Send dummy ChangeCipherSpec messages from the TLSv1.3 server

If the client has requested middle box compatibility mode by sending
a non-empty legacy_session_id, the server must send a dummy CCS right
after its first handshake message. This means right after ServerHello
or HelloRetryRequest.

Two important improvements over the backed-out diffr: make sure that
First: client and server can send their dummy CCS at the correct moment
(right before the next flight or right after the current flight).
Second: as jsing noted, we also need to deal with the corner case that
tls13_send_dummy_ccs() can return TLS13_IO_WANT_POLLOUT.

with/ok jsing

1 files changed, 14 insertions, 1 deletions

diff --git a/src/lib/libssl/tls13_handshake.c b/src/lib/libssl/tls13_handshake.c
index a09659bffc..11fc1db9f9 100644
--- a/src/lib/libssl/tls13_handshake.c
+++ b/src/lib/libssl/tls13_handshake.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: tls13_handshake.c,v 1.61 2020/05/10 16:56:11 jsing Exp $      */
+/*      $OpenBSD: tls13_handshake.c,v 1.62 2020/05/10 17:13:29 tb Exp $ */
 /*
  * Copyright (c) 2018-2019 Theo Buehler <tb@openbsd.org>
  * Copyright (c) 2019 Joel Sing <jsing@openbsd.org>
@@ -102,6 +102,7 @@ static const struct tls13_handshake_action state_machine[] = {
                 .sender = TLS13_HS_SERVER,
                 .send = tls13_server_hello_retry_request_send,
                 .recv = tls13_server_hello_retry_request_recv,
+                .sent = tls13_server_hello_retry_request_sent,
         },
         [SERVER_ENCRYPTED_EXTENSIONS] = {
                 .handshake_type = TLS13_MT_ENCRYPTED_EXTENSIONS,
@@ -391,6 +392,10 @@ tls13_handshake_send_action(struct tls13_ctx *ctx,
                 if ((ret = tls13_send_dummy_ccs(ctx->rl)) != TLS13_IO_SUCCESS)
                         return ret;
                 ctx->send_dummy_ccs = 0;
+                if (ctx->send_dummy_ccs_after) {
+                        ctx->send_dummy_ccs_after = 0;
+                        return TLS13_IO_SUCCESS;
+                }
         }
 
         /* If we have no handshake message, we need to build one. */
@@ -428,6 +433,14 @@ tls13_handshake_send_action(struct tls13_ctx *ctx,
         if (action->sent != NULL && !action->sent(ctx))
                 return TLS13_IO_FAILURE;
 
+        if (ctx->send_dummy_ccs_after) {
+                ctx->send_dummy_ccs = 1;
+                if ((ret = tls13_send_dummy_ccs(ctx->rl)) != TLS13_IO_SUCCESS)
+                        return ret;
+                ctx->send_dummy_ccs = 0;
+                ctx->send_dummy_ccs_after = 0;
+        }
+
         return TLS13_IO_SUCCESS;
 }