Implement '--compress migrate' to migrate to non-compression setup

This option allow migration to a non compression server config while
still retraining compatibility with client that have a compression
setting in their config.

For existing setups that used to have comp-lzo no or another
compression setting in their configs it is a difficult to migrate to
a setup without compression without replacing all client configs at
once especially if OpenVPN 2.3 or earlier clients are in the mix that
do not support pushing stub-v2. Even with OpenVPN 2.4 and later clients
that support pushing this is not a satisfying solution as the clients
log occ mismatches and the "push stub-v2" needs to be in the server
config "forever".

If the new migrate option to compress is set and  a client is detected
that indicates that compression is used (via OCC), the server will
automatically add ``--push compress stub-v2`` to the client specific
configuration if stub-v2 is supported by the client and otherwise
switch to ``comp-lzo no`` and add ``--push comp-lzo`` to the client
specific configuration.

Patch v2: better commit message/man page, add USE_COMP ifdefs, various
          style fixes

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20210324220853.31246-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21801.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

doc/man-sections/protocol-options.rst

@@ -84,10 +84,10 @@ configured in a compatible way between both the local and remote side.
 --compress algorithm
   **DEPRECATED** Enable a compression algorithm.  Compression is generally
   not recommended.  VPN tunnels which use compression are susceptible to
-  the VORALCE attack vector.
+  the VORALCE attack vector. See also the :code:`migrate` parameter below.
 
   The ``algorithm`` parameter may be :code:`lzo`, :code:`lz4`,
-  :code:`lz4-v2`, :code:`stub`, :code:`stub-v2` or empty.
+  :code:`lz4-v2`, :code:`stub`, :code:`stub-v2`, :code:`migrate` or empty.
   LZO and LZ4 are different compression algorithms, with LZ4 generally
   offering the best performance with least CPU usage.
 
@@ -106,6 +106,15 @@ configured in a compatible way between both the local and remote side.
   Note: the :code:`stub` (or empty) option is NOT compatible with the older
   option ``--comp-lzo no``.
 
+  Using :code:`migrate` as compression algorithm enables a special migration mode.
+  It allows migration away from the ``--compress``/``--comp-lzo`` options to no compression.
+  This option sets the server to no compression mode and the server behaves identical to
+  a server without a compression option for all clients without a compression in their
+  config. However, if a client is detected that indicates that compression is used (via OCC),
+  the server will automatically add ``--push compress stub-v2`` to the client specific
+  configuration if supported by the client and otherwise switch to ``comp-lzo no``
+  and add ``--push comp-lzo`` to the client specific configuration.
+
   ***Security Considerations***
 
   Compression and encryption is a tricky combination. If an attacker knows

src/openvpn/comp.h

@@ -58,6 +58,7 @@
 #define COMP_F_ADVERTISE_STUBS_ONLY (1<<3) /* tell server that we only support compression stubs */
 #define COMP_F_ALLOW_STUB_ONLY      (1<<4) /* Only accept stub compression, even with COMP_F_ADVERTISE_STUBS_ONLY
                                             * we still accept other compressions to be pushed */
+#define COMP_F_MIGRATE              (1<<5) /* push stub-v2 or comp-lzo no when we see a client with comp-lzo in occ */
 
 
 /*

src/openvpn/multi.c

@@ -2469,6 +2469,47 @@ multi_client_connect_early_setup(struct multi_context *m,
     multi_client_connect_setenv(m, mi);
 }
 
