plug-ins: Disallow multiple deferred authentication plug-ins

The plug-in API in OpenVPN 2.x is not designed for running multiple
deferred authentication processes in parallel. The authentication
results of such configurations are not to be trusted.  For now we bail
out when this is discovered with an error in the log.

CVE: 2022-0547
Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20220313193154.9350-3-openvpn@sf.lists.topphemmelig.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23931.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Change-Id: Ie5d1496e1e00b0f1e5be741785b8022161073867

diff --git a/src/openvpn/plugin.c b/src/openvpn/plugin.c
index ddd9e85..619ec15 100644 (file)
--- a/src/openvpn/plugin.c
+++ b/src/openvpn/plugin.c
@@ -806,7 +806,7 @@ plugin_call_ssl(const struct plugin_list *pl,
         const int n = plugin_n(pl);
         bool success = false;
         bool error = false;
-        bool deferred = false;
+        bool deferred_auth_done = false;
 
         setenv_del(es, "script_type");
         envp = make_env_array(es, false, &gc);
@@ -831,7 +831,34 @@ plugin_call_ssl(const struct plugin_list *pl,
                     break;
 
                 case OPENVPN_PLUGIN_FUNC_DEFERRED:
-                    deferred = true;
+                    if ((type == OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY)
+                        && deferred_auth_done)
+                    {
+                        /*
+                         * Do not allow deferred auth if a deferred auth has
+                         * already been started.  This should allow a single
+                         * deferred auth call to happen, with one or more
+                         * auth calls with an instant authentication result.
+                         *
+                         * The plug-in API is not designed for multiple
+                         * deferred authentications to happen, as the
+                         * auth_control_file file will be shared across all
+                         * the plug-ins.
+                         *
+                         * Since this is considered a critical configuration
+                         * error, we bail out and exit the OpenVPN process.
+                         */
+                        error = true;
+                        msg(M_FATAL,
+                            "Exiting due to multiple authentication plug-ins "
+                            "performing deferred authentication.  Only one "
+                            "authentication plug-in doing deferred auth is "
+                            "allowed.  Ignoring the result and stopping now, "
+                            "the current authentication result is not to be "
+                            "trusted.");
+                        break;
+                    }
+                    deferred_auth_done = true;
                     break;
 
                 default:
@@ -855,7 +882,7 @@ plugin_call_ssl(const struct plugin_list *pl,
         {
             return OPENVPN_PLUGIN_FUNC_ERROR;
         }
-        else if (deferred)
+        else if (deferred_auth_done)
         {
             return OPENVPN_PLUGIN_FUNC_DEFERRED;
         }