Implement exit notification via control channel

Current exit notification relies on data channel messages with specific
prefix. Adding these to new data channel modules (DCO) adds unncessary
complexity for the data for messages that from their idea belong to the
control channel anyway.

This patch adds announcing support for control channel and sending/receving
it. We use the simple EXIT message for this.

Patch v2: add comment about protocol-flags to be not a user visible option,
          fix various grammar mistakes, remove unused argument to
          receive_exit_message

Patch v3: rename data_channel_crypto_flags to imported_protocol_flags
          add tls-ekm to protocol-flags.

Patch v4: rebase, use a buffer for the code that prepares the push reply

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Heiko Hund <heiko@ist.eigentlich.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220914165041.2658423-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25209.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

12 files changed, 129 insertions, 9 deletions

diff --git a/doc/man-sections/client-options.rst b/doc/man-sections/client-options.rst
index 8e0e4f1..5a90689 100644
--- a/doc/man-sections/client-options.rst
+++ b/doc/man-sections/client-options.rst
@@ -220,9 +220,14 @@ configuration.
   immediately close its client instance object rather than waiting for a
   timeout.
 
+  If both server and client support sending this message using the control
+  channel, the message will be sent as control-channel message. Otherwise
+  the message is sent as data-channel message, which will be ignored by
+  data-channel offloaded peers.
+
   The **n** parameter (default :code:`1` if not present) controls the
   maximum number of attempts that the client will try to resend the exit
-  notification message.
+  notification message if messages are sent in data-channel mode.
 
   In UDP server mode, send :code:`RESTART` control channel command to
   connected clients. The ``n`` parameter (default :code:`1` if not present)
diff --git a/src/openvpn/crypto.h b/src/openvpn/crypto.h
index 98e2c76..5ea8890 100644
--- a/src/openvpn/crypto.h
+++ b/src/openvpn/crypto.h
@@ -264,6 +264,11 @@ struct crypto_options
     /**< Bit-flag indicating that we do not allow clients that do
      * not support resending the wrapped client key (WKc) with the
      * third packet of the three-way handshake */
+#define CO_USE_CC_EXIT_NOTIFY       (1<<6)
+    /**< Bit-flag indicating that explicit exit notifies should be
+     * sent via the control channel instead of using an OCC message
+     */
+
     unsigned int flags;         /**< Bit-flags determining behavior of
                                  *   security operation functions. */
 };
diff --git a/src/openvpn/forward.c b/src/openvpn/forward.c
index 468eeee..6a45b9e 100644
--- a/src/openvpn/forward.c
+++ b/src/openvpn/forward.c
@@ -260,6 +260,10 @@ check_incoming_control_channel(struct context *c)
         {
             receive_auth_pending(c, &buf);
         }
+        else if (buf_string_match_head_str(&buf, "EXIT"))
+        {
+            receive_exit_message(c);
+        }
         else
         {
             msg(D_PUSH_ERRORS, "WARNING: Received unknown control message: %s", BSTR(&buf));
diff --git a/src/openvpn/multi.c b/src/openvpn/multi.c
index 1bbeab7..b9b087e 100644
--- a/src/openvpn/multi.c
+++ b/src/openvpn/multi.c
@@ -1801,10 +1801,15 @@ multi_client_set_protocol_options(struct context *c)
 #ifdef HAVE_EXPORT_KEYING_MATERIAL
     if (proto & IV_PROTO_TLS_KEY_EXPORT)
     {
-        o->data_channel_crypto_flags |= CO_USE_TLS_KEY_MATERIAL_EXPORT;
+        o->imported_protocol_flags |= CO_USE_TLS_KEY_MATERIAL_EXPORT;
     }
 #endif
 
+    if (proto & IV_PROTO_CC_EXIT_NOTIFY)
+    {
+        o->imported_protocol_flags |= CO_USE_CC_EXIT_NOTIFY;
+    }
+
     /* Select cipher if client supports Negotiable Crypto Parameters */
 
     /* if we have already created our key, we cannot *change* our own
diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 45fbb63..e44993c 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -3341,7 +3341,7 @@ pre_connect_restore(struct options *o, struct gc_arena *gc)
 
     o->push_continuation = 0;
     o->push_option_types_found = 0;
-    o->data_channel_crypto_flags = 0;
+    o->imported_protocol_flags = 0;
 }
 
