Keep the DTLS timer running after the end of the handshake if appropriate

During a full handshake the server is the last one to "speak". The timer
should continue to run until we know that the client has received our last
flight (e.g. because we receive some application data).
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6170)

ssl/record/rec_layer_d1.c

@@ -444,6 +444,19 @@ int dtls1_read_bytes(SSL *s, int type, int *recvd_type, unsigned char *buf,
             && SSL3_RECORD_get_length(rr) != 0)
         s->rlayer.alert_count = 0;
 
+    if (SSL3_RECORD_get_type(rr) != SSL3_RT_HANDSHAKE
+            && SSL3_RECORD_get_type(rr) != SSL3_RT_CHANGE_CIPHER_SPEC
+            && !SSL_in_init(s)
+            && (s->d1->next_timeout.tv_sec != 0
+                || s->d1->next_timeout.tv_usec != 0)) {
+        /*
+         * The timer is still running but we've received something that isn't
+         * handshake data - so the peer must have finished processing our
+         * last handshake flight. Stop the timer.
+         */
+        dtls1_stop_timer(s);
+    }
+
     /* we now have a packet which can be read and processed */
 
     if (s->s3->change_cipher_spec /* set when we receive ChangeCipherSpec,

ssl/statem/statem_lib.c

@@ -1057,6 +1057,15 @@ WORK_STATE tls_finish_handshake(SSL *s, WORK_STATE wst, int clearbufs, int stop)
             CRYPTO_atomic_add(&s->ctx->stats.sess_accept_good, 1, &discard,
                               s->ctx->lock);
             s->handshake_func = ossl_statem_accept;
+
+            if (SSL_IS_DTLS(s) && !s->hit) {
+                /*
+                 * We are finishing after the client. We start the timer going
+                 * in case there are any retransmits of our final flight
+                 * required.
+                 */
+                dtls1_start_timer(s);
+            }
         } else {
             /*
              * In TLSv1.3 we update the cache as part of processing the
@@ -1071,6 +1080,15 @@ WORK_STATE tls_finish_handshake(SSL *s, WORK_STATE wst, int clearbufs, int stop)
             s->handshake_func = ossl_statem_connect;
             CRYPTO_atomic_add(&s->session_ctx->stats.sess_connect_good, 1,
                               &discard, s->session_ctx->lock);
+
+            if (SSL_IS_DTLS(s) && s->hit) {
+                /*
+                 * We are finishing after the server. We start the timer going
+                 * in case there are any retransmits of our final flight
+                 * required.
+                 */
+                dtls1_start_timer(s);
+            }
         }
 
         if (SSL_IS_DTLS(s)) {