Make compression asymmetric by default and add warnings

This commit introduces the allow-compression option that allow
changing the new default to the previous default or to a stricter
version.

Warning for comp-lzo/compress are not generated in the post option check
(options_postprocess_mutate) since these warnings should also be shown
on pushed options. Moving the showing the warning showing for
allow-compression to options_postprocess_mutate will complicate the
option handling without giving any other benefit.

Patch V2: fix spelling and grammer (thanks tincantech), also fix
   uncompressiable to incompressible in three other instances in the
   source code

Patch V3: fix overlong lines. Do not allow compression to be pushed

Patch V4: rename COMP_F_NO_ASYM to COMP_F_ALLOW_COMPRESS, fix style.
          The logic of warnings etc in options.c has not been changed
          since adding all the code to mutate_options would a lot more
          and more complicated code and after discussion we decided that
          it is okay as is.

Patch V5: Reword warnings, rebase on master

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <20200626110554.3690-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20138.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

doc/openvpn.8

@@ -2545,26 +2545,54 @@ 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 be compressed. All other variants always add one extra
+framing byte 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", "stub\-v2", 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 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
+controlling 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 is not a feasible option.
+
+The default of asym has been chosen to maximise compatibility with existing
+configuration while at the same time phasing out compression in existing
+deployment by disabling compression on the uplink, effectively completely disabling
+compression if both client and server are upgraded.
 
 .\"*********************************************************
 .TP

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_ALLOW_COMPRESS))
     {
         const size_t ps = PAYLOAD_SIZE(frame);
         int zlen_max = ps + COMP_EXTRA_BUFFER(ps);

src/openvpn/comp.c

@@ -127,7 +127,7 @@ void
 comp_add_to_extra_buffer(struct frame *frame)
 {
     /* Leave room for compression buffer to expand in worst case scenario
-     * where data is totally uncompressible */
+     * where data is totally incompressible */
     frame_add_to_extra_buffer(frame, COMP_EXTRA_BUFFER(EXPANDED_SIZE(frame)));
 }
 

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_ALLOW_COMPRESS       (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)
 {

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_ALLOW_COMPRESS))
     {
         return false;
     }

src/openvpn/mtu.h

@@ -45,7 +45,7 @@
  *      ifconfig $1 10.1.0.2 pointopoint 10.1.0.1 mtu 1450
  *
  *    Compression overflow bytes is the worst-case size expansion that would be
- *    expected if we tried to compress mtu + extra_frame bytes of uncompressible data.
+ *    expected if we tried to compress mtu + extra_frame bytes of incompressible data.
  */
 
 /*

src/openvpn/options.c

@@ -357,9 +357,10 @@ static const char usage_message[] =
 #endif
 #if defined(USE_COMP)
     "--compress alg  : Use compression algorithm alg\n"
+    "--allow-compression: Specify whether 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"
+    "                  packet for incompressible data.\n"
     "--comp-noadapt  : Don't use adaptive compression when --comp-lzo\n"
     "                  is specified.\n"
 #endif
@@ -4978,11 +4979,11 @@ options_string_import(struct options *options,
 #if P2MP
 
 #define VERIFY_PERMISSION(mask) {                                            \
-    if (!verify_permission(p[0], file, line, (mask), permission_mask,        \
-                           option_types_found, msglevel, options, is_inline))\
-    {                                                                        \
-        goto err;                                                            \
-    }                                                                        \
+        if (!verify_permission(p[0], file, line, (mask), permission_mask,        \
+                               option_types_found, msglevel, options, is_inline)) \
+        {                                                                        \
+            goto err;                                                            \
+        }                                                                        \
 }
 
 static bool
@@ -5115,6 +5116,25 @@ 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_ALLOW_COMPRESS))
+        {
+            msg(M_WARN, "WARNING: Compression for receiving enabled. "
+                "Compression has been used in the past to break encryption. "
+                "Sent packets are not compressed unless \"allow-compression yes\" "
+                "is also set.");
+        }
+    }
+}
+
 static void
 add_option(struct options *options,
            char *p[],
@@ -7606,29 +7626,80 @@ 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_GENERAL);
+
+        if (streq(p[1], "no"))
+        {
+            options->comp.flags =
+                COMP_F_ALLOW_STUB_ONLY|COMP_F_ADVERTISE_STUBS_ONLY;
+            if (comp_non_stub_enabled(&options->comp))
+            {
+                msg(msglevel, "'--allow-compression no' conflicts with "
+                    " enabling compression");
+            }
+        }
+        else if (options->comp.flags & COMP_F_ALLOW_STUB_ONLY)
+        {
+            /* Also printed on a push to hint at configuration problems */
+            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_ALLOW_COMPRESS;
+        }
+        else if (streq(p[1], "yes"))
+        {
+            msg(M_WARN, "WARNING: Compression for sending and receiving enabled. Compression has "
+                "been used in the past to break encryption. 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_ALLOW_COMPRESS;
+        }
+        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)
+        {
+            /* Also printed on a push to hint at configuration problems */
+            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
             {
@@ -7639,12 +7710,17 @@ 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;
     }
@@ -7656,30 +7732,36 @@ 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)
+            {
+                /* Also printed on a push to hint at configuration problems */
+                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
@@ -7691,8 +7773,9 @@ 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])