+/**
+ *  Do the necessary modification for doing the compress migrate. This is
+ *  implemented as a connect handler as it fits the modify config for a client
+ *  paradigm and also is early enough in the chain to be overwritten by another
+ *  ccd/script to do compression on a special client.
+ */
+static enum client_connect_return
+multi_client_connect_compress_migrate(struct multi_context *m,
+                                      struct multi_instance *mi,
+                                      bool deferred,
+                                      unsigned int *option_types_found)
+{
+#ifdef USE_COMP
+    struct options *o = &mi->context.options;
+    const char *const peer_info = mi->context.c2.tls_multi->peer_info;
+
+    if (!peer_info)
+    {
+        return CC_RET_SUCCEEDED;
+    }
+
+    if (o->comp.flags & COMP_F_MIGRATE && mi->context.c2.tls_multi->remote_usescomp)
+    {
+        if(strstr(peer_info, "IV_COMP_STUBv2=1"))
+        {
+            push_option(o, "compress stub-v2", M_USAGE);
+        }
+        else
+        {
+            /* Client is old and does not support STUBv2 but since it
+             * announced comp-lzo via OCC we assume it uses comp-lzo, so
+             * switch to that and push the uncompressed variant. */
+            push_option(o, "comp-lzo no", M_USAGE);
+            o->comp.alg = COMP_ALG_STUB;
+            *option_types_found |= OPT_P_COMP;
+        }
+    }
+#endif
+    return CC_RET_SUCCEEDED;
+}
+
 /**
  * Try to source a dynamic config file from the
  * --client-config-dir directory.
@@ -2537,6 +2578,7 @@ typedef enum client_connect_return (*multi_client_connect_handler)
     bool from_deferred, unsigned int *option_types_found);
 
 static const multi_client_connect_handler client_connect_handlers[] = {
+    multi_client_connect_compress_migrate,
     multi_client_connect_source_ccd,
     multi_client_connect_call_plugin_v1,
     multi_client_connect_call_plugin_v2,

src/openvpn/options.c

@@ -7783,6 +7783,12 @@ add_option(struct options *options,
                 options->comp.alg = COMP_ALGV2_UNCOMPRESSED;
                 options->comp.flags |= COMP_F_ADVERTISE_STUBS_ONLY;
             }
+            else if (streq(p[1], "migrate"))
+            {
+                options->comp.alg = COMP_ALG_UNDEF;
+                options->comp.flags = COMP_F_MIGRATE;
+
+            }
             else if (options->comp.flags & COMP_F_ALLOW_STUB_ONLY)
             {
                 /* Also printed on a push to hint at configuration problems */

src/openvpn/ssl.c

@@ -60,6 +60,7 @@
 #include "ssl_verify.h"
 #include "ssl_backend.h"
 #include "ssl_ncp.h"
+#include "ssl_util.h"
 #include "auth_token.h"
 
 #include "memdbg.h"
@@ -2233,12 +2234,24 @@ error:
     return ret;
 }
 
+#ifdef USE_COMP
+static bool
+write_compat_local_options(struct buffer *buf, const char *options)
+{
+    struct gc_arena gc = gc_new();
+    const char *local_options = options_string_compat_lzo(options, &gc);
+    bool ret = write_string(buf, local_options, TLS_OPTIONS_LEN);
+    gc_free(&gc);
+    return ret;
+}
+#endif
+
 /**
  * Handle the writing of key data, peer-info, username/password, OCC
  * to the TLS control channel (cleartext).
  */
 static bool
-key_method_2_write(struct buffer *buf, struct tls_session *session)
+key_method_2_write(struct buffer *buf, struct tls_multi *multi, struct tls_session *session)
 {
     struct key_state *ks = &session->key[KS_PRIMARY];      /* primary key */
 
@@ -2264,6 +2277,17 @@ key_method_2_write(struct buffer *buf, struct tls_session *session)
 
     /* write options string */
     {
+#ifdef USE_COMP
+        if (multi->remote_usescomp && session->opt->mode == MODE_SERVER
+           && multi->opt.comp_options.flags & COMP_F_MIGRATE)
+        {
+            if (!write_compat_local_options(buf, session->opt->local_options))
+            {
+                goto error;
+            }
+        }
+        else
+#endif
         if (!write_string(buf, session->opt->local_options, TLS_OPTIONS_LEN))
         {
             goto error;
@@ -2458,6 +2482,7 @@ key_method_2_read(struct buffer *buf, struct tls_multi *multi, struct tls_sessio
     free(multi->remote_ciphername);
     multi->remote_ciphername =
         options_string_extract_option(options, "cipher", NULL);
+    multi->remote_usescomp = strstr(options, ",comp-lzo,");
 
