
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token' if it's not initialized yet. The service is conditionalized so that it is run only when all of the
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below apply:</para>
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<itemizedlist>
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<listitem><para>A boot loader is used that implements the <ulink
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url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink> (which defines the 'system
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token' concept).</para></listitem>
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<listitem><para>Either a 'system token' was not set yet, or the boot loader has not passed the OS a
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random seed yet (and thus most likely has been missing the random seed file in the
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ESP).</para></listitem>
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<listitem><para>The system is not running in a VM environment. This case is explicitly excluded since
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on VM environments the ESP backing storage and EFI variable storage is typically not physically
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separated and hence booting the same OS image in multiple instances would replicate both, thus reusing
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the same random seed and 'system token' among all instances, which defeats its purpose. Note that it's
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still possible to use boot loader random seed provisioning in this mode, but the automatic logic
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implemented by this service has no effect then, and the user instead has to manually invoke the
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<command>bootctl random-seed</command> acknowledging these restrictions.</para></listitem>
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</itemizedlist>
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<para>For further details see
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<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, regarding
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the command this service invokes.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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@ -28,13 +28,14 @@
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manager. It provides a graphical menu to select the entry to boot and an editor for the kernel command
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line. <command>systemd-boot</command> supports systems with UEFI firmware only.</para>
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<para>systemd-boot loads boot entry information from the EFI system partition (ESP), usually mounted at
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<filename>/efi/</filename>, <filename>/boot/</filename>, or <filename>/boot/efi/</filename> during OS
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runtime, as well as from the Extended Boot Loader partition if it exists (usually mounted to
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<filename>/boot/</filename>). Configuration file fragments, kernels, initrds and other EFI images to boot
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generally need to reside on the ESP or the Extended Boot Loader partition. Linux kernels must be built
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with <option>CONFIG_EFI_STUB</option> to be able to be directly executed as an EFI image. During boot
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systemd-boot automatically assembles a list of boot entries from the following sources:</para>
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<para><command>systemd-boot</command> loads boot entry information from the EFI system partition (ESP),
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usually mounted at <filename>/efi/</filename>, <filename>/boot/</filename>, or
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<filename>/boot/efi/</filename> during OS runtime, as well as from the Extended Boot Loader partition if
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it exists (usually mounted to <filename>/boot/</filename>). Configuration file fragments, kernels,
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initrds and other EFI images to boot generally need to reside on the ESP or the Extended Boot Loader
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partition. Linux kernels must be built with <option>CONFIG_EFI_STUB</option> to be able to be directly
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executed as an EFI image. During boot <command>systemd-boot</command> automatically assembles a list of
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boot entries from the following sources:</para>
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<itemizedlist>
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<listitem><para>Boot entries defined with <ulink
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@ -57,17 +58,50 @@
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<listitem><para>A reboot into the UEFI firmware setup option, if supported by the firmware</para></listitem>
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</itemizedlist>
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<para><citerefentry><refentrytitle>kernel-install</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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may be used to copy kernel images onto the ESP or the Extended Boot Loader Partition and to generate
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description files compliant with the Boot Loader
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Specification. <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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<para><command>systemd-boot</command> supports the following features:</para>
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<itemizedlist>
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<listitem><para>Basic boot manager configuration changes (such as timeout
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configuration, default boot entry selection, …) may be made directly from the boot loader UI at
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boot-time, as well as during system runtime with EFI variables.</para></listitem>
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<listitem><para>The boot manager integrates with the <command>systemctl</command> command to implement
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features such as <command>systemctl reboot --boot-loader-entry=…</command> (for rebooting into a
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specific boot menu entry, i.e. "reboot into Windows") and <command>systemctl reboot
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--boot-loader-menu=…</command> (for rebooting into the boot loader menu), by implementing the <ulink
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url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>. See
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<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
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details.</para></listitem>
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<listitem><para>An EFI variable set by the boot loader informs the OS about the ESP partition used
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during boot. This is then used to automatically mount the correct ESP partition to
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<filename>/efi/</filename> or <filename>/boot/</filename> during OS runtime. See
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<citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for details.</para></listitem>
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<listitem><para>The boot manager provides information about the boot time spent in UEFI firmware using
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the <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>. This
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information can be displayed using
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<citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
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</para></listitem>
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<listitem><para>The boot manager implements boot counting and automatic fallback to older, working boot
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entries on failure. See <ulink url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot
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Assessment</ulink>.</para></listitem>
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<listitem><para>The boot manager optionally reads a random seed from the ESP partition, combines it
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with a 'system token' stored in a persistant EFI variable and derives a random seed to use by the OS as
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entropy pool initializaton, providing a full entropy pool during early boot.</para></listitem>
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</itemizedlist>
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<para><citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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may be used from a running system to locate the ESP and the Extended Boot Loader Partition, list
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available entries, and install <command>systemd-boot</command> itself.</para>
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<para>systemd-boot will provide information about the time spent in UEFI firmware using the <ulink
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url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>. This information can be displayed
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using <citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
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</para>
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<para><citerefentry><refentrytitle>kernel-install</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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may be used to copy kernel images onto the ESP or the Extended Boot Loader Partition and to generate
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description files compliant with the Boot Loader
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Specification.</para>
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</refsect1>
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<refsect1>
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@ -238,7 +272,9 @@
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Loader Specification</ulink> are read from <filename>/loader/entries/</filename> on the ESP and the
