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921fc451cb
This renames systemd-boot-system-token.service to
systemd-boot-random-seed.service and conditions it less strictly.
Previously, the job of the service was to write a "system token" EFI
variable if it was missing. It called "bootctl --graceful random-seed"
for that. With this change we condition it more liberally: instead of
calling it only when the "system token" EFI variable isn't set, we call
it whenever a boot loader interface compatible boot loader is used. This
means, previously it was invoked on the first boot only: now it is
invoked at every boot.
This doesn#t change the command that is invoked. That's because
previously already the "bootctl --graceful random-seed" did two things:
set the system token if not set yet *and* refresh the random seed in the
ESP. Previousy we put the focus on the former, now we shift the focus to
the latter.
With this simple change we can replace the logic
f913c784ad added, but from a service that
can run much later and doesn't keep the ESP pinned.
100 lines
5.8 KiB
XML
100 lines
5.8 KiB
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/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd-boot-random-seed.service" conditional='HAVE_GNU_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-random-seed.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-random-seed.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-random-seed.service</refname>
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<refpurpose>Refresh boot loader random seed at boot</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>systemd-boot-random-seed.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-random-seed.service</filename> is a system service that automatically
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refreshes the boot loader random seed stored in the EFI System Partition (ESP), from the Linux kernel
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entropy pool. The boot loader random seed is primarily consumed and updated by
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<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> from the
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UEFI environemnt (or
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<citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry> if the
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former is not used, but the latter is), and passed as initial RNG seed to the OS. It is an effective way
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to ensure the OS comes up with a random pool that is fully initialized.</para>
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<para>The service also automatically generates a 'system token' to store in an EFI variable in the
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system's NVRAM. The boot loader may then combine the on-disk random seed and the system token by
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cryptographic hashing, and pass it to the OS it boots as initialization seed for its entropy pool. Note:
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the random seed stored in the ESP is refreshed on <emphasis>every</emphasis> reboot ensuring that
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multiple subsequent boots will boot with different seeds. On the other hand, the system token is
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generated randomly <emphasis>once</emphasis>, and then persistently stored in the system's EFI variable
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storage, ensuring the same disk image won't result in the same series of boot loader seed values if used
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on multiple systems in parallel.</para>
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<para>The <filename>systemd-boot-random-seed.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 a boot
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loader is used that implements the <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader
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Interface</ulink>.</para> <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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<para>Note the relationship between <filename>systemd-boot-random-seed.service</filename> and
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<citerefentry><refentrytitle>systemd-random-seed</refentrytitle><manvolnum>8</manvolnum></citerefentry>. The
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former maintains the random seed consumed and updated by the boot environment (i.e. by
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<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or
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<citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>), the
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latter maintains a random seed consumed and updated by the OS itself. The former ensures that the OS has
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a filled entropy pool already during earliest boot when regular disk access is not available yet
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(i.e. when the OS random seed cannot be loaded yet). The latter is processed much later, once writable
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disk access is available. Thus it cannot be used to seed the initial boot phase, but typically has much
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higher quality of entropy. Both files are consumed and updated at boot, but at different
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times. Specifically:</para>
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<orderedlist>
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<listitem><para>In UEFI mode, the <filename>systemd-boot</filename> or
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<filename>systemd-stub</filename> components load the boot loader random seed off the ESP, hash it with
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available entropy and the system token, and then update it on disk. A derived seed is passed to the
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kernel which writes it to its entropy pool.</para></listitem>
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<listitem><para>In userspace the <filename>systemd-random-seed.service</filename> service loads the OS
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random seed, writes it to the kernel entropy pool, and then updates it on disk with a new value derived
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from the kernel entropy pool.</para></listitem>
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<listitem><para>In userspace the <filename>systemd-boot-random-seed.service</filename> service updates
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the boot loader random seed with a new value derived from the kernel kernel entropy pool.</para></listitem>
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</orderedlist>
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<para>This logic should ensure that the kernel's entropy pool is seeded during earliest bool already, if
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possible, but the highest quality entropy is propagated back to both on-disk seeds.</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>random</refentrytitle><manvolnum>4</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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<citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-random-seed.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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