[M] Change in openvpn[master]: oob: Answer SERVER_PROBE on the server (P_CONTROL_OOB_V1)

Attention is currently required from: stipa.

Hello plaisthos, 

I'd like you to reexamine a change. Please visit

    http://gerrit.openvpn.net/c/openvpn/+/1744?usp=email

to look at the new patch set (#9).


Change subject: oob: Answer SERVER_PROBE on the server (P_CONTROL_OOB_V1)
......................................................................

oob: Answer SERVER_PROBE on the server (P_CONTROL_OOB_V1)

Make a --mode server UDP listener answer an out-of-band SERVER_PROBE without
creating a session:

  - tls_pre_decrypt_lite() accepts P_CONTROL_OOB_V1 and returns the new
    VERDICT_VALID_OOB_V1 verdict.
  - tls_wrap_oob_standalone() builds a session-less P_CONTROL_OOB_V1 packet:
    opcode + session id + the usual tls-auth/tls-crypt wrapping around a bare
    TLV payload (no reliability/ACK fields), mirroring tls_reset_standalone().
  - oob_client_reply_write() writes the PROBE_REPLY message (message-type
    header + probe_reply TLV).
  - do_pre_decrypt_check() handles the new verdict: it asks
    oob_build_probe_reply() whether to answer, and if so sends a PROBE_REPLY
    via send_probe_reply() (synchronous, stateless, like the HMAC reset path)
    and returns false so no session is created. The verdict joins the reset
    verdicts in the reflect_filter rate-limit check, so probes cannot be used
    to make the server answer faster than a reset flood could.

The reply's own session id is the stateless SYN-cookie used by the three-way
handshake, so the responder keeps no state and the reply can later serve as
the handshake reset (connect_lifetime advertisement).

Change-Id: I930d3789e0313aa0c3bc51ee5fd1d108343d59f0
Signed-off-by: Lev Stipakov <[email protected]>
---
M src/openvpn/mudp.c
M src/openvpn/oob.c
M src/openvpn/oob.h
M src/openvpn/ssl_pkt.c
M src/openvpn/ssl_pkt.h
5 files changed, 118 insertions(+), 4 deletions(-)


  git pull ssh://gerrit.openvpn.net:29418/openvpn refs/changes/44/1744/9

diff --git a/src/openvpn/mudp.c b/src/openvpn/mudp.c
index 2be085f..2209407 100644
--- a/src/openvpn/mudp.c
+++ b/src/openvpn/mudp.c
@@ -32,6 +32,7 @@
 
 #include "memdbg.h"
 #include "ssl_pkt.h"
+#include "oob.h"
 
 #ifdef HAVE_SYS_INOTIFY_H
 #include <sys/inotify.h>
@@ -87,6 +88,34 @@
                           "Reset packet from client, sending HMAC based reset challenge", sock);
 }
 
+/* Send an out-of-band PROBE_REPLY back to the source of a SERVER_PROBE,
+ * synchronously and without keeping any state, mirroring the reset path. */
+static void
+send_probe_reply(struct multi_context *m, struct tls_pre_decrypt_state *state,
+                 struct tls_auth_standalone *tas, const struct oob_probe_reply *reply,
+                 struct session_id *own_sid, struct link_socket *sock)
+{
+    struct gc_arena gc = gc_new();
+
+    /* Build the payload of the reply (message-type header + probe_reply TLV) */
+    struct buffer payload = alloc_buf_gc(128, &gc);
+    if (!oob_client_reply_write(&payload, reply))
+    {
+        gc_free(&gc);
+        return;
+    }
+
+    /* OOB replies use the same control-channel wrapping as the request */
+    reset_packet_id_send(&state->tls_wrap_tmp.opt.packet_id.send);
+    state->tls_wrap_tmp.opt.packet_id.rec.initialized = true;
+
+    struct buffer buf = tls_wrap_oob_standalone(&state->tls_wrap_tmp, tas, own_sid, &payload);
+    send_standalone_reply(m, &buf, "Server Probe", "Server probe from client, sending probe reply",
+                          sock);
+
+    gc_free(&gc);
+}
+
 
 /* Returns true if this packet should create a new session */
 static bool
@@ -105,7 +134,8 @@
     struct openvpn_sockaddr *from = &m->top.c2.from.dest;
     int handwindow = m->top.options.handshake_window;
 