     /* In OCC we send '[null-cipher]' instead 'none' */
     if (multi->remote_ciphername
@@ -2507,7 +2532,18 @@ key_method_2_read(struct buffer *buf, struct tls_multi *multi, struct tls_sessio
     if (!session->opt->disable_occ
         && !options_cmp_equal(options, session->opt->remote_options))
     {
-        options_warning(options, session->opt->remote_options);
+        const char *remote_options = session->opt->remote_options;
+#ifdef USE_COMP
+        if (multi->opt.comp_options.flags & COMP_F_MIGRATE && multi->remote_usescomp)
+        {
+            msg(D_SHOW_OCC, "Note: 'compress migrate' detected remote peer "
+                            "with compression enabled.");
+            remote_options = options_string_compat_lzo(remote_options, &gc);
+        }
+#endif
+
+        options_warning(options, remote_options);
+
         if (session->opt->ssl_flags & SSLF_OPT_VERIFY)
         {
             msg(D_TLS_ERRORS, "Option inconsistency warnings triggering disconnect due to --opt-verify");
@@ -2824,7 +2860,7 @@ tls_process(struct tls_multi *multi,
         if (!buf->len && ((ks->state == S_START && !session->opt->server)
                           || (ks->state == S_GOT_KEY && session->opt->server)))
         {
-            if (!key_method_2_write(buf, session))
+            if (!key_method_2_write(buf, multi, session))
             {
                 goto error;
             }

src/openvpn/ssl_common.h

@@ -572,6 +572,7 @@ struct tls_multi
     bool use_peer_id;
 
     char *remote_ciphername;    /**< cipher specified in peer's config file */
+    bool remote_usescomp;       /**< remote announced comp-lzo in OCC string */
 
     /*
      * Our session objects.

src/openvpn/ssl_util.c

@@ -75,3 +75,44 @@ extract_iv_proto(const char *peer_info)
     }
     return 0;
 }
+
+const char *
+options_string_compat_lzo(const char *options, struct gc_arena *gc)
+{
+    /* Example string without and with comp-lzo, i.e. input/output of this function */
+    /* w/o comp: 'V4,dev-type tun,link-mtu 1457,tun-mtu 1400,proto UDPv4,auth SHA1,keysize 128,key-method 2,tls-server' */
+    /* comp-lzo: 'V4,dev-type tun,link-mtu 1458,tun-mtu 1400,proto UDPv4,comp-lzo,auth SHA1,keysize 128,key-method 2,tls-server' */
+
+    /* Note: since this function is used only in a very limited scope it makes
+     * assumptions how the string looks. Since we locally generated the string
+     * we can make these assumptions */
+
+    /* Check that the link-mtu string is in options */
+    const char *tmp = strstr(options, ",link-mtu");
+    if (!tmp)
+    {
+        return options;
+    }
+
+    /* Get old link_mtu size */
+    int link_mtu;
+    if (sscanf(tmp, ",link-mtu %d,", &link_mtu) != 1 || link_mtu < 100 || link_mtu > 9900)
+    {
+        return options;
+    }
+
+    /* 1 byte for the possibility of 999 to 1000 and 1 byte for the null
+     * terminator */
+    struct buffer buf = alloc_buf_gc(strlen(options) + strlen(",comp-lzo") + 2, gc);
+
+    buf_write(&buf, options, (int)(tmp - options));
+
+    /* Increase link-mtu by one for the comp-lzo opcode */
+    buf_printf(&buf, ",link-mtu %d", link_mtu + 1);
+
+    tmp += strlen(",link-mtu ") + (link_mtu < 1000 ? 3 : 4);
+
+    buf_printf(&buf, "%s,comp-lzo", tmp);
+
+    return BSTR(&buf);
+}

src/openvpn/ssl_util.h

@@ -54,4 +54,19 @@ extract_var_peer_info(const char *peer_info,
  */
 unsigned int
 extract_iv_proto(const char *peer_info);
+
+/**
+ * Takes a locally produced OCC string for TLS server mode and modifies as
+ * if the option comp-lzo was enabled. This is to send a client in
+ * comp-lzo migrate mode the expected OCC string.
+ *
+ * Note: This function expects the string to be in the locally generated
+ * format and does not accept arbitrary strings.
+ *
+ * @param options   the locally generated OCC string
+ * @param gc        gc_arena to allocate the returned string in
+ * @return          the modified string or options on error
+ */
+const char *
+options_string_compat_lzo(const char *options, struct gc_arena *gc);
 #endif

tests/unit_tests/openvpn/Makefile.am

@@ -6,7 +6,7 @@ if HAVE_LD_WRAP_SUPPORT
 test_binaries += argv_testdriver buffer_testdriver
 endif
 