 static void
@@ -8496,11 +8496,13 @@ add_option(struct options *options,
     }
     else if (streq(p[0], "key-derivation") && p[1])
     {
+        /* NCP only option that is pushed by the server to enable EKM,
+         * should not be used by normal users in config files*/
         VERIFY_PERMISSION(OPT_P_NCP)
 #ifdef HAVE_EXPORT_KEYING_MATERIAL
         if (streq(p[1], "tls-ekm"))
         {
-            options->data_channel_crypto_flags |= CO_USE_TLS_KEY_MATERIAL_EXPORT;
+            options->imported_protocol_flags |= CO_USE_TLS_KEY_MATERIAL_EXPORT;
         }
         else
 #endif
@@ -8508,6 +8510,30 @@ add_option(struct options *options,
             msg(msglevel, "Unknown key-derivation method %s", p[1]);
         }
     }
+    else if (streq(p[0], "protocol-flags") && p[1])
+    {
+        /* NCP only option that is pushed by the server to enable protocol
+         * features that are negotiated, should not be used by normal users
+         * in config files */
+        VERIFY_PERMISSION(OPT_P_NCP)
+        for (size_t j = 1; j < MAX_PARMS && p[j] != NULL; j++)
+        {
+            if (streq(p[j], "cc-exit"))
+            {
+                options->imported_protocol_flags |= CO_USE_CC_EXIT_NOTIFY;
+            }
+#ifdef HAVE_EXPORT_KEYING_MATERIAL
+            else if (streq(p[j], "tls-ekm"))
+            {
+                options->imported_protocol_flags |= CO_USE_TLS_KEY_MATERIAL_EXPORT;
+            }
+#endif
+            else
+            {
+                msg(msglevel, "Unknown protocol-flags flag: %s", p[j]);
+            }
+        }
+    }
     else if (streq(p[0], "prng") && p[1] && !p[3])
     {
         msg(M_WARN, "NOTICE: --prng option ignored (SSL library PRNG is used)");
diff --git a/src/openvpn/options.h b/src/openvpn/options.h
index b70cd42..f438cd2 100644
--- a/src/openvpn/options.h
+++ b/src/openvpn/options.h
@@ -681,7 +681,7 @@ struct options
     bool allow_recursive_routing;
 
     /* data channel crypto flags set by push/pull. Reuses the CO_* crypto_flags */
-    unsigned int data_channel_crypto_flags;
+    unsigned int imported_protocol_flags;
 };
 
 #define streq(x, y) (!strcmp((x), (y)))
diff --git a/src/openvpn/push.c b/src/openvpn/push.c
index 4cbc89e..47a7992 100644
--- a/src/openvpn/push.c
+++ b/src/openvpn/push.c
@@ -180,6 +180,21 @@ server_pushed_signal(struct context *c, const struct buffer *buffer, const bool
 }
 
 void
+receive_exit_message(struct context *c)
+{
+    dmsg(D_STREAM_ERRORS, "Exit message received by peer");
+    c->sig->signal_received = SIGTERM;
+    c->sig->signal_text = "remote-exit";
+#ifdef ENABLE_MANAGEMENT
+    if (management)
+    {
+        management_notify(management, "info", c->sig->signal_text, "EXIT");
+    }
+#endif
+}
+
+
+void
 server_pushed_info(struct context *c, const struct buffer *buffer,
                    const int adv)
 {
@@ -606,10 +621,30 @@ prepare_push_reply(struct context *c, struct gc_arena *gc,
     {
         push_option_fmt(gc, push_list, M_USAGE, "cipher %s", o->ciphername);
     }
-    if (o->data_channel_crypto_flags & CO_USE_TLS_KEY_MATERIAL_EXPORT)
+
+    struct buffer proto_flags = alloc_buf_gc(128, gc);
+
+    if (o->imported_protocol_flags & CO_USE_CC_EXIT_NOTIFY)
+    {
+        buf_printf(&proto_flags, " cc-exit");
+
+        /* if the cc exit flag is supported, pushing tls-ekm via protocol-flags
+         * is also supported */
+        if (o->imported_protocol_flags & CO_USE_TLS_KEY_MATERIAL_EXPORT)
+        {
+            buf_printf(&proto_flags, " tls-ekm");
+        }
+    }
+    else if (o->imported_protocol_flags & CO_USE_TLS_KEY_MATERIAL_EXPORT)
     {
         push_option_fmt(gc, push_list, M_USAGE, "key-derivation tls-ekm");
     }
+
+    if (buf_len(&proto_flags) > 0)
+    {
+        push_option_fmt(gc, push_list, M_USAGE, "protocol-flags%s", buf_str(&proto_flags));
+    }
+
     return true;
 }
 
diff --git a/src/openvpn/push.h b/src/openvpn/push.h
index 62fad4a..7138055 100644
--- a/src/openvpn/push.h
+++ b/src/openvpn/push.h
@@ -48,6 +48,8 @@ void receive_auth_failed(struct context *c, const struct buffer *buffer);
 
 void server_pushed_signal(struct context *c, const struct buffer *buffer, const bool restart, const int adv);
 