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Extended Boot Loader partition. Unified kernel boot entries following the <ulink
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url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink> are read from
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<filename>/EFI/Linux/</filename> on the ESP and the Extended Boot Loader partition.</para>
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<filename>/EFI/Linux/</filename> on the ESP and the Extended Boot Loader partition. Optionally, a random
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seed for early boot entropy pool provisioning is stored in <filename>/loader/random-seed</filename> in
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the ESP.</para>
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</refsect1>
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<refsect1>
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@ -346,10 +382,39 @@
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<listitem><para>Information about the time spent in various parts of the boot loader. Set by the boot
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loader. Use <citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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to view this data. These variables are defined by the <ulink
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url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para></listitem>
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to view this data. </para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>LoaderRandomSeed</varname></term>
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<listitem><para>A binary random seed <command>systemd-boot</command> may optionally pass to the
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OS. This is a volatile EFI variable that is hashed at boot from the combination of a random seed
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stored in the ESP (in <filename>/loader/random-seed</filename>) and a "system token" persistently
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stored in the EFI variable <varname>LoaderSystemToken</varname> (see below). During early OS boot the
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system manager reads this variable and passes it to the OS kernel's random pool, crediting the full
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entropy it contains. This is an efficient way to ensure the system starts up with a fully initialized
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kernel random pool — as early as the initial RAM disk phase. <command>systemd-boot</command> reads
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the random seed from the ESP, combines it with the "system token", and both derives a new random seed
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to update in-place the seed stored in the ESP, and the random seed to pass to the OS from it via
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SHA256 hashing in counter mode. This ensures that different physical systems that boot the same
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"golden" OS image — i.e. containing the same random seed file in the ESP — will still pass a
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different random seed to the OS. It is made sure the random seed stored in the ESP is fully
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overwritten before the OS is booted, to ensure different random seed data is used between subsequent
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boots.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>LoaderSystemToken</varname></term>
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<listitem><para>A binary random data field, that is used for generating the random see to pass to the
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OS (see above). Note that this random data is generally only generated once, during OS installation,
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and is then never updated again.</para></listitem>
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</varlistentry>
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</variablelist>
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<para>Many of these variables are defined by the <ulink
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url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
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</refsect1>
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<refsect1>
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@ -413,6 +478,7 @@
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<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>loader.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-bless-boot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-boot-system-token.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>kernel-install</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<ulink url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink>,
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<ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>
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@ -29,21 +29,60 @@
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<refsect1>
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<title>Description</title>
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<para><filename>systemd-random-seed.service</filename> is a
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service that restores the random seed of the system at early boot
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and saves it at shutdown. See
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<citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
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for details. Saving/restoring the random seed across boots
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increases the amount of available entropy early at boot. On disk
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the random seed is stored in
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<filename>/var/lib/systemd/random-seed</filename>.</para>
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<para><filename>systemd-random-seed.service</filename> is a service that loads an on-disk random seed
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into the kernel entropy pool during boot and saves it at shutdown. See
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<citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry> for
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details. By default, no entropy is credited when the random seed is written into the kernel entropy pool,
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but this may be changed with <varname>$SYSTEMD_RANDOM_SEED_CREDIT</varname>, see below. On disk the random
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seed is stored in <filename>/var/lib/systemd/random-seed</filename>.</para>
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<para>Note that this service runs relatively late during the early boot phase, i.e. generally after the
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initial RAM disk (initrd) completed its work, and the <filename>/var/</filename> file system has been
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mounted writable. Many system services require entropy much earlier than this — this service is hence of
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limited use for complex system. It is recommended to use a boot loader that can pass an initial random
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seed to the kernel to ensure that entropy is available from earliest boot on, for example
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<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>, with
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its <command>bootctl random-seed</command> functionality.</para>
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<para>When loading the random seed from disk its file is immediately updated with a new seed retrieved
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from the kernel, in order to ensure no two boots operate with the same random seed. This new seed is
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retrieved synchronously from the kernel, which means the service will not complete start-up until the
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random pool is fully initialized. On entropy-starved systems this may take a while. This functionality is
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intended to be used as synchronization point for ordering services that require an initialized entropy
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pool to function securely (i.e. services that access <filename>/dev/urandom</filename> without any
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further precautions).</para>
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||||
|
||||
<para>Care should be taken when creating OS images that are replicated to multiple systems: if the random
|
||||
seed file is included unmodified each system will initialize its entropy pool with the same data, and
|
||||
thus — if otherwise entropy-starved — generate the same or at least guessable random seed streams. As a
|
||||
safety precaution crediting entropy is thus disabled by default. It is recommended to remove the random
|
||||
seed from OS images intended for replication on multiple systems, in which case it is safe to enable
|
||||
entropy crediting, see below.</para>
|
||||
</refsect1>
|
||||
|
||||
<refsect1>
|
||||
<title>Environment</title>
|
||||
|
||||
<variablelist class='environment-variables'>
|
||||
<varlistentry>
|
||||
<term><varname>$SYSTEMD_RANDOM_SEED_CREDIT</varname></term>
|
||||
<listitem><para>By default, <filename>systemd-random-seed.service</filename> does not credit any
|
||||
entropy when loading the random seed. With this option this behaviour may be changed: it either takes
|
||||
a boolean parameter or the special string <literal>force</literal>. Defaults to false, in which case
|
||||
no entropy is credited. If true, entropy is credited if the random seed file and system state pass
|
||||
various superficial concisistency checks. If set to <literal>force</literal> entropy is credited,
|
||||
regardless of these checks, as long as the random seed file exists.</para></listitem>
|
||||
</varlistentry>
|
||||
</variablelist>
|
||||
</refsect1>
|
||||
|
||||
<refsect1>
|
||||
<title>See Also</title>
|
||||
<para>
|
||||
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
|
||||
<citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
|
||||
<citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>,
|
||||
<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
|
||||
<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>4</manvolnum></citerefentry>
|
||||
</para>
|
||||
</refsect1>
|
||||
|
||||
|
||||
Loading…
Reference in New Issue
Block a user