Only schedule_exit() once

If an exit has already been scheduled we should not schedule it again.
Otherwise, the exit signal is never emitted if the peer reschedules the
exit before the timeout occurs.

schedule_exit() now only takes the context as argument. The signal is
hard coded to SIGTERM, and the interval is read directly from the
context options.

Furthermore, schedule_exit() now returns a bool signifying whether an
exit was scheduled; false if exit is already scheduled. The call sites
are updated accordingly. A notable difference is that management is only
notified *once* when an exit is scheduled - we no longer notify
management on redundant exit.

This patch was assigned a CVE number after already reviewed and ACKed,
because it was discovered that a misbehaving client can use the (now
fixed) server behaviour to avoid being disconnected by means of a
managment interface "client-kill" command - the security issue here is
"client can circumvent security policy set by management interface".

This only affects previously authenticated clients, and only management
client-kill, so normal renegotion / AUTH_FAIL ("your session ends") is not
affected.

CVE: 2024-28882

Change-Id: I9457f005f4ba970502e6b667d9dc4299a588d661
Signed-off-by: Reynir Björnsson <reynir@reynir.dk>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Message-Id: <20240516120434.23499-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28679.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

src/openvpn/forward.c

@@ -514,17 +514,24 @@ check_server_poll_timeout(struct context *c)
 }
 
 /*
- * Schedule a signal n_seconds from now.
+ * Schedule a SIGTERM signal c->options.scheduled_exit_interval seconds from now.
  */
-void
-schedule_exit(struct context *c, const int n_seconds, const int signal)
+bool
+schedule_exit(struct context *c)
 {
+    const int n_seconds = c->options.scheduled_exit_interval;
+    /* don't reschedule if already scheduled. */
+    if (event_timeout_defined(&c->c2.scheduled_exit))
+    {
+        return false;
+    }
     tls_set_single_session(c->c2.tls_multi);
     update_time();
     reset_coarse_timers(c);
     event_timeout_init(&c->c2.scheduled_exit, n_seconds, now);
-    c->c2.scheduled_exit_signal = signal;
+    c->c2.scheduled_exit_signal = SIGTERM;
     msg(D_SCHED_EXIT, "Delayed exit in %d seconds", n_seconds);
+    return true;
 }
 
 /*

src/openvpn/forward.h

@@ -303,7 +303,7 @@ void reschedule_multi_process(struct context *c);
 
 void process_ip_header(struct context *c, unsigned int flags, struct buffer *buf);
 
-void schedule_exit(struct context *c, const int n_seconds, const int signal);
+bool schedule_exit(struct context *c);
 
 static inline struct link_socket_info *
 get_link_socket_info(struct context *c)

src/openvpn/push.c

@@ -204,7 +204,11 @@ receive_exit_message(struct context *c)
      * */
     if (c->options.mode == MODE_SERVER)
     {
-        schedule_exit(c, c->options.scheduled_exit_interval, SIGTERM);
+        if (!schedule_exit(c))
+        {
+            /* Return early when we don't need to notify management */
+            return;
+        }
     }
     else
     {
@@ -391,7 +395,7 @@ __attribute__ ((format(__printf__, 4, 5)))
 void
 send_auth_failed(struct context *c, const char *client_reason)
 {
-    if (event_timeout_defined(&c->c2.scheduled_exit))
+    if (!schedule_exit(c))
     {
         msg(D_TLS_DEBUG, "exit already scheduled for context");
         return;
@@ -401,8 +405,6 @@ send_auth_failed(struct context *c, const char *client_reason)
     static const char auth_failed[] = "AUTH_FAILED";
     size_t len;
 
-    schedule_exit(c, c->options.scheduled_exit_interval, SIGTERM);
-
     len = (client_reason ? strlen(client_reason)+1 : 0) + sizeof(auth_failed);
     if (len > PUSH_BUNDLE_SIZE)
     {
@@ -492,7 +494,7 @@ send_auth_pending_messages(struct tls_multi *tls_multi,
 void
 send_restart(struct context *c, const char *kill_msg)
 {
-    schedule_exit(c, c->options.scheduled_exit_interval, SIGTERM);
+    schedule_exit(c);
     send_control_channel_string(c, kill_msg ? kill_msg : "RESTART", D_PUSH);
 }