diff mbox series

[Openvpn-devel,v2,release/2.4] Fix broken fragment/mssfix with NCP

Message ID 1563892487-12063-1-git-send-email-lstipakov@gmail.com
State Accepted
Headers show
Series [Openvpn-devel,v2,release/2.4] Fix broken fragment/mssfix with NCP | expand

Commit Message

Lev Stipakov July 23, 2019, 4:34 a.m. UTC 
From: Lev Stipakov <lev@openvpn.net>

This is the 2.4 backport of master patch (commit d22ba6b).

NCP negotiation replaces worst cast crypto overhead
with actual one in data channel frame. That frame
params are used by mssfix.

Fragment frame still contains worst case overhead.
Because of that TCP packets are fragmented, since
MSS value exceeds max fragment size.

Fix by replacing worst case crypto overhead with
actual one for fragment frame, as it is done for data
channel frame.

Trac #1140

Signed-off-by: Lev Stipakov <lev@openvpn.net>
---
 v2: fix --disable-fragment build

 src/openvpn/forward.c |  3 +++
 src/openvpn/init.c    | 12 +++++++++++-
 src/openvpn/openvpn.h |  1 +
 src/openvpn/push.c    |  9 ++++++++-
 src/openvpn/ssl.c     | 19 ++++++++++++++++++-
 src/openvpn/ssl.h     | 13 ++++++++-----
 6 files changed, 49 insertions(+), 8 deletions(-)

Comments

Gert Doering Oct. 29, 2019, 11:31 p.m. UTC | #1 
Hi,

On Tue, Jul 23, 2019 at 05:34:47PM +0300, Lev Stipakov wrote:
> From: Lev Stipakov <lev@openvpn.net>
> 
> This is the 2.4 backport of master patch (commit d22ba6b).
> 
> NCP negotiation replaces worst cast crypto overhead
> with actual one in data channel frame. That frame
> params are used by mssfix.
> 
> Fragment frame still contains worst case overhead.
> Because of that TCP packets are fragmented, since
> MSS value exceeds max fragment size.
> 
> Fix by replacing worst case crypto overhead with
> actual one for fragment frame, as it is done for data
> channel frame.

Thanks for the backport.

Unfortunately, I can not reproduce the problem *without* the patch,
so I wonder if the issue got fixed by something else as side-effect?

If I do what you describe in the ticket:

run

 openvpn --fragment 1300 --mssfix

and grep out the MSS lines, I get (for IPv4, IPv6 numbers are -20 bytes):

with NCP:      MSS: 1460 -> 1203
--ncp-disable: MSS: 1284 -> 1211

... and the very same numbers from master...


OpenVPN versions tested:

OpenVPN 2.4.7 [git:release/2.4/416532f8e4125adb+] 
OpenVPN 2.5_git [git:master/b5fe104ddbbdf59b+]


So, I'm wondering how to proceed here...

gert
Lev Stipakov Oct. 30, 2019, 1:24 a.m. UTC | #2 
Hi,


> with NCP:      MSS: 1460 -> 1203
> --ncp-disable: MSS: 1284 -> 1211
>
> ... and the very same numbers from master...
>


> So, I'm wondering how to proceed here...
>

As it turns out, the fix is correct but explanation is not quite clear.

The problem is not in incorrect MSS value (it is correct), but
inside openvpn's fragmentation logic, which decides
"to fragment or not to fragment".

Without this patch, fragmentation logic incorrectly uses max crypto
overhead when calculating packet size. It exceeds fragment size and
openvpn peforms fragmentation:

> sudo tcpdump port 1194
13:59:06.956394 IP server.fi.openvpn > nat2.panoulu.net.openvpn: UDP,
length 652
13:59:06.956489 IP server.fi.openvpn > nat2.panoulu.net.openvpn: UDP,
length 648

With this patch, an actual crypto overhead is used and no unnecessary
fragmentation is performed:

> sudo tcpdump port 1194
13:58:08.685915 IP server.fi.openvpn > nat2.panoulu.net.openvpn: UDP,
length 1272
13:58:08.686007 IP server.fi.openvpn > nat2.panoulu.net.openvpn: UDP,
length 1272

I will send v3 with a better explanation.
diff mbox series

Patch

diff --git a/src/openvpn/forward.c b/src/openvpn/forward.c
index 65f790f..84bb584 100644
--- a/src/openvpn/forward.c
+++ b/src/openvpn/forward.c
@@ -873,6 +873,9 @@  process_incoming_link_part1(struct context *c, struct link_socket_info *lsi, boo
                 if (is_hard_reset(opcode, c->options.key_method))
                 {
                     c->c2.frame = c->c2.frame_initial;
+#ifdef ENABLE_FRAGMENT
+                    c->c2.frame_fragment = c->c2.frame_fragment_initial;
+#endif
                 }
 
