[Openvpn-devel] Make compression asymmetric by default and add warnings

diff --git a/doc/openvpn.8 b/doc/openvpn.8
index de1a1928..21f42591 100644
--- a/doc/openvpn.8
+++ b/doc/openvpn.8
@@ -2508,26 +2508,54 @@  Enable a compression algorithm.
 
 The
 .B algorithm
-parameter may be "lzo", "lz4", or empty.  LZO and LZ4
-are different compression algorithms, with LZ4 generally
+parameter may be "lzo", "lz4", "lz4-v2", "stub", "stub-v2" or empty.
+LZO and LZ4 are different compression algorithms, with LZ4 generally
 offering the best performance with least CPU usage.
 For backwards compatibility with OpenVPN versions before v2.4, use "lzo"
 (which is identical to the older option "\-\-comp\-lzo yes").
 
+The "lz4-v2" and "stub-v2" variants implement a better framing that does not add
+overhead when packets cannot compressed. All other variants always add one extra
+compared to no compression framing.
+
 If the
 .B algorithm
-parameter is empty, compression will be turned off, but the packet
-framing for compression will still be enabled, allowing a different
-setting to be pushed later.
+parameter is "stub", "stubv2", or empty, compression will be turned off, but
+the packet framing for compression will still be enabled, allowing a different
+setting to be pushed later. Additionally, "stub" and "stub-v2" will disable
+announcing lzo and lz4 compression support for via "IV_" variables to the server.
+
 
 .B Security Considerations
 
 Compression and encryption is a tricky combination.  If an attacker knows or is
 able to control (parts of) the plaintext of packets that contain secrets, the
 attacker might be able to extract the secret if compression is enabled.  See
-e.g. the CRIME and BREACH attacks on TLS which also leverage compression to
-break encryption.  If you are not entirely sure that the above does not apply
-to your traffic, you are advised to *not* enable compression.
+e.g. the CRIME and BREACH attacks on TLS and VORACLE on VPNs which also leverage
+compression to break encryption.  If you are not entirely sure that the above does
+not apply to your traffic, you are advised to *not* enable compression.
+
+.\"*********************************************************
+.TP
+.B \-\-allow-compression [mode]
+As described in
+\.B \-\-compress
+option, compression is potentially dangerous option. This option allows
+control the behaviour of OpenVPN when compression is used and allowed.
+.B mode
+may be "yes", "no", or "asym" (default).
+
+With allow-compression set to "no", OpenVPN will refuse any non stub
+compression. With "yes" OpenVPN will send and receive compressed packets.
+With "asym", the default, OpenVPN will only decompress (downlink) packets but
+not compress (uplink) packets. This also allows migrating to disable compression
+when changing both server and client configurations to remove compression at the
+same time options is not feasible.
+
+The default of asym has been chosen to maximise compatibility with existing
+configuration while at the same time phasing phasing out compression in existing
+deployment by disabling compression on the uplink, effectively completely disabling
+compression if both client and server are upgraded.
 
 .\"*********************************************************
 .TP
diff --git a/src/openvpn/comp-lz4.c b/src/openvpn/comp-lz4.c
index f2916bdd..2f6ce3f3 100644
--- a/src/openvpn/comp-lz4.c
+++ b/src/openvpn/comp-lz4.c
@@ -70,8 +70,9 @@  do_lz4_compress(struct buffer *buf,
 {
     /*
      * In order to attempt compression, length must be at least COMPRESS_THRESHOLD.
+     * and asymmetric compression must be disabled
      */
-    if (buf->len >= COMPRESS_THRESHOLD)
+    if (buf->len >= COMPRESS_THRESHOLD && (compctx->flags & COMP_F_NO_ASYM))
     {
         const size_t ps = PAYLOAD_SIZE(frame);
         int zlen_max = ps + COMP_EXTRA_BUFFER(ps);
diff --git a/src/openvpn/comp.h b/src/openvpn/comp.h
index 0dadd1e6..a1a19ed1 100644
--- a/src/openvpn/comp.h
+++ b/src/openvpn/comp.h
@@ -52,10 +52,12 @@ 
  */
 
