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lxc: linux Container library

(C) Copyright Canonical Inc. 2019

Authors:
Venkata Harshavardhan Reddy Allu <venkataharshavardhan_ven@srmuniv.edu.in>

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
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<refentry>

  <docinfo><date>@LXC_GENERATE_DATE@</date></docinfo>

  <refmeta>
    <refentrytitle>pam_cgfs</refentrytitle>
    <manvolnum>8</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>pam_cgfs</refname>

    <refpurpose>
      cgroup management for unprivileged LXC containers.
    </refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <cmdsynopsis>
      <command>pam_cgfs.so</command>
      <arg choice="req">-c <replaceable>kernel_controller,name=named_controller</replaceable></arg>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>
      LXC has supported fully unprivileged containers since LXC 1.0.
      Fully unprivileged containers are the safest containers and are run by
      normal (non-root) users.  This is achieved by using user namespaces by
      mapping between a range of UIDs and GIDs on the host to a different
      (unprivileged) range of UIDs and GIDs in the container. That means the uid
      0 (root) in the container is mapped to an unprivileged user id (something
      like 1000000) outside of the container and only has rights on resources
      that it owns itself.
    </para>
    <para>
      Cgroup management of fully unprivileged containers means restricting the
      resources used by these containers like limiting the CPU usage of a
      container, or the number of processes it is allowed to spawn, or the
      memory it is allowed to consume. It is clear that the fully
      unprivileged containers are run by normal users and there is a need to
      limit and manage resource consumption among the containers.
      But unprivileged cgroup management is not easy with most init systems.
      So, the pam_cgfs.so came into existence.
    </para>

    <para>
      The <command>pam_cgfs.so</command> module can handle both pure cgroupfs v1
      (<filename>/sys/fs/cgroup/$controller</filename>) and pure cgroupfs
      v2 (<filename>/sys/fs/cgroup</filename>), as well as mixed mounts,
      where some controllers are mounted in a standard cgroupfs v1 hierarchy
      (<filename>/sys/fs/cgroup/$controller</filename>) and others in
      cgroupfs v2 hierarchy (<filename>/sys/fs/cgroup/unified</filename>).
      Writeable cgroups are either created for all controllers or, if specified,
      for only controllers listed as arguments on the command line.
    </para>

    <para>
      The cgroup created <filename>user/$user/n</filename> will be for the nth
      session under cgroup kernel controller hierarchy.
    </para>

    <para>
      Systems with a systemd init system are treated specifically, both with
      respect to cgroupfs v1 and cgroupfs v2. For  both, cgroupfs v1 and
      cgroupfs v2, the module checks whether systemd already placed the user in
      a cgroup it created <filename>user.slice/user-$uid/session-n.scope
      </filename> by checking whether $uid == login uid. If so, the login
      user chown the <filename>session-n.scope</filename>, else a cgroup is
      created as outlined above (<filename>user/$user/n</filename>) and chown it
      to login uid. If the init system has already placed the login user inside
      a session specific group, the <command>pam_cgfs.so</command> module is
      smart enough to detect it and re-use the cgroup.
    </para>

    <para>
      In essence,  the <command>pam_cgfs.so</command> module takes care of
      placing unprivileged (non-root) users into writable cgroups at login
      and also cleaning up these cgroup hierarchies on logout, so they are free
      to delegate resources to containers as needed that have been provided to
      them.
    </para>

  </refsect1>

  <refsect1>

    <title>Options</title>

    <variablelist>

	  <varlistentry>
	    <term> <option>-c <replaceable>controller-list</replaceable></option> </term>
	   <listitem>
	    <para>
        Takes a string argument which sets the list of kernel controllers and
        named controllers delimited by commas in-between “,”. Named controllers
        need to be specified in the form “name=$namedcontroller”. Can use “all”
        enable all cgroup resource controller hierarchies. Specifying “all” and
        other controllers explicitly returns PAM_SESSION_ERR.
      </para>
	   </listitem>
	  </varlistentry>

    </variablelist>

  </refsect1>

  <refsect1>
    <title>Module types provided</title>
    <para>
      Only <option>session</option> module type is provided (and needed).
    </para>
  </refsect1>

  <refsect1>

    <title>Return Values</title>

    <variablelist>

      <varlistentry>
        <term>PAM_SUCCESS</term>
        <listitem>
          <para>
            Writeable cgroups have been created for the user.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>PAM_SESSION_ERR</term>
        <listitem>
          <para>
            Failed to create writable cgroups for the user.
          </para>
        </listitem>
      </varlistentry>

    </variablelist>

  </refsect1>

  <refsect1>

    <title>Files</title>

    <variablelist>

      <varlistentry>
        <term>/etc/pam.d/common-session{,-noninteractive}</term>
        <listitem>
          <para>
            Default configuration is added at the end of these files.
          </para>
        </listitem>
      </varlistentry>

    </variablelist>

  </refsect1>

  <refsect1>
    <title>EXAMPLE</title>
    <programlisting>
session optional	pam_cgfs.so -c freezer,memory,named=systemd
# default configuration
# user writable cgroups are created under freezer, memory and named cgroup systemd hierarchies.
# /sys/fs/cgroup/$controller/user/$user/n for freezer,memory.
# /sys/fs/cgroup/systemd/user.slice/user-$uid/session-n.scope for systemd.

session optional	pam_cgfs.so -c all
# user writable cgroups are created under all cgroup controllers.

session optional	pam_cgfs.so -c all,memory,freezer
# invalid argument and returns PAM_SESSION_ERR
    </programlisting>
  </refsect1>

  <refsect1>
    <title>See Also</title>

    <simpara>
      <citerefentry>
      <refentrytitle><command>lxc-cgroup</command></refentrytitle>
      <manvolnum>1</manvolnum>
      </citerefentry>,

      <citerefentry>
      <refentrytitle><command>cgroups</command></refentrytitle>
      <manvolnum>7</manvolnum>
      </citerefentry>,

      <citerefentry>
      <refentrytitle><command>user_namespaces</command></refentrytitle>
      <manvolnum>7</manvolnum>
      </citerefentry>,

      <citerefentry>
      <refentrytitle><command>namespaces</command></refentrytitle>
      <manvolnum>7</manvolnum>
      </citerefentry>,

      <citerefentry>
      <refentrytitle><command>pam</command></refentrytitle>
      <manvolnum>8</manvolnum>
      </citerefentry>
    </simpara>


  </refsect1>


  <refsect1>
    <title>Author</title>
    <para>Venkata Harshavardhan Reddy Allu <email>venkataharshavardhan_ven@srmuniv.edu.in</email></para>
  </refsect1>

</refentry>

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