Allow the TLS session to send out TLS alerts

Previous OpenVPN versions shut down the TLS control channel immediately
when encountering an error. This also meant that we would not send out
TLS alerts to notify a client about potential problems like mismatching
TLS versions or having no common cipher.

This commit adds a new key_state S_ERROR_PRE which still allows to
send out the remaining TLS packets of the control session which are
typically the alert message and then going to S_ERROR. We do not
wait for retries. So this is more a one-shot notify but that is
acceptable in this situation.

Sending out alerts is a slight compromise in security as alerts give
out a bit of information that otherwise is not given
out. But since all other consumers TLS implementations are already doing this
and TLS implementations (nowadays) are very careful not to leak (sensitive)
information by alerts and since the user experience is much better with
alerts, this compromise is worth it.

Change-Id: I0ad48915004ddee587e97c8ed190ba8ee989e48d
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20240408124933.243991-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28540.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

Changes.rst

@@ -1,5 +1,14 @@
 Overview of changes in 2.7
 ==========================
+New features
+------------
+TLS alerts
+    OpenVPN 2.7 will send out TLS alerts to peers informing them if the TLS
+    session shuts down or when the TLS implementation informs the peer about
+    an error in the TLS session (e.g. mismatching TLS versions). This improves
+    the user experience as the client shows an error instead of running into
+    a timeout when the server just stops responding completely.
+
 Deprecated features
 -------------------
 ``secret`` support has been removed by default.

src/openvpn/ssl.c

@@ -690,6 +690,9 @@ state_name(int state)
         case S_ERROR:
             return "S_ERROR";
 
+        case S_ERROR_PRE:
+            return "S_ERROR_PRE";
+
         case S_GENERATED_KEYS:
             return "S_GENERATED_KEYS";
 
@@ -2682,6 +2685,25 @@ write_outgoing_tls_ciphertext(struct tls_session *session, bool *continue_tls_pr
     return true;
 }
 
+static bool
+check_outgoing_ciphertext(struct key_state *ks, struct tls_session *session,
+                          bool *continue_tls_process)
+{
+    /* Outgoing Ciphertext to reliable buffer */
+    if (ks->state >= S_START)
+    {
+        struct buffer *buf = reliable_get_buf_output_sequenced(ks->send_reliable);
+        if (buf)
+        {
+            if (!write_outgoing_tls_ciphertext(session, continue_tls_process))
+            {
+                return false;
+            }
+        }
+    }
+    return true;
+}
+
 static bool
 tls_process_state(struct tls_multi *multi,
                   struct tls_session *session,
@@ -2706,7 +2728,7 @@ tls_process_state(struct tls_multi *multi,
     }
 
     /* Are we timed out on receive? */
-    if (now >= ks->must_negotiate && ks->state < S_ACTIVE)
+    if (now >= ks->must_negotiate && ks->state >= S_UNDEF && ks->state < S_ACTIVE)
     {
         msg(D_TLS_ERRORS,
             "TLS Error: TLS key negotiation failed to occur within %d seconds (check your network connectivity)",
@@ -2760,6 +2782,16 @@ tls_process_state(struct tls_multi *multi,
         return false;
     }
 
+    if (ks->state == S_ERROR_PRE)
+    {
+        /* When we end up here, we had one last chance to send an outstanding
+         * packet that contained an alert. We do not ensure that this packet
+         * has been successfully delivered  (ie wait for the ACK etc)
+         * but rather stop processing now */
+        ks->state = S_ERROR;
+        return false;
+    }
+
     /* Write incoming ciphertext to TLS object */
     struct reliable_entry *entry = reliable_get_entry_sequenced(ks->rec_reliable);
     if (entry)
@@ -2840,29 +2872,31 @@ tls_process_state(struct tls_multi *multi,
             dmsg(D_TLS_DEBUG, "Outgoing Plaintext -> TLS");
         }
     }
-
-    /* Outgoing Ciphertext to reliable buffer */
-    if (ks->state >= S_START)
+    if (!check_outgoing_ciphertext(ks, session, &continue_tls_process))
     {
-        buf = reliable_get_buf_output_sequenced(ks->send_reliable);
-        if (buf)
-        {
-            if (!write_outgoing_tls_ciphertext(session, &continue_tls_process))
-            {
-                goto error;
-            }
-        }
+        goto error;
     }
 