-test_binaries += crypto_testdriver packet_id_testdriver auth_token_testdriver ncp_testdriver
+test_binaries += crypto_testdriver packet_id_testdriver auth_token_testdriver ncp_testdriver misc_testdriver
 if HAVE_LD_WRAP_SUPPORT
 test_binaries += tls_crypt_testdriver
 endif
@@ -127,3 +127,14 @@ ncp_testdriver_SOURCES = test_ncp.c mock_msg.c \
 	$(openvpn_srcdir)/packet_id.c \
 	$(openvpn_srcdir)/platform.c \
 	$(openvpn_srcdir)/ssl_util.c
+
+misc_testdriver_CFLAGS  = @TEST_CFLAGS@ \
+	-I$(openvpn_includedir) -I$(compat_srcdir) -I$(openvpn_srcdir)
+
+misc_testdriver_LDFLAGS = @TEST_LDFLAGS@
+
+misc_testdriver_SOURCES = test_misc.c mock_msg.c \
+    mock_get_random.c \
+	$(openvpn_srcdir)/buffer.c \
+	$(openvpn_srcdir)/ssl_util.c \
+	$(openvpn_srcdir)/platform.c

tests/unit_tests/openvpn/test_misc.c

@@ -0,0 +1,83 @@
+/*
+ *  OpenVPN -- An application to securely tunnel IP networks
+ *             over a single UDP port, with support for SSL/TLS-based
+ *             session authentication and key exchange,
+ *             packet encryption, packet authentication, and
+ *             packet compression.
+ *
+ *  Copyright (C) 2021 Arne Schwabe <arne@rfc2549.org>
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License version 2
+ *  as published by the Free Software Foundation.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License along
+ *  with this program; if not, write to the Free Software Foundation, Inc.,
+ *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#elif defined(_MSC_VER)
+#include "config-msvc.h"
+#endif
+
+#include "syshead.h"
+
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdarg.h>
+#include <string.h>
+#include <setjmp.h>
+#include <cmocka.h>
+
+#include "ssl_util.h"
+
+static void
+test_compat_lzo_string(void **state)
+{
+    struct gc_arena gc = gc_new();
+
+    const char* input = "V4,dev-type tun,link-mtu 1457,tun-mtu 1400,proto UDPv4,auth SHA1,keysize 128,key-method 2,tls-server";
+
+    const char* output = options_string_compat_lzo(input, &gc);
+
+    assert_string_equal(output, "V4,dev-type tun,link-mtu 1458,tun-mtu 1400,proto UDPv4,auth SHA1,keysize 128,key-method 2,tls-server,comp-lzo");
+
+    /* This string is has a much too small link-mtu so we should fail on it" */
+    input = "V4,dev-type tun,link-mtu 2,tun-mtu 1400,proto UDPv4,auth SHA1,keysize 128,key-method 2,tls-server";
+
+    output = options_string_compat_lzo(input, &gc);
+
+    assert_string_equal(input, output);
+
+    /* not matching at all */
+    input = "V4,dev-type tun";
+    output = options_string_compat_lzo(input, &gc);
+
+    assert_string_equal(input, output);
+
+
+    input = "V4,dev-type tun,link-mtu 999,tun-mtu 1400,proto UDPv4,auth SHA1,keysize 128,key-method 2,tls-server";
+    output = options_string_compat_lzo(input, &gc);
+
+    /* 999 -> 1000, 3 to 4 chars */
+    assert_string_equal(output, "V4,dev-type tun,link-mtu 1000,tun-mtu 1400,proto UDPv4,auth SHA1,keysize 128,key-method 2,tls-server,comp-lzo");
+
+};
+
+const struct CMUnitTest misc_tests[] = {
+    cmocka_unit_test(test_compat_lzo_string),
+};
+
+int
+main(void)
+{
+    return cmocka_run_group_tests(misc_tests, NULL, NULL);
+}