core: drop CAP_MKNOD when PrivateDevices= is set

man/systemd.exec.xml

@@ -908,8 +908,11 @@
                                 <filename>/dev/sda</filename>. This is
                                 useful to securely turn off physical
                                 device access by the executed
-                                process. Defaults to
-                                false.</para></listitem>
+                                process. Defaults to false. Note that
+                                enabling this option implies that
+                                <constant>CAP_MKNOD</constant> is
+                                removed from the capability bounding
+                                set for the unit.</para></listitem>
                         </varlistentry>
 
                         <varlistentry>

src/core/unit.c

@@ -2830,6 +2830,9 @@ int unit_exec_context_patch_defaults(Unit *u, ExecContext *c) {
              !set_isempty(c->address_families)))
                 c->no_new_privileges = true;
 
+        if (c->private_devices)
+                c->capability_bounding_set_drop |= (uint64_t) 1ULL << (uint64_t) CAP_MKNOD;
+
         return 0;
 }