Defer sending a KeyUpdate until after pending writes are complete

If we receive a KeyUpdate message (update requested) from the peer while
we are in the middle of a write, we should defer sending the responding
KeyUpdate message until after the current write is complete. We do this
by waiting to send the KeyUpdate until the next time we write and there is
no pending write data.

This does imply a subtle change in behaviour. Firstly the responding
KeyUpdate message won't be sent straight away as it is now. Secondly if
the peer sends multiple KeyUpdates without us doing any writing then we
will only send one response, as opposed to previously where we sent a
response for each KeyUpdate received.

Fixes #8677

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/8773)

ssl/record/rec_layer_s3.c

@@ -377,6 +377,13 @@ int ssl3_write_bytes(SSL *s, int type, const void *buf_, size_t len,
 
     s->rlayer.wnum = 0;
 
+    /*
+     * If we are supposed to be sending a KeyUpdate then go into init unless we
+     * have writes pending - in which case we should finish doing that first.
+     */
+    if (wb->left == 0 && s->key_update != SSL_KEY_UPDATE_NONE)
+        ossl_statem_set_in_init(s, 1);
+
     /*
      * When writing early data on the server side we could be "in_init" in
      * between receiving the EoED and the CF - but we don't want to handle those

ssl/statem/statem_clnt.c

@@ -474,12 +474,6 @@ static WRITE_TRAN ossl_statem_client13_write_transition(SSL *s)
         return WRITE_TRAN_CONTINUE;
 
     case TLS_ST_CR_KEY_UPDATE:
-        if (s->key_update != SSL_KEY_UPDATE_NONE) {
-            st->hand_state = TLS_ST_CW_KEY_UPDATE;
-            return WRITE_TRAN_CONTINUE;
-        }
-        /* Fall through */
-
     case TLS_ST_CW_KEY_UPDATE:
     case TLS_ST_CR_SESSION_TICKET:
     case TLS_ST_CW_FINISHED:

ssl/statem/statem_lib.c

@@ -643,12 +643,9 @@ MSG_PROCESS_RETURN tls_process_key_update(SSL *s, PACKET *pkt)
     /*
      * If we get a request for us to update our sending keys too then, we need
      * to additionally send a KeyUpdate message. However that message should
-     * not also request an update (otherwise we get into an infinite loop). We
-     * ignore a request for us to update our sending keys too if we already
-     * sent close_notify.
+     * not also request an update (otherwise we get into an infinite loop).
      */
-    if (updatetype == SSL_KEY_UPDATE_REQUESTED
-            && (s->shutdown & SSL_SENT_SHUTDOWN) == 0)
+    if (updatetype == SSL_KEY_UPDATE_REQUESTED)
         s->key_update = SSL_KEY_UPDATE_NOT_REQUESTED;
 
     if (!tls13_update_key(s, 0)) {

ssl/statem/statem_srvr.c

@@ -503,12 +503,6 @@ static WRITE_TRAN ossl_statem_server13_write_transition(SSL *s)
         return WRITE_TRAN_CONTINUE;
 
     case TLS_ST_SR_KEY_UPDATE:
-        if (s->key_update != SSL_KEY_UPDATE_NONE) {
-            st->hand_state = TLS_ST_SW_KEY_UPDATE;
-            return WRITE_TRAN_CONTINUE;
-        }
-        /* Fall through */
-
     case TLS_ST_SW_KEY_UPDATE:
         st->hand_state = TLS_ST_OK;
         return WRITE_TRAN_CONTINUE;