-    if (verdict == VERDICT_VALID_RESET_V3 || verdict == VERDICT_VALID_RESET_V2)
+    if (verdict == VERDICT_VALID_RESET_V3 || verdict == VERDICT_VALID_RESET_V2
+        || verdict == VERDICT_VALID_OOB_V1)
     {
         /* Check if we are still below our limit for sending out
          * responses */
@@ -203,6 +233,31 @@
 
         return ret;
     }
+    else if (verdict == VERDICT_VALID_OOB_V1)
+    {
+        /* Out-of-band server probe. state->newbuf points at the TLV payload
+         * (read_control_auth has stripped the opcode, session id and any
+         * tls-auth/tls-crypt wrapping). Answer it without creating a session. */
+        struct oob_probe_reply reply;
+        if (!oob_build_probe_reply(&state->newbuf, (uint64_t)now, (uint64_t)handwindow,
+                                   &state->peer_session_id, &reply))
+        {
+            /* malformed or replayed/stale probe: silently drop */
+            return false;
+        }
+
+        /* Our session id is a stateless SYN cookie (the same HMAC the three-way
+         * handshake uses): we keep no per-probe state, and the reply can later
+         * also serve as the server's CONTROL_HARD_RESET_SERVER_V2, so a client
+         * can start the handshake from it (the connect_lifetime advertisement). */
+        struct session_id sid =
+            calculate_session_id_hmac(state->peer_session_id, from, hmac, handwindow, 0);
+
+        send_probe_reply(m, state, tas, &reply, &sid, sock);
+
+        /* An OOB probe never creates a session */
+        return false;
+    }
 
     /* VERDICT_INVALID */
     return false;
diff --git a/src/openvpn/oob.c b/src/openvpn/oob.c
index 930b213..c841d59 100644
--- a/src/openvpn/oob.c
+++ b/src/openvpn/oob.c
@@ -106,6 +106,12 @@
 }
 
 bool
+oob_client_reply_write(struct buffer *buf, const struct oob_probe_reply *reply)
+{
+    return buf_write_u16(buf, OOB_MSG_PROBE_REPLY) && oob_probe_reply_write(buf, reply);
+}
+
+bool
 oob_timestamp_in_window(uint64_t probe_ts, uint64_t now, uint64_t window_secs)
 {
     uint64_t diff = (now > probe_ts) ? (now - probe_ts) : (probe_ts - now);
diff --git a/src/openvpn/oob.h b/src/openvpn/oob.h
index 1afabbd..49d64e8 100644
--- a/src/openvpn/oob.h
+++ b/src/openvpn/oob.h
@@ -126,6 +126,12 @@
 bool oob_server_probe_read(struct buffer *payload, struct oob_probe_parameter *param);
 
 /**
+ * Write a complete PROBE_REPLY message (message-type header + probe_reply TLV)
+ * to buf. Sent by the server.
+ */
+bool oob_client_reply_write(struct buffer *buf, const struct oob_probe_reply *reply);
+
+/**
  * Check whether a probe timestamp is within an acceptable window around the
  * current time. Used to cheaply drop replayed or implausibly-timed probes
  * before doing any further work (see the probe_parameter timestamp rationale
diff --git a/src/openvpn/ssl_pkt.c b/src/openvpn/ssl_pkt.c
index 0d882d9..335e26f 100644
--- a/src/openvpn/ssl_pkt.c
+++ b/src/openvpn/ssl_pkt.c
@@ -313,7 +313,8 @@
 
     /* Allow only the reset packet or the first packet of the actual handshake. */
     if (op != P_CONTROL_HARD_RESET_CLIENT_V2 && op != P_CONTROL_HARD_RESET_CLIENT_V3
-        && op != P_CONTROL_V1 && op != P_CONTROL_WKC_V1 && op != P_ACK_V1)
+        && op != P_CONTROL_V1 && op != P_CONTROL_WKC_V1 && op != P_ACK_V1
+        && !opcode_is_oob(op))
     {
         /*
          * This can occur due to bogus data or DoS packets.
@@ -388,6 +389,10 @@
     {
         return VERDICT_VALID_WKC_V1;
     }
+    else if (opcode_is_oob(op))
+    {
+        return VERDICT_VALID_OOB_V1;
+    }
     else
     {
         return VERDICT_VALID_RESET_V2;
@@ -442,6 +447,29 @@
     return buf;
 }
 
