From 58ec3bb4aac77131118dbbc39a65181e7847adee Mon Sep 17 00:00:00 2001
From: David Sommerseth <davids@openvpn.net>
Date: Tue, 15 Mar 2022 16:53:43 +0100
Subject: [PATCH] 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 discovered with an error in the log.

This is a backport of commit 282ddbac54f8d4923844f699 (master), taking
the different man-page format into account.  The code change is the same.

CVE: 2022-0547
Signed-off-by: David Sommerseth <davids@openvpn.net>

Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220315155344.37787-3-openvpn@sf.lists.topphemmelig.net>
URL: https://www.mail-archive.com/search?l=mid&q=20220315155344.37787-3-openvpn@sf.lists.topphemmelig.net
Signed-off-by: Gert Doering <gert@greenie.muc.de>
---
 doc/openvpn.8        | 13 +++++++++++++
 src/openvpn/plugin.c | 33 ++++++++++++++++++++++++++++++---
 2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/doc/openvpn.8 b/doc/openvpn.8
index 598d5fce5..7f773b695 100644
--- a/doc/openvpn.8
+++ b/doc/openvpn.8
@@ -2805,6 +2805,19 @@ function (such as tls\-verify, auth\-user\-pass\-verify, or
 client\-connect), then
 every module and script must return success (0) in order for
 the connection to be authenticated.
+
+.INDENT 7.0
+.TP
+.B \fBWARNING\fP:
+Plug\-ins may do deferred execution, meaning the plug\-in will
+return the control back to the main OpenVPN process and provide
+the plug\-in result later on via a different thread or process.
+OpenVPN does \fBNOT\fP support multiple authentication plug\-ins
+\fBwhere more than one plugin\fP tries to do deferred authentication.
+If this behaviour is detected, OpenVPN will shut down upon first
+authentication.
+.UNINDENT
+.UNINDENT
 .\"*********************************************************
 .TP
 .B \-\-keying\-material\-exporter label len
diff --git a/src/openvpn/plugin.c b/src/openvpn/plugin.c
index 0ab99ab5c..5ba1c2470 100644
--- a/src/openvpn/plugin.c
+++ b/src/openvpn/plugin.c
@@ -809,7 +809,7 @@ 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);
@@ -834,7 +834,34 @@ 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:
@@ -858,7 +885,7 @@ 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;
         }