+void receive_exit_message(struct context *c);
+
 void server_pushed_info(struct context *c, const struct buffer *buffer,
                         const int adv);
 
diff --git a/src/openvpn/sig.c b/src/openvpn/sig.c
index e06edd2..65cd25c 100644
--- a/src/openvpn/sig.c
+++ b/src/openvpn/sig.c
@@ -321,20 +321,46 @@ print_status(const struct context *c, struct status_output *so)
     gc_free(&gc);
 }
 
+
+/* Small helper function to determine if we should send the exit notification
+ * via control channel */
+static inline bool
+cc_exit_notify_enabled(struct context *c)
+{
+    /* Check if we have TLS active at all */
+    if (!c->c2.tls_multi)
+    {
+        return false;
+    }
+
+    const struct key_state *ks = get_primary_key(c->c2.tls_multi);
+    return (ks->crypto_options.flags & CO_USE_CC_EXIT_NOTIFY);
+}
+
 /*
  * Handle the triggering and time-wait of explicit
  * exit notification.
  */
-
 static void
 process_explicit_exit_notification_init(struct context *c)
 {
     msg(M_INFO, "SIGTERM received, sending exit notification to peer");
+    /* init the timeout to send the OCC_EXIT messages if cc exit is not
+     * enabled and also to exit after waiting for retries of resending of
+     * exit messages */
     event_timeout_init(&c->c2.explicit_exit_notification_interval, 1, 0);
     reset_coarse_timers(c);
+
     signal_reset(c->sig);
     halt_non_edge_triggered_signals();
     c->c2.explicit_exit_notification_time_wait = now;
+
+    /* Check if we are in TLS mode and should send the notification via data
+     * channel */
+    if (cc_exit_notify_enabled(c))
+    {
+        send_control_channel_string(c, "EXIT", D_PUSH);
+    }
 }
 
 void
@@ -351,7 +377,7 @@ process_explicit_exit_notification_timer_wakeup(struct context *c)
             c->sig->signal_received = SIGTERM;
             c->sig->signal_text = "exit-with-notification";
         }
-        else
+        else if (!cc_exit_notify_enabled(c))
         {
             c->c2.occ_op = OCC_EXIT;
         }
diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
index 33e145b..d3f7a02 100644
--- a/src/openvpn/ssl.c
+++ b/src/openvpn/ssl.c
@@ -1719,7 +1719,7 @@ tls_session_update_crypto_params(struct tls_multi *multi,
     }
 
     /* Import crypto settings that might be set by pull/push */
-    session->opt->crypto_flags |= options->data_channel_crypto_flags;
+    session->opt->crypto_flags |= options->imported_protocol_flags;
 
     return tls_session_update_crypto_params_do_work(multi, session, options,
                                                     frame, frame_fragment, lsi);
@@ -1969,6 +1969,9 @@ push_peer_info(struct buffer *buf, struct tls_session *session)
         /* support for the --dns option */
         iv_proto |= IV_PROTO_DNS_OPTION;
 
+        /* support for exit notify via control channel */
+        iv_proto |= IV_PROTO_CC_EXIT_NOTIFY;
+
         /* support for receiving push_reply before sending
          * push request, also signal that the client wants
          * to get push-reply messages without without requiring a round
diff --git a/src/openvpn/ssl.h b/src/openvpn/ssl.h
index 76b1b67..12ffd44 100644
--- a/src/openvpn/ssl.h
+++ b/src/openvpn/ssl.h
@@ -96,6 +96,10 @@
 /** Supports the --dns option introduced in version 2.6 */
 #define IV_PROTO_DNS_OPTION      (1<<6)
 
+/** Support for explicit exit notify via control channel
+ *  This also includes support for the protocol-flags pushed option */
+#define IV_PROTO_CC_EXIT_NOTIFY  (1<<7)
+
 /* Default field in X509 to be username */
 #define X509_USERNAME_FIELD_DEFAULT "CN"
 
diff --git a/src/openvpn/ssl_ncp.c b/src/openvpn/ssl_ncp.c
index a58ced5..fe84919 100644
--- a/src/openvpn/ssl_ncp.c
+++ b/src/openvpn/ssl_ncp.c
@@ -419,6 +419,11 @@ p2p_ncp_set_options(struct tls_multi *multi, struct tls_session *session)
         multi->peer_id = 0x76706e; /* 'v' 'p' 'n' */
     }
 
+    if (iv_proto_peer & IV_PROTO_CC_EXIT_NOTIFY)
+    {
+        session->opt->crypto_flags |= CO_USE_CC_EXIT_NOTIFY;
+    }
+
 #if defined(HAVE_EXPORT_KEYING_MATERIAL)
     if (iv_proto_peer & IV_PROTO_TLS_KEY_EXPORT)
     {