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This adds a small, socket-activated Varlink daemon that can delegate UID ranges for user namespaces to clients asking for it. The primary call is AllocateUserRange() where the user passes in an uninitialized userns fd, which is then set up. There are other calls that allow assigning a mount fd to a userns allocated that way, to set up permissions for a cgroup subtree, and to allocate a veth for such a user namespace. Since the UID assignments are supposed to be transitive, i.e. not permanent, care is taken to ensure that users cannot create inodes owned by these UIDs, so that persistancy cannot be acquired. This is implemented via a BPF-LSM module that ensures that any member of a userns allocated that way cannot create files unless the mount it operates on is owned by the userns itself, or is explicitly allowelisted. BPF LSM program with contributions from Alexei Starovoitov.
82 lines
4.3 KiB
XML
82 lines
4.3 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-nsresourced.service" conditional='ENABLE_NSRESOURCED'>
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<refentryinfo>
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<title>systemd-nsresourced.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-nsresourced.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-nsresourced.service</refname>
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<refname>systemd-nsresourced</refname>
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<refpurpose>User Namespace Resource Delegation Service</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>systemd-nsresourced.service</filename></para>
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<para><filename>/usr/lib/systemd/systemd-nsresourced</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><command>systemd-nsresourced</command> is a system service that permits transient delegation of a a
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UID/GID range to a user namespace (see <citerefentry
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project='man-pages'><refentrytitle>user_namespaces</refentrytitle><manvolnum>7</manvolnum></citerefentry>)
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allocated by a client, via a Varlink IPC API.</para>
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<para>Unprivileged clients may allocate a user namespace, and then request a UID/GID range to be assigned
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to it via this service. The user namespace may then be used to run containers and other sandboxes, and/or
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apply it to an id-mapped mount.</para>
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<para>Allocations of UIDs/GIDs this way are transient: when a user namespace goes away, its UID/GID range
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is returned to the pool of available ranges. In order to ensure that clients cannot gain persistency in
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their transient UID/GID range a BPF-LSM based policy is enforced that ensures that user namespaces set up
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this way can only write to file systems they allocate themselves or that are explicitly allowlisted via
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<command>systemd-nsresourced</command>.</para>
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<para><command>systemd-nsresourced</command> automatically ensures that any registered UID ranges show up
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in the system's NSS database via the <ulink url="https://systemd.io/USER_GROUP_API">User/Group Record
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Lookup API via Varlink</ulink>.</para>
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<para>Currently, only UID/GID ranges consisting of either exactly 1 or exactly 65536 UIDs/GIDs can be
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registered with this service. Moreover, UIDs and GIDs are always allocated together, and
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symmetrically.</para>
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<para>The service provides API calls to allowlist mounts (referenced via their mount file descriptors as
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per Linux <function>fsmount()</function> API), to pass ownership of a cgroup subtree to the user
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namespace and to delegate a virtual Ethernet device pair to the user namespace. When used in combination
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this is sufficient to implement fully unprivileged container environments, as implemented by
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<citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>, fully
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unprivileged <varname>RootImage=</varname> (see
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>) or
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fully unprivileged disk image tools such as
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<citerefentry><refentrytitle>systemd-dissect</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
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<para>This service provides one <ulink url="https://varlink.org/">Varlink</ulink> service:
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<constant>io.systemd.NamespaceResource</constant> allows registering user namespaces, and assign mounts,
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cgroups and network interfaces to it.</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>systemd-mountfsd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-dissect</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>user_namespaces</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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