 /* Compression flags */
-#define COMP_F_ADAPTIVE   (1<<0) /* COMP_ALG_LZO only */
-#define COMP_F_ASYM       (1<<1) /* only downlink is compressed, not uplink */
-#define COMP_F_SWAP       (1<<2) /* initial command byte is swapped with last byte in buffer to preserve payload alignment */
+#define COMP_F_ADAPTIVE             (1<<0) /* COMP_ALG_LZO only */
+#define COMP_F_NO_ASYM              (1<<1) /* not only downlink is compressed but also uplink */
+#define COMP_F_SWAP                 (1<<2) /* initial command byte is swapped with last byte in buffer to preserve payload alignment */
 #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 */
 
 
 /*
@@ -188,6 +190,14 @@  comp_enabled(const struct compress_options *info)
     return info->alg != COMP_ALG_UNDEF;
 }
 
+static inline bool
+comp_non_stub_enabled (const struct compress_options *info)
+{
+    return info->alg != COMP_ALGV2_UNCOMPRESSED &&
+        info->alg != COMP_ALG_STUB &&
+        info->alg != COMP_ALG_UNDEF;
+}
+
 static inline bool
 comp_unswapped_prefix(const struct compress_options *info)
 {
diff --git a/src/openvpn/lzo.c b/src/openvpn/lzo.c
index e3be6adf..61bf0c12 100644
--- a/src/openvpn/lzo.c
+++ b/src/openvpn/lzo.c
@@ -123,7 +123,7 @@  lzo_compress_uninit(struct compress_context *compctx)
 static inline bool
 lzo_compression_enabled(struct compress_context *compctx)
 {
-    if (compctx->flags & COMP_F_ASYM)
+    if (!(compctx->flags & COMP_F_NO_ASYM))
     {
         return false;
     }
diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 61fa9833..8b5c96c8 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -349,6 +349,7 @@  static const char usage_message[] =
 #endif
 #if defined(USE_COMP)
     "--compress alg  : Use compression algorithm alg\n"
+    "--allow-compression: Specify wether compression should be allowed\n"
 #if defined(ENABLE_LZO)
     "--comp-lzo      : Use LZO compression -- may add up to 1 byte per\n"
     "                  packet for uncompressible data.\n"
@@ -4952,6 +4953,18 @@  set_user_script(struct options *options,
 #endif
 }
 
+static void show_compression_warning(struct compress_options *info)
+{
+    if (comp_non_stub_enabled(info))
+    {
+        /* Check if already displayed the strong warning and enabled full compression */
+        if (!(info->flags & COMP_F_NO_ASYM))
+        {
+            msg(M_WARN, "WARNING: compression allows attacks that break encryption. "
+                        "Enabling decompression of received packet only. Sent packets are not compressed.");
+        }
+    }
+}
 
 static void
 add_option(struct options *options,
@@ -7368,29 +7381,72 @@  add_option(struct options *options,
     }
 #endif
 #if defined(USE_COMP)
+    else if (streq(p[0], "allow-compression") && p[1] && !p[2])
+    {
+        VERIFY_PERMISSION(OPT_P_COMP);
+
+        if (streq(p[1], "no"))
+        {
+            options->comp.flags = COMP_F_ALLOW_STUB_ONLY;
+            if (comp_non_stub_enabled(&options->comp)) {
+                msg(msglevel, "allow-compression no conflicts with enabling compression");
+                /* we could manipulate the algorithm to a stub/no variant but the only scenario
+                 * that the previous message does not abort the program is when pushed from a server
+                 * and the server probably has a very confused configuration, so keep whatever we have */
+            }
+        }
+        else if (options->comp.flags & COMP_F_ALLOW_STUB_ONLY)
+        {
+            msg(msglevel, "Cannot set allow-compression to '%s' after set to 'no'", p[1]);
+            goto err;
+        }
+        else if (streq(p[1], "asym"))
+        {
+            options->comp.flags &= ~COMP_F_NO_ASYM;
+        }
+        else if (streq(p[1], "yes"))
+        {
+            msg(M_WARN, "WARNING: Allowing compression allows attacks that break encryption. "
+                "Using allow-compression yes is strongly discouraged for common usage. "
+                "See --compress in the manual page for more information");
+            options->comp.flags |= COMP_F_NO_ASYM;
+        }
+        else
+        {
+          msg(msglevel, "bad allow-compression option: %s -- must be 'yes', 'no', or 'asym'", p[1]);
+          goto err;
+        }
+    }
     else if (streq(p[0], "comp-lzo") && !p[2])
     {
         VERIFY_PERMISSION(OPT_P_COMP);
 
