core/execute: add the magic character '!' to allow privileged execution (#3493)

This patch implements the new magic character '!'. By putting '!' in front
of a command, systemd executes it with full privileges ignoring paramters
such as User, Group, SupplementaryGroups, CapabilityBoundingSet,
AmbientCapabilities, SecureBits, SystemCallFilter, SELinuxContext,
AppArmorProfile, SmackProcessLabel, and RestrictAddressFamilies.

Fixes partially https://github.com/systemd/systemd/issues/3414
Related to https://github.com/coreos/rkt/issues/2482

Testing:
1. Create a user 'bob'
2. Create the unit file /etc/systemd/system/exec-perm.service
   (You can use the example below)
3. sudo systemctl start ext-perm.service
4. Verify that the commands starting with '!' were not executed as bob,
   4.1 Looking to the output of ls -l /tmp/exec-perm
   4.2 Each file contains the result of the id command.

`````````````````````````````````````````````````````````````````
[Unit]
Description=ext-perm

[Service]
Type=oneshot
TimeoutStartSec=0
User=bob
ExecStartPre=!/usr/bin/sh -c "/usr/bin/rm /tmp/exec-perm*" ;
    /usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-start-pre"
ExecStart=/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-start" ;
    !/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-star-2"
ExecStartPost=/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-start-post"
ExecReload=/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-reload"
ExecStop=!/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-stop"
ExecStopPost=/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-stop-post"

[Install]
WantedBy=multi-user.target]
`````````````````````````````````````````````````````````````````

1 files changed, 10 insertions, 5 deletions

diff --git a/src/core/load-fragment.c b/src/core/load-fragment.c
index 2d8f6296c8..17c72aed88 100644
--- a/src/core/load-fragment.c
+++ b/src/core/load-fragment.c
@@ -596,7 +596,7 @@ int config_parse_exec(
         p = rvalue;
         do {
                 _cleanup_free_ char *path = NULL, *firstword = NULL;
-                bool separate_argv0 = false, ignore = false;
+                bool separate_argv0 = false, ignore = false, privileged = false;
                 _cleanup_free_ ExecCommand *nce = NULL;
                 _cleanup_strv_free_ char **n = NULL;
                 size_t nlen = 0, nbufsize = 0;
@@ -610,14 +610,18 @@ int config_parse_exec(
                         return 0;
 
                 f = firstword;
-                for (i = 0; i < 2; i++) {
-                        /* We accept an absolute path as first argument, or
-                         * alternatively an absolute prefixed with @ to allow
-                         * overriding of argv[0]. */
+                for (i = 0; i < 3; i++) {
+                        /* We accept an absolute path as first argument.
+                         * If it's prefixed with - and the path doesn't exist,
+                         * we ignore it instead of erroring out;
+                         * if it's prefixed with @, we allow overriding of argv[0];
+                         * and if it's prefixed with !, it will be run with full privileges */
                         if (*f == '-' && !ignore)
                                 ignore = true;
                         else if (*f == '@' && !separate_argv0)
                                 separate_argv0 = true;
+                        else if (*f == '!' && !privileged)
+                                privileged = true;
                         else
                                 break;
                         f++;
@@ -715,6 +719,7 @@ int config_parse_exec(
                 nce->argv = n;
                 nce->path = path;
                 nce->ignore = ignore;
+                nce->privileged = privileged;
 
                 exec_command_append_list(e, nce);