                 interval_action(&c->c2.tmp_int);
diff --git a/src/openvpn/init.c b/src/openvpn/init.c
index d3785ca..37b832a 100644
--- a/src/openvpn/init.c
+++ b/src/openvpn/init.c
@@ -2294,9 +2294,18 @@  do_deferred_options(struct context *c, const unsigned int found)
         {
             tls_poor_mans_ncp(&c->options, c->c2.tls_multi->remote_ciphername);
         }
+        struct frame *frame_fragment = NULL;
+#ifdef ENABLE_FRAGMENT
+        if (c->options.ce.fragment)
+        {
+            frame_fragment = &c->c2.frame_fragment;
+        }
+#endif
+
         /* Do not regenerate keys if server sends an extra push reply */
         if (!session->key[KS_PRIMARY].crypto_options.key_ctx_bi.initialized
-            && !tls_session_update_crypto_params(session, &c->options, &c->c2.frame))
+            && !tls_session_update_crypto_params(session, &c->options, &c->c2.frame,
+                                                 frame_fragment))
         {
             msg(D_TLS_ERRORS, "OPTIONS ERROR: failed to import crypto options");
             return false;
@@ -3035,6 +3044,7 @@  do_init_frame(struct context *c)
      */
     c->c2.frame_fragment = c->c2.frame;
     frame_subtract_extra(&c->c2.frame_fragment, &c->c2.frame_fragment_omit);
+    c->c2.frame_fragment_initial = c->c2.frame_fragment;
 #endif
 
 #if defined(ENABLE_FRAGMENT) && defined(ENABLE_OCC)
diff --git a/src/openvpn/openvpn.h b/src/openvpn/openvpn.h
index 7736183..ed7975c 100644
--- a/src/openvpn/openvpn.h
+++ b/src/openvpn/openvpn.h
@@ -269,6 +269,7 @@  struct context_2
     /* Object to handle advanced MTU negotiation and datagram fragmentation */
     struct fragment_master *fragment;
     struct frame frame_fragment;
+    struct frame frame_fragment_initial;
     struct frame frame_fragment_omit;
 #endif
 
diff --git a/src/openvpn/push.c b/src/openvpn/push.c
index dd5bd41..ba2fbe4 100644
--- a/src/openvpn/push.c
+++ b/src/openvpn/push.c
@@ -287,11 +287,18 @@  incoming_push_message(struct context *c, const struct buffer *buffer)
     {
         if (c->options.mode == MODE_SERVER)
         {
+            struct frame *frame_fragment = NULL;
+#ifdef ENABLE_FRAGMENT
+            if (c->options.ce.fragment)
+            {
+                frame_fragment = &c->c2.frame_fragment;
+            }
+#endif
             struct tls_session *session = &c->c2.tls_multi->session[TM_ACTIVE];
             /* Do not regenerate keys if client send a second push request */
             if (!session->key[KS_PRIMARY].crypto_options.key_ctx_bi.initialized
                 && !tls_session_update_crypto_params(session, &c->options,
-                                                     &c->c2.frame))
+                                                     &c->c2.frame, frame_fragment))
             {
                 msg(D_TLS_ERRORS, "TLS Error: initializing data channel failed");
                 goto error;
diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
index 9696e9b..7dcd962 100644
--- a/src/openvpn/ssl.c
+++ b/src/openvpn/ssl.c
@@ -1962,7 +1962,8 @@  cleanup:
 
 bool
 tls_session_update_crypto_params(struct tls_session *session,
-                                 struct options *options, struct frame *frame)
+                                 struct options *options, struct frame *frame,
+                                 struct frame *frame_fragment)
 {
     if (!session->opt->server
         && 0 != strcmp(options->ciphername, session->opt->config_ciphername)
@@ -2006,6 +2007,22 @@  tls_session_update_crypto_params(struct tls_session *session,
     frame_init_mssfix(frame, options);
     frame_print(frame, D_MTU_INFO, "Data Channel MTU parms");
 
+    /*
+     * mssfix uses data channel framing, which at this point contains
+     * actual overhead. Fragmentation logic uses frame_fragment, which
+     * still contains worst case overhead. Replace it with actual overhead
+     * to prevent unneeded fragmentation.
+     */
+
+    if (frame_fragment)
+    {
+        frame_remove_from_extra_frame(frame_fragment, crypto_max_overhead());
+        crypto_adjust_frame_parameters(frame_fragment, &session->opt->key_type,
+                                       options->use_iv, options->replay, packet_id_long_form);
+        frame_set_mtu_dynamic(frame_fragment, options->ce.fragment, SET_MTU_UPPER_BOUND);
+        frame_print(frame_fragment, D_MTU_INFO, "Fragmentation MTU parms");
+    }
+
     return tls_session_generate_data_channel_keys(session);
 }
 
diff --git a/src/openvpn/ssl.h b/src/openvpn/ssl.h
index 8066789..6672d43 100644
--- a/src/openvpn/ssl.h
+++ b/src/openvpn/ssl.h
@@ -475,15 +475,18 @@  void tls_update_remote_addr(struct tls_multi *multi,
  * Update TLS session crypto parameters (cipher and auth) and derive data
  * channel keys based on the supplied options.
  *
- * @param session       The TLS session to update.
- * @param options       The options to use when updating session.
- * @param frame         The frame options for this session (frame overhead is
- *                      adjusted based on the selected cipher/auth).
+ * @param session         The TLS session to update.
+ * @param options         The options to use when updating session.
+ * @param frame           The frame options for this session (frame overhead is
+ *                        adjusted based on the selected cipher/auth).
+ * @param frame_fragment  The fragment frame options.
  *
  * @return true if updating succeeded, false otherwise.
  */
 bool tls_session_update_crypto_params(struct tls_session *session,
-                                      struct options *options, struct frame *frame);
+                                      struct options *options,
+                                      struct frame *frame,
+                                      struct frame *frame_fragment);
 
 /**
  * "Poor man's NCP": Use peer cipher if it is an allowed (NCP) cipher.