audit: log failed attempts to change audit_pid configuration

Failed attempts to change the audit_pid configuration are not presently
logged.  One case is an attempt to starve an old auditd by starting up
a new auditd when the old one is still alive and active.  The other
case is an attempt to orphan a new auditd when an old auditd shuts
down.

Log both as AUDIT_CONFIG_CHANGE messages with failure result.
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <pmoore@redhat.com>

kernel/audit.c

@@ -882,11 +882,15 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 			int new_pid = s.pid;
 			pid_t requesting_pid = task_tgid_vnr(current);
 
-			if ((!new_pid) && (requesting_pid != audit_pid))
+			if ((!new_pid) && (requesting_pid != audit_pid)) {
+				audit_log_config_change("audit_pid", new_pid, audit_pid, 0);
 				return -EACCES;
+			}
 			if (audit_pid && new_pid &&
-			    audit_replace(requesting_pid) != -ECONNREFUSED)
+			    audit_replace(requesting_pid) != -ECONNREFUSED) {
+				audit_log_config_change("audit_pid", new_pid, audit_pid, 0);
 				return -EEXIST;
+			}
 			if (audit_enabled != AUDIT_OFF)
 				audit_log_config_change("audit_pid", new_pid, audit_pid, 1);
 			audit_pid = new_pid;