Add 'allow-compression stub-only' internally for DCO

This changes the "no" setting of allow-compression to also refuse framing
if DCO is active.  This is important for our DCO implementations as these
do not implement framing.

This behaviour surfaced when a commercial VPN provider was pushing
"comp-lzo no" to a client with DCO. While we are technically at fault here
for announcing comp-lzo no support by announcing IV_LZO_STUB=1, the
VPN provider continues to push "comp-lzo no" even in absense of that
flag.

As the new default we default to 'allow-compression no' if DCO is
enabled and to 'allow-compression stub' otherwise.

This will now also bail out if the server pushes a compression setting that
we do not support as mismatching compression is almost never a working
connection. In the case of lz4-v2 and lzo-v2 you might have a connection
that works mostly but some packets will be dropped since they compressed
which is not desirable either since it becomes very hard to debug.

Patch v2: bail out if server pushes an unsupported method. Also include this
          bail out logic when OpenVPN is compiled without compression support.

Patch v3: always parse all compression option and move logic to check method
Patch v4: fix for not setting correct default for non-dco

Change-Id: Ibd0c77af24e2214b3055d585dc23a4b06dccd414
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230324100640.1340535-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26509.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 4117d950788eebfaf6c9b5dde278e3a81b9e805d)

4 files changed, 46 insertions, 21 deletions

diff --git a/doc/man-sections/protocol-options.rst b/doc/man-sections/protocol-options.rst
index 248f65c..8134157 100644
--- a/doc/man-sections/protocol-options.rst
+++ b/doc/man-sections/protocol-options.rst
@@ -25,7 +25,9 @@ configured in a compatible way between both the local and remote side.
       compression at the same time is not a feasible option.
 
   :code:`no`  (default)
-      OpenVPN will refuse any non-stub compression.
+      OpenVPN will refuse any compression.  If data-channel offloading
+      is enabled, OpenVPN will additionally also refuse compression
+      framing (stub).
 
   :code:`yes`
       OpenVPN will send and receive compressed packets.
diff --git a/src/openvpn/comp.c b/src/openvpn/comp.c
index d6d8029..c7a562f 100644
--- a/src/openvpn/comp.c
+++ b/src/openvpn/comp.c
@@ -134,36 +134,51 @@ comp_print_stats(const struct compress_context *compctx, struct status_output *s
 void
 comp_generate_peer_info_string(const struct compress_options *opt, struct buffer *out)
 {
-    if (opt)
+    if (!opt || opt->flags & COMP_F_ALLOW_NOCOMP_ONLY)
+    {
+        return;
+    }
+
+    bool lzo_avail = false;
+    if (!(opt->flags & COMP_F_ADVERTISE_STUBS_ONLY))
     {
-        bool lzo_avail = false;
-        if (!(opt->flags & COMP_F_ADVERTISE_STUBS_ONLY))
-        {
 #if defined(ENABLE_LZ4)
-            buf_printf(out, "IV_LZ4=1\n");
-            buf_printf(out, "IV_LZ4v2=1\n");
+        buf_printf(out, "IV_LZ4=1\n");
+        buf_printf(out, "IV_LZ4v2=1\n");
 #endif
 #if defined(ENABLE_LZO)
-            buf_printf(out, "IV_LZO=1\n");
-            lzo_avail = true;
+        buf_printf(out, "IV_LZO=1\n");
+        lzo_avail = true;
 #endif
-        }
-        if (!lzo_avail)
-        {
-            buf_printf(out, "IV_LZO_STUB=1\n");
-        }
-        buf_printf(out, "IV_COMP_STUB=1\n");
-        buf_printf(out, "IV_COMP_STUBv2=1\n");
     }
+    if (!lzo_avail)
+    {
+        buf_printf(out, "IV_LZO_STUB=1\n");
+    }
+    buf_printf(out, "IV_COMP_STUB=1\n");
+    buf_printf(out, "IV_COMP_STUBv2=1\n");
 }
 
 bool
 check_compression_settings_valid(struct compress_options *info, int msglevel)
 {
+    /*
+     * We also allow comp-stub-v2 here as it technically allows escaping of
+     * weird mac address and IPv5 protocol but practically always is used
+     * as an way to disable all framing.
+     */
+    if (info->alg != COMP_ALGV2_UNCOMPRESSED && info->alg != COMP_ALG_UNDEF
+        && (info->flags & COMP_F_ALLOW_NOCOMP_ONLY))
+    {
+        msg(msglevel, "Compression or compression stub framing is not allowed "
+            "since data-channel offloading is enabled.");
+        return false;
+    }
+
     if ((info->flags & COMP_F_ALLOW_STUB_ONLY) && comp_non_stub_enabled(info))
     {
         msg(msglevel, "Compression is not allowed since allow-compression is "
-            "set to 'no'");
+            "set to 'stub-only'");
         return false;
     }
 #ifndef ENABLE_LZ4
diff --git a/src/openvpn/comp.h b/src/openvpn/comp.h
index 8636727..71b350d 100644
--- a/src/openvpn/comp.h
+++ b/src/openvpn/comp.h
@@ -60,7 +60,7 @@
                                             * 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 */
 #define COMP_F_ALLOW_ASYM           (1<<6) /* Compression was explicitly set to allow asymetric compression */
-
+#define COMP_F_ALLOW_NOCOMP_ONLY    (1<<7) /* Do not allow compression framing (breaks DCO) */
 
 /*
  * Length of prepended prefix on compressed packets
diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 435e1ca..0ccff72 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -3644,10 +3644,12 @@ options_set_backwards_compatible_options(struct options *o)
      *
      * Disable compression by default starting with 2.6.0 if no other
      * compression related option has been explicitly set */
-    if (!comp_non_stub_enabled(&o->comp) && !need_compatibility_before(o, 20600)
-        && (o->comp.flags == 0))
+    if (!need_compatibility_before(o, 20600) && (o->comp.flags == 0))
     {
-        o->comp.flags = COMP_F_ALLOW_STUB_ONLY|COMP_F_ADVERTISE_STUBS_ONLY;
+        if (!comp_non_stub_enabled(&o->comp))
+        {
+            o->comp.flags = COMP_F_ALLOW_STUB_ONLY | COMP_F_ADVERTISE_STUBS_ONLY;
+        }
     }
 #endif
 }
@@ -3749,6 +3751,12 @@ options_postprocess_mutate(struct options *o, struct env_set *es)
         o->tuntap_options.disable_dco = !dco_check_option(D_DCO, o)
                                         || !dco_check_startup_option(D_DCO, o);
     }
+#ifdef USE_COMP
+    if (dco_enabled(o))
+    {
+        o->comp.flags |= COMP_F_ALLOW_NOCOMP_ONLY;
+    }
+#endif
 
 #ifdef _WIN32
     if (dco_enabled(o))