     return continue_tls_process;
 error:
     tls_clear_error();
-    ks->state = S_ERROR;
+
+    /* Shut down the TLS session but do a last read from the TLS
+     * object to be able to read potential TLS alerts */
+    key_state_ssl_shutdown(&ks->ks_ssl);
+    check_outgoing_ciphertext(ks, session, &continue_tls_process);
+
+    /* Put ourselves in the pre error state that will only send out the
+     * control channel packets but nothing else */
+    ks->state = S_ERROR_PRE;
+
     msg(D_TLS_ERRORS, "TLS Error: TLS handshake failed");
     INCR_ERROR;
-    return false;
-
+    return true;
 }
+
+
+
 /*
  * This is the primary routine for processing TLS stuff inside the
  * the main event loop.  When this routine exits
@@ -2970,7 +3004,7 @@ tls_process(struct tls_multi *multi,
     }
 
     /* When should we wake up again? */
-    if (ks->state >= S_INITIAL)
+    if (ks->state >= S_INITIAL || ks->state == S_ERROR_PRE)
     {
         compute_earliest_wakeup(wakeup,
                                 reliable_send_timeout(ks->send_reliable));
@@ -3123,7 +3157,7 @@ tls_multi_process(struct tls_multi *multi,
              session_id_print(&ks->session_id_remote, &gc),
              print_link_socket_actual(&ks->remote_addr, &gc));
 
-        if (ks->state >= S_INITIAL && link_socket_actual_defined(&ks->remote_addr))
+        if ((ks->state >= S_INITIAL || ks->state == S_ERROR_PRE) &&  link_socket_actual_defined(&ks->remote_addr))
         {
             struct link_socket_actual *tla = NULL;
 
@@ -3201,7 +3235,8 @@ tls_multi_process(struct tls_multi *multi,
             {
                 msg(D_TLS_ERRORS, "TLS Error: generate_key_expansion failed");
                 ks->authenticated = KS_AUTH_FALSE;
-                ks->state = S_ERROR;
+                key_state_ssl_shutdown(&ks->ks_ssl);
+                ks->state = S_ERROR_PRE;
             }
 
             /* Update auth token on the client if needed on renegotiation

src/openvpn/ssl_backend.h

@@ -362,6 +362,13 @@ void tls_ctx_personalise_random(struct tls_root_ctx *ctx);
 void key_state_ssl_init(struct key_state_ssl *ks_ssl,
                         const struct tls_root_ctx *ssl_ctx, bool is_server, struct tls_session *session);
 
+/**
+ * Sets a TLS session to be shutdown state, so the TLS library will generate
+ * a shutdown alert.
+ */
+void
+key_state_ssl_shutdown(struct key_state_ssl *ks_ssl);
+
 /**
  * Free the SSL channel part of the given key state.
  *

src/openvpn/ssl_common.h

@@ -74,7 +74,10 @@
  *
  * @{
  */
-#define S_ERROR          -1     /**< Error state.  */
+#define S_ERROR         (-2)     /**< Error state.  */
+#define S_ERROR_PRE     (-1)     /**< Error state but try to send out alerts
+                                  *  before killing the keystore and moving
+                                  *  it to S_ERROR */
 #define S_UNDEF           0     /**< Undefined state, used after a \c
                                  *   key_state is cleaned up. */
 #define S_INITIAL         1     /**< Initial \c key_state state after

src/openvpn/ssl_mbedtls.c

@@ -1276,6 +1276,14 @@ key_state_ssl_init(struct key_state_ssl *ks_ssl,
                         ssl_bio_read, NULL);
 }
 
+
+void
+key_state_ssl_shutdown(struct key_state_ssl *ks_ssl)
+{
+    mbedtls_ssl_send_alert_message(ks_ssl->ctx, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                   MBEDTLS_SSL_ALERT_MSG_CLOSE_NOTIFY);
+}
+
 void
 key_state_ssl_free(struct key_state_ssl *ks_ssl)
 {

src/openvpn/ssl_openssl.c

@@ -1940,6 +1940,12 @@ key_state_ssl_init(struct key_state_ssl *ks_ssl, const struct tls_root_ctx *ssl_
     BIO_set_ssl(ks_ssl->ssl_bio, ks_ssl->ssl, BIO_NOCLOSE);
 }
 
+void
+key_state_ssl_shutdown(struct key_state_ssl *ks_ssl)
+{
+    SSL_set_shutdown(ks_ssl->ssl, SSL_SENT_SHUTDOWN | SSL_RECEIVED_SHUTDOWN);
+}
+
 void
 key_state_ssl_free(struct key_state_ssl *ks_ssl)
 {