core: imply NNP and SUID/SGID restriction for DynamicUser=yes service

Let's be safe, rather than sorry. This way DynamicUser=yes services can
neither take benefit of, nor create SUID/SGID binaries.

Given that DynamicUser= is a recent addition only we should be able to
get away with turning this on, even though this is strictly speaking a
binary compatibility breakage.

(cherry picked from commit bf65b7e0c9fc215897b676ab9a7c9d1c688143ba)
Resolves: #1687512

diff --git a/man/systemd.exec.xml b/man/systemd.exec.xml
index 45ed1864f883d83c93041865bd3938f0f7ccc07e..bdaed68162a6a93056623cef2aca6362ba4cd982 100644 (file)
--- a/man/systemd.exec.xml
+++ b/man/systemd.exec.xml
@@ -229,7 +229,9 @@
         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. Moreover <varname>ProtectSystem=strict</varname> and
+        cannot leave files around after unit termination. Furthermore <varname>NoNewPrivileges=</varname> and
+        <varname>RestrictSUIDSGID=</varname> are implicitly enabled to ensure that processes invoked cannot take benefit
+        or create SUID/SGID files or directories. Moreover <varname>ProtectSystem=strict</varname> and
         <varname>ProtectHome=read-only</varname> are implied, thus prohibiting the service to write to arbitrary file
         system locations. In order to allow the service to write to certain directories, they have to be whitelisted
         using <varname>ReadWritePaths=</varname>, but care must be taken so that UID/GID recycling doesn't create
@@ -357,11 +359,12 @@ CapabilityBoundingSet=~CAP_B CAP_C</programlisting>
         <varname>RestrictAddressFamilies=</varname>, <varname>RestrictNamespaces=</varname>,
         <varname>PrivateDevices=</varname>, <varname>ProtectKernelTunables=</varname>,
         <varname>ProtectKernelModules=</varname>, <varname>MemoryDenyWriteExecute=</varname>,
-        <varname>RestrictRealtime=</varname>, <varname>RestrictSUIDSGID=</varname> or
-        <varname>LockPersonality=</varname> are specified. Note that even if this setting is overridden by
-        them, <command>systemctl show</command> shows the original value of this setting. Also see <ulink
+        <varname>RestrictRealtime=</varname>, <varname>RestrictSUIDSGID=</varname>,
+        <varname>DynamicUser=</varname> or <varname>LockPersonality=</varname> are specified. Note that even
+        if this setting is overridden by them, <command>systemctl show</command> shows the original value of
+        this setting. Also see <ulink
         url="https://www.kernel.org/doc/html/latest/userspace-api/no_new_privs.html">No New Privileges
-        Flag</ulink>.  </para></listitem>
+        Flag</ulink>.</para></listitem>
       </varlistentry>
 
       <varlistentry>
@@ -1288,7 +1291,8 @@ RestrictNamespaces=~cgroup net</programlisting>
         identity of other users, it is recommended to restrict creation of SUID/SGID files to the few
         programs that actually require them. Note that this restricts marking of any type of file system
         object with these bits, including both regular files and directories (where the SGID is a different
-        meaning than for files, see documentation). Defaults to off.</para></listitem>
+        meaning than for files, see documentation). This option is implied if <varname>DynamicUser=</varname>
+        is enabled. Defaults to off.</para></listitem>
       </varlistentry>
 
       <varlistentry>

diff --git a/src/core/unit.c b/src/core/unit.c
index 115739f4c60d23aa2b1f859a155bee7549440a23..e1f5e6f7bdc0ddd093502b3b70a254660395ef53 100644 (file)
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -4161,14 +4161,20 @@ int unit_patch_contexts(Unit *u) {
                                         return -ENOMEM;
                         }
 
-                        /* If the dynamic user option is on, let's make sure that the unit can't leave its UID/GID
-                         * around in the file system or on IPC objects. Hence enforce a strict sandbox. */
+                        /* If the dynamic user option is on, let's make sure that the unit can't leave its
+                         * UID/GID around in the file system or on IPC objects. Hence enforce a strict
+                         * sandbox. */
 
                         ec->private_tmp = true;
                         ec->remove_ipc = true;
                         ec->protect_system = PROTECT_SYSTEM_STRICT;
                         if (ec->protect_home == PROTECT_HOME_NO)
                                 ec->protect_home = PROTECT_HOME_READ_ONLY;
+
+                        /* Make sure this service can neither benefit from SUID/SGID binaries nor create
+                         * them. */
+                        ec->no_new_privileges = true;
+                        ec->restrict_suid_sgid = true;
                 }
         }