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192 lines
10 KiB
XML
192 lines
10 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.image-policy">
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<refentryinfo>
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<title>systemd.image-policy</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd.image-policy</refentrytitle>
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<manvolnum>7</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd.image-policy</refname>
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<refpurpose>Disk Image Dissection Policy</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<para>In systemd, whenever a disk image (DDI) implementing the <ulink
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url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable
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Partitions Specification</ulink> is activated, a policy may be specified controlling which partitions to
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mount and what kind of cryptographic protection to require. Such a disk image dissection policy is a
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string that contains per-partition-type rules, separated by colons (<literal>:</literal>). The individual
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rules consist of a partition identifier, an equal sign (<literal>=</literal>), and one or more flags
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which may be set per partition. If multiple flags are specified per partition they are separated by a
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plus sign (<literal>+</literal>).</para>
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<para>The partition identifiers currently defined are: <option>root</option>, <option>usr</option>,
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<option>home</option>, <option>srv</option>, <option>esp</option>, <option>xbootldr</option>,
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<option>swap</option>, <option>root-verity</option>, <option>root-verity-sig</option>,
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<option>usr-verity</option>, <option>usr-verity-sig</option>, <option>tmp</option>,
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<option>var</option>. These identifiers match the relevant partition types in the Discoverable Partitions
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Specification, but are agnostic to CPU architectures. If the partition identifier is left empty it
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defines the <emphasis>default</emphasis> policy for partitions defined in the Discoverable Partitions
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Specification for which no policy flags are explicitly listed in the policy string.</para>
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<para>The following partition policy flags are defined that dictate the existence/absence, the use, and
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the protection level of partitions:</para>
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<itemizedlist>
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<listitem><para><option>unprotected</option> for partitions that shall exist and be used, but shall
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come without cryptographic protection, lacking both Verity authentication and LUKS
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encryption.</para></listitem>
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<listitem><para><option>verity</option> for partitions that shall exist and be used, with Verity
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authentication. (Note: if a DDI image carries a data partition, along with a Verity partition and a
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signature partition for it, and only the <option>verity</option> flag is set – and
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<option>signed</option> is not –, then the image will be set up with Verity, but the signature data will
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not be used. Or in other words: any DDI with a set of partitions that qualify for
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<option>signature</option> also implicitly qualifies for <option>verity</option>, and in fact
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<option>unprotected</option>).</para></listitem>
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<listitem><para><option>signed</option> for partitions that shall exist and be used, with Verity
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authentication, which are also accompanied by a PKCS#7 signature of the Verity root
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hash.</para></listitem>
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<listitem><para><option>encrypted</option> for partitions which shall exist and be used and are
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encrypted with LUKS.</para></listitem>
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<listitem><para><option>unused</option> for partitions that shall exist but shall not be
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used.</para></listitem>
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<listitem><para><option>absent</option> for partitions that shall not exist on the
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image.</para></listitem>
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</itemizedlist>
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<para>By setting a combination of the flags above, alternatives can be declared. For example the
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combination <literal>unused+absent</literal> means: the partition may exist (in which case it shall not
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be used) or may be absent. The combination of
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<literal>unprotected+verity+signed+encrypted+unused+absent</literal> may be specified via the special
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shortcut <literal>open</literal>, and indicates that the partition may exist or may be absent, but if it
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exists is used, regardless of the protection level.</para>
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<para>As special rule: if none of the flags above are set for a listed partition identifier, the default
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policy of <option>open</option> is implied, i.e. setting none of these flags listed above means
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effectively all flags listed above will be set.</para>
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<para>The following partition policy flags are defined that dictate the state of specific GPT partition
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flags:</para>
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<itemizedlist>
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<listitem><para><option>read-only-off</option>, <option>read-only-on</option> to require that the
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partitions have the read-only partition flag off or on.</para></listitem>
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<listitem><para><option>growfs-off</option>, <option>growfs-on</option> to require that the
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partitions have the growfs partition flag off or on.</para></listitem>
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</itemizedlist>
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<para>If both <option>read-only-off</option> and <option>read-only-on</option> are set for a partition,
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then the state of the read-only flag on the partition is not dictated by the policy. Setting neither flag
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is equivalent to setting both, i.e. setting neither of these two flags means effectively both will be
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set. A similar logic applies to <option>growfs-off</option>/<option>growfs-on</option>.</para>
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<para>If partitions are not listed within an image policy string, the default policy flags are applied
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(configurable via an empty partition identifier, see above). If no default policy flags are configured in
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the policy string, it is implied to be <literal>absent+unused</literal>, except for the Verity partition
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and their signature partitions where the policy is automatically derived from minimal protection level of
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the data partition they protect, as encoded in the policy.</para>
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</refsect1>
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<refsect1>
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<title>Special Policies</title>
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<para>The special image policy string <literal>*</literal> is short for "use everything", i.e. is
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equivalent to:</para>
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<programlisting>=verity+signed+encrypted+unprotected+unused+absent</programlisting>
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<para>The special image policy string <literal>-</literal> is short for "use nothing", i.e. is equivalent
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to:</para>
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<programlisting>=unused+absent</programlisting>
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<para>The special image policy string <literal>~</literal> is short for "everything must be absent",
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i.e. is equivalent to:</para>
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<programlisting>=absent</programlisting>
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</refsect1>
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<refsect1>
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<title>Use</title>
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<para>Most systemd components that support operating with disk images support a
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<option>--image-policy=</option> command line option to specify the image policy to use, and default to
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relatively open policies by default (typically the <literal>*</literal> policy, as described above),
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under the assumption that trust in disk images is established before the images are passed to the program
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in question.</para>
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<para>For the host image itself
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<citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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is responsible for processing the GPT partition table and making use of the included discoverable
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partitions. It accepts an image policy via the kernel command line option
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<option>systemd.image-policy=</option>.</para>
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<para>Note that image policies do not dictate how the components will mount and use disk images — they
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only dictate which parts to avoid and which protection level and arrangement to require while
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mounting/using them. For example,
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<citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> only
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cares for the <filename>/usr/</filename> and <filename>/opt/</filename> trees inside a disk image, and
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thus ignores any <filename>/home/</filename> partitions (and similar) in all cases, which might be
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included in the image, regardless whether the configured image policy would allow access to it or
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not. Similar,
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<citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry> is not
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going to make use of any discovered swap device, regardless if the policy would allow that or not.</para>
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<para>Use the <command>image-policy</command> command of the
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<citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>8</manvolnum></citerefentry> tool
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to analyze image policy strings, and determine what a specific policy string means for a specific
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partition.</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<para>The following image policy string dictates one read-only Verity-enabled <filename>/usr/</filename>
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partition must exist, plus encrypted root and swap partitions. All other partitions are ignored:</para>
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<programlisting>usr=verity+read-only-on:root=encrypted:swap=encrypted</programlisting>
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<para>The following image policy string dictates an encrypted, writable root file system, and optional
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<filename>/srv/</filename> file system that must be encrypted if it exists and no swap partition may
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exist:</para>
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<programlisting>root=encrypted+read-only-off:srv=encrypted+absent:swap=absent</programlisting>
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<para>The following image policy string dictates a single root partition that may be encrypted, but
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doesn't have to be, and ignores swap partitions, and uses all other partitions if they are available, possibly with encryption.</para>
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<programlisting>root=unprotected+encrypted:swap=absent+unused:=unprotected+encrypted+absent</programlisting>
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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>systemd-dissect</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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