+struct buffer
+tls_wrap_oob_standalone(struct tls_wrap_ctx *ctx, struct tls_auth_standalone *tas,
+                        struct session_id *own_sid, const struct buffer *payload)
+{
+    /* Copy buffer here to point at the same data but allow tls_wrap_control
+     * to potentially change buf to point to another buffer without
+     * modifying the buffer in tas */
+    struct buffer buf = tas->workbuf;
+    ASSERT(buf_init(&buf, tas->frame.buf.headroom));
+
+    /* Out-of-band messages carry the payload directly, with no reliability
+     * or ACK fields. */
+    ASSERT(buf_copy(&buf, payload));
+
+    uint8_t header = (uint8_t)(P_CONTROL_OOB_V1 << P_OPCODE_SHIFT);
+
+    /* Add tls-auth/tls-crypt wrapping, this might replace buf with
+     * ctx->work */
+    tls_wrap_control(ctx, header, &buf, own_sid);
+
+    return buf;
+}
+
 hmac_ctx_t *
 session_id_hmac_init(void)
 {
diff --git a/src/openvpn/ssl_pkt.h b/src/openvpn/ssl_pkt.h
index 25e50bf..3ab09db 100644
--- a/src/openvpn/ssl_pkt.h
+++ b/src/openvpn/ssl_pkt.h
@@ -62,8 +62,9 @@
  * control channel session (e.g. a server probe). Inherently unreliable:
  * there is no protocol-level retransmission.
  *
- * It is not legal on an established session; see opcode_valid_in_session()
- * below. */
+ * On the server this is accepted via the tls_pre_decrypt_lite() allowlist and
+ * handled in the new-connection path (it never creates a session). It is not
+ * legal on an established session; see opcode_valid_in_session() below. */
 #define P_CONTROL_OOB_V1 12
 
 /* define the range of defined opcodes, in- and out-of-band. Note this is not
@@ -136,6 +137,9 @@
     VERDICT_VALID_ACK_V1,
     /** The packet is a valid control packet with appended wrapped client key */
     VERDICT_VALID_WKC_V1,
+    /** This packet is a valid out-of-band control message (e.g. a server
+     * probe). It does not belong to a session and must not create one. */
+    VERDICT_VALID_OOB_V1,
     /** the packet failed on of the various checks */
     VERDICT_INVALID
 };
@@ -264,6 +268,21 @@
                                    struct session_id *own_sid, struct session_id *remote_sid,
                                    uint8_t header, bool request_resend_wkc);
 
+/**
+ * Wrap an already-built out-of-band payload (e.g. probe-reply TLVs) into a
+ * standalone, session-less P_CONTROL_OOB_V1 packet: it prepends the opcode and
+ * own_sid and applies the same tls-auth/tls-crypt wrapping as a regular
+ * control packet, but carries no reliability/ACK fields.
+ *
+ * @param ctx       tls wrapping context (from the pre-decrypt state)
+ * @param tas       standalone auth context providing the work buffer
+ * @param own_sid   session id to use as our session id in the header
+ * @param payload   the OOB message payload (TLV stream) to wrap
+ * @return          the wrapped packet buffer, ready to send
+ */
+struct buffer tls_wrap_oob_standalone(struct tls_wrap_ctx *ctx, struct tls_auth_standalone *tas,
+                                      struct session_id *own_sid, const struct buffer *payload);
+
 
 /**
  * Extracts a control channel message from buf and adjusts the size of

-- 
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Gerrit-MessageType: newpatchset
Gerrit-Project: openvpn
Gerrit-Branch: master
Gerrit-Change-Id: I930d3789e0313aa0c3bc51ee5fd1d108343d59f0
Gerrit-Change-Number: 1744
Gerrit-PatchSet: 9
Gerrit-Owner: stipa <[email protected]>
Gerrit-Reviewer: plaisthos <[email protected]>
Gerrit-CC: flichtenheld <[email protected]>
Gerrit-CC: openvpn-devel <[email protected]>
Gerrit-Attention: stipa <[email protected]>

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