Don't fail on an out-of-order CCS in DTLS

Fixes #4929
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6170)

ssl/statem/statem.c

@@ -589,10 +589,8 @@ static SUB_STATE_RETURN read_state_machine(SSL *s)
              * Validate that we are allowed to move to the new state and move
              * to that state if so
              */
-            if (!transition(s, mt)) {
-                check_fatal(s, SSL_F_READ_STATE_MACHINE);
+            if (!transition(s, mt))
                 return SUB_STATE_ERROR;
-            }
 
             if (s->s3->tmp.message_size > max_message_size(s)) {
                 SSLfatal(s, SSL_AD_ILLEGAL_PARAMETER, SSL_F_READ_STATE_MACHINE,

ssl/statem/statem_clnt.c

@@ -375,6 +375,20 @@ int ossl_statem_client_read_transition(SSL *s, int mt)
 
  err:
     /* No valid transition found */
+    if (SSL_IS_DTLS(s) && mt == SSL3_MT_CHANGE_CIPHER_SPEC) {
+        BIO *rbio;
+
+        /*
+         * CCS messages don't have a message sequence number so this is probably
+         * because of an out-of-order CCS. We'll just drop it.
+         */
+        s->init_num = 0;
+        s->rwstate = SSL_READING;
+        rbio = SSL_get_rbio(s);
+        BIO_clear_retry_flags(rbio);
+        BIO_set_retry_read(rbio);
+        return 0;
+    }
     SSLfatal(s, SSL3_AD_UNEXPECTED_MESSAGE,
              SSL_F_OSSL_STATEM_CLIENT_READ_TRANSITION,
              SSL_R_UNEXPECTED_MESSAGE);

ssl/statem/statem_srvr.c

@@ -277,6 +277,20 @@ int ossl_statem_server_read_transition(SSL *s, int mt)
 
  err:
     /* No valid transition found */
+    if (SSL_IS_DTLS(s) && mt == SSL3_MT_CHANGE_CIPHER_SPEC) {
+        BIO *rbio;
+
+        /*
+         * CCS messages don't have a message sequence number so this is probably
+         * because of an out-of-order CCS. We'll just drop it.
+         */
+        s->init_num = 0;
+        s->rwstate = SSL_READING;
+        rbio = SSL_get_rbio(s);
+        BIO_clear_retry_flags(rbio);
+        BIO_set_retry_read(rbio);
+        return 0;
+    }
     SSLfatal(s, SSL3_AD_UNEXPECTED_MESSAGE,
              SSL_F_OSSL_STATEM_SERVER_READ_TRANSITION,
              SSL_R_UNEXPECTED_MESSAGE);