+        /* All lzo variants do not use swap */
+        options->comp.flags &= ~COMP_F_SWAP;
 #if defined(ENABLE_LZO)
         if (p[1] && streq(p[1], "no"))
 #endif
         {
             options->comp.alg = COMP_ALG_STUB;
-            options->comp.flags = 0;
+            options->comp.flags &= ~COMP_F_ADAPTIVE;
         }
 #if defined(ENABLE_LZO)
+        else if (options->comp.flags & COMP_F_ALLOW_STUB_ONLY)
+        {
+            msg(msglevel, "Cannot set comp-lzo to '%s', allow-compression is set to  'no'", p[1]);
+            goto err;
+        }
         else if (p[1])
         {
             if (streq(p[1], "yes"))
             {
                 options->comp.alg = COMP_ALG_LZO;
-                options->comp.flags = 0;
+                options->comp.flags &= ~COMP_F_ADAPTIVE;
             }
             else if (streq(p[1], "adaptive"))
             {
                 options->comp.alg = COMP_ALG_LZO;
-                options->comp.flags = COMP_F_ADAPTIVE;
+                options->comp.flags |= COMP_F_ADAPTIVE;
             }
             else
             {
@@ -7401,12 +7457,15 @@  add_option(struct options *options,
         else
         {
             options->comp.alg = COMP_ALG_LZO;
-            options->comp.flags = COMP_F_ADAPTIVE;
+            options->comp.flags |= COMP_F_ADAPTIVE;
         }
+        show_compression_warning(&options->comp);
 #endif /* if defined(ENABLE_LZO) */
     }
     else if (streq(p[0], "comp-noadapt") && !p[1])
     {
+        /* We do not need to check here if we allow compression since
+         * it only modifies a flag if compression is enabled */
         VERIFY_PERMISSION(OPT_P_COMP);
         options->comp.flags &= ~COMP_F_ADAPTIVE;
     }
@@ -7418,30 +7477,34 @@  add_option(struct options *options,
             if (streq(p[1], "stub"))
             {
                 options->comp.alg = COMP_ALG_STUB;
-                options->comp.flags = (COMP_F_SWAP|COMP_F_ADVERTISE_STUBS_ONLY);
+                options->comp.flags |= (COMP_F_SWAP|COMP_F_ADVERTISE_STUBS_ONLY);
             }
             else if (streq(p[1], "stub-v2"))
             {
                 options->comp.alg = COMP_ALGV2_UNCOMPRESSED;
-                options->comp.flags = COMP_F_ADVERTISE_STUBS_ONLY;
+                options->comp.flags |= COMP_F_ADVERTISE_STUBS_ONLY;
+            }
+            else if (options->comp.flags & COMP_F_ALLOW_STUB_ONLY)
+            {
+              msg(msglevel, "Cannot set compress to '%s', allow-compression is set to  'no'", p[1]);
+              goto err;
             }
 #if defined(ENABLE_LZO)
             else if (streq(p[1], "lzo"))
             {
                 options->comp.alg = COMP_ALG_LZO;
-                options->comp.flags = 0;
+                options->comp.flags &= ~(COMP_F_ADAPTIVE | COMP_F_SWAP);
             }
 #endif
 #if defined(ENABLE_LZ4)
             else if (streq(p[1], "lz4"))
             {
                 options->comp.alg = COMP_ALG_LZ4;
-                options->comp.flags = COMP_F_SWAP;
+                options->comp.flags |= COMP_F_SWAP;
             }
             else if (streq(p[1], "lz4-v2"))
             {
                 options->comp.alg = COMP_ALGV2_LZ4;
-                options->comp.flags = 0;
             }
 #endif
             else
@@ -7453,8 +7516,9 @@  add_option(struct options *options,
         else
         {
             options->comp.alg = COMP_ALG_STUB;
-            options->comp.flags = COMP_F_SWAP;
+            options->comp.flags |= COMP_F_SWAP;
         }
+      show_compression_warning(&options->comp);
     }
 #endif /* USE_COMP */
     else if (streq(p[0], "show-ciphers") && !p[1])
@@ -8345,4 +8409,4 @@  add_option(struct options *options,
     }
 err:
     gc_free(&gc);
-}
+}
\ No newline at end of file