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diff --git a/man/systemd-boot-random-seed.service.xml b/man/systemd-boot-random-seed.service.xml new file mode 100644 index 0000000..87e2e27 --- /dev/null +++ b/man/systemd-boot-random-seed.service.xml @@ -0,0 +1,101 @@ +<?xml version='1.0'?> <!--*-nxml-*--> +<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" + "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> +<!-- SPDX-License-Identifier: LGPL-2.1-or-later --> + +<refentry id="systemd-boot-random-seed.service" conditional='ENABLE_BOOTLOADER' + xmlns:xi="http://www.w3.org/2001/XInclude"> + + <refentryinfo> + <title>systemd-boot-random-seed.service</title> + <productname>systemd</productname> + </refentryinfo> + + <refmeta> + <refentrytitle>systemd-boot-random-seed.service</refentrytitle> + <manvolnum>8</manvolnum> + </refmeta> + + <refnamediv> + <refname>systemd-boot-random-seed.service</refname> + <refpurpose>Refresh boot loader random seed at boot</refpurpose> + </refnamediv> + + <refsynopsisdiv> + <para><filename>systemd-boot-random-seed.service</filename></para> + </refsynopsisdiv> + + <refsect1> + <title>Description</title> + + <para><filename>systemd-boot-random-seed.service</filename> is a system service that automatically + refreshes the boot loader random seed stored in the EFI System Partition (ESP), from the Linux kernel + entropy pool. The boot loader random seed is primarily consumed and updated by + <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> from the + UEFI environment (or + <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry> if the + former is not used, but the latter is), and passed as initial RNG seed to the OS. It is an effective way + to ensure the OS comes up with a random pool that is fully initialized.</para> + + <para>The service also automatically generates a 'system token' to store in an EFI variable in the + system's NVRAM. The boot loader may then combine the on-disk random seed and the system token by + cryptographic hashing, and pass it to the OS it boots as initialization seed for its entropy pool. Note: + the random seed stored in the ESP is refreshed on <emphasis>every</emphasis> reboot ensuring that + multiple subsequent boots will boot with different seeds. On the other hand, the system token is + generated randomly <emphasis>once</emphasis>, and then persistently stored in the system's EFI variable + storage, ensuring the same disk image won't result in the same series of boot loader seed values if used + on multiple systems in parallel.</para> + + <para>The <filename>systemd-boot-random-seed.service</filename> unit invokes the <command>bootctl + random-seed</command> command, which updates the random seed in the ESP, and initializes the system + token if it's not initialized yet. The service is conditionalized so that it is run only when a boot + loader is used that implements the <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader + Interface</ulink>.</para> <para>For further details see + <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, regarding + the command this service invokes.</para> + + <para>Note the relationship between <filename>systemd-boot-random-seed.service</filename> and + <citerefentry><refentrytitle>systemd-random-seed</refentrytitle><manvolnum>8</manvolnum></citerefentry>. The + former maintains the random seed consumed and updated by the boot environment (i.e. by + <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or + <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>), the + latter maintains a random seed consumed and updated by the OS itself. The former ensures that the OS has + a filled entropy pool already during earliest boot when regular disk access is not available yet + (i.e. when the OS random seed cannot be loaded yet). The latter is processed much later, once writable + disk access is available. Thus it cannot be used to seed the initial boot phase, but typically has much + higher quality of entropy. Both files are consumed and updated at boot, but at different + times. Specifically:</para> + + <orderedlist> + <listitem><para>In UEFI mode, the + <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or + <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry> + components load the boot loader random seed from the ESP, hash it with available entropy and the system + token, and then update it on disk. A derived seed is passed to the kernel which writes it to its + entropy pool.</para></listitem> + + <listitem><para>In userspace the <filename>systemd-random-seed.service</filename> service loads the OS + random seed, writes it to the kernel entropy pool, and then updates it on disk with a new value derived + from the kernel entropy pool.</para></listitem> + + <listitem><para>In userspace the <filename>systemd-boot-random-seed.service</filename> service updates + the boot loader random seed with a new value derived from the kernel entropy pool.</para></listitem> + </orderedlist> + + <para>This logic should ensure that the kernel's entropy pool is seeded during earliest bool already, if + possible, but the highest quality entropy is propagated back to both on-disk seeds.</para> + </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>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, + <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>, + <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>, + <citerefentry><refentrytitle>systemd-random-seed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> + </para> + </refsect1> + +</refentry> |