core: add RemoveIPC= setting

This adds the boolean RemoveIPC= setting to service, socket, mount and swap
units (i.e.  all unit types that may invoke processes). if turned on, and the
unit's user/group is not root, all IPC objects of the user/group are removed
when the service is shut down. The life-cycle of the IPC objects is hence bound
to the unit life-cycle.

This is particularly relevant for units with dynamic users, as it is essential
that no objects owned by the dynamic users survive the service exiting. In
fact, this patch adds code to imply RemoveIPC= if DynamicUser= is set.

In order to communicate the UID/GID of an executed process back to PID 1 this
adds a new "user lookup" socket pair, that is inherited into the forked
processes, and closed before the exec(). This is needed since we cannot do NSS
from PID 1 due to deadlock risks, However need to know the used UID/GID in
order to clean up IPC owned by it if the unit shuts down.

1 files changed, 32 insertions, 28 deletions

diff --git a/man/systemd.exec.xml b/man/systemd.exec.xml
index bf82326096..bcedebd5bb 100644
--- a/man/systemd.exec.xml
+++ b/man/systemd.exec.xml
@@ -160,14 +160,14 @@
         use. However, UID/GIDs are recycled after a unit is terminated. Care should be taken that any processes running
         as part of a unit for which dynamic users/groups are enabled do not leave files or directories owned by these
         users/groups around, as a different unit might get the same UID/GID assigned later on, and thus gain access to
-        these files or directories. If <varname>DynamicUser=</varname> is enabled, <varname>PrivateTmp=</varname> is
-        implied. This ensures that the lifetime of temporary files created by the executed processes is bound to the
-        runtime of the service, and hence the lifetime of the dynamic user/group. Since <filename>/tmp</filename> and
-        <filename>/var/tmp</filename> are usually the only world-writable directories on a system this ensures that a
-        unit making use of dynamic user/group allocation cannot leave files around after unit termination. Use
-        <varname>RuntimeDirectory=</varname> (see below) in order to assign a writable runtime directory to a service,
-        owned by the dynamic user/group and removed automatically when the unit is terminated. Defaults to
-        off.</para></listitem>
+        these files or directories. If <varname>DynamicUser=</varname> is enabled, <varname>RemoveIPC=</varname> and
+        <varname>PrivateTmp=</varname> are implied. This ensures that the lifetime of IPC objects and temporary files
+        created by the executed processes is bound to the runtime of the service, and hence the lifetime of the dynamic
+        user/group. Since <filename>/tmp</filename> and <filename>/var/tmp</filename> are usually the only
+        world-writable directories on a system this ensures that a unit making use of dynamic user/group allocation
+        cannot leave files around after unit termination. Use <varname>RuntimeDirectory=</varname> (see below) in order
+        to assign a writable runtime directory to a service, owned by the dynamic user/group and removed automatically
+        when the unit is terminated. Defaults to off.</para></listitem>
       </varlistentry>
 
       <varlistentry>
@@ -186,6 +186,18 @@
       </varlistentry>
 
       <varlistentry>
+        <term><varname>RemoveIPC=</varname></term>
+
+        <listitem><para>Takes a boolean parameter. If set, all System V and POSIX IPC objects owned by the user and
+        group the processes of this unit are run as are removed when the unit is stopped. This setting only has an
+        effect if at least one of <varname>User=</varname>, <varname>Group=</varname> and
+        <varname>DynamicUser=</varname> are used. It has no effect on IPC objects owned by the root user. Specifically,
+        this removes System V semaphores, as well as System V and POSIX shared memory segments and message queues. If
+        multiple units use the same user or group the IPC objects are removed when the last of these units is
+        stopped. This setting is implied if <varname>DynamicUser=</varname> is set.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
         <term><varname>Nice=</varname></term>
 
         <listitem><para>Sets the default nice level (scheduling
@@ -920,27 +932,19 @@
       <varlistentry>
         <term><varname>PrivateTmp=</varname></term>
 
-        <listitem><para>Takes a boolean argument. If true, sets up a
-        new file system namespace for the executed processes and
-        mounts private <filename>/tmp</filename> and
-        <filename>/var/tmp</filename> directories inside it that is
-        not shared by processes outside of the namespace. This is
-        useful to secure access to temporary files of the process, but
-        makes sharing between processes via <filename>/tmp</filename>
-        or <filename>/var/tmp</filename> impossible. If this is
-        enabled, all temporary files created by a service in these
-        directories will be removed after the service is stopped.
-        Defaults to false. It is possible to run two or more units
-        within the same private <filename>/tmp</filename> and
-        <filename>/var/tmp</filename> namespace by using the
+        <listitem><para>Takes a boolean argument. If true, sets up a new file system namespace for the executed
+        processes and mounts private <filename>/tmp</filename> and <filename>/var/tmp</filename> directories inside it
+        that is not shared by processes outside of the namespace. This is useful to secure access to temporary files of
+        the process, but makes sharing between processes via <filename>/tmp</filename> or <filename>/var/tmp</filename>
+        impossible. If this is enabled, all temporary files created by a service in these directories will be removed
+        after the service is stopped.  Defaults to false. It is possible to run two or more units within the same
+        private <filename>/tmp</filename> and <filename>/var/tmp</filename> namespace by using the
         <varname>JoinsNamespaceOf=</varname> directive, see
-        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-        for details. Note that using this setting will disconnect
-        propagation of mounts from the service to the host
-        (propagation in the opposite direction continues to work).
-        This means that this setting may not be used for services
-        which shall be able to install mount points in the main mount
-        namespace.</para></listitem>
+        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
+        details. Note that using this setting will disconnect propagation of mounts from the service to the host
+        (propagation in the opposite direction continues to work).  This means that this setting may not be used for
+        services which shall be able to install mount points in the main mount namespace. This setting is implied if
+        <varname>DynamicUser=</varname> is set.</para></listitem>
       </varlistentry>
 
       <varlistentry>