Don't calculate the Finished MAC twice

In <= TLSv1.2 a Finished message always comes immediately after a CCS
except in the case of NPN where there is an additional message between
the CCS and Finished. Historically we always calculated the Finished MAC
when we processed the CCS. However to deal with NPN we also calculated it
when we receive the Finished message. Really this should only have been
done if we hand negotiated NPN.

This simplifies the code to only calculate the MAC when we receive the
Finished. In 1.1.1 we need to do it this way anyway because there is no
CCS (except in middlebox compat mode) in TLSv1.3.

Coincidentally, this commit also fixes the fact that no-nextprotoneg does
not currently work in master.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5285)

ssl/s3_msg.c

@@ -12,9 +12,6 @@
 int ssl3_do_change_cipher_spec(SSL *s)
 {
     int i;
-    size_t finish_md_len;
-    const char *sender;
-    size_t slen;
 
     if (s->server)
         i = SSL3_CHANGE_CIPHER_SERVER_READ;
@@ -36,26 +33,6 @@ int ssl3_do_change_cipher_spec(SSL *s)
     if (!s->method->ssl3_enc->change_cipher_state(s, i))
         return 0;
 
-    /*
-     * we have to record the message digest at this point so we can get it
-     * before we read the finished message
-     */
-    if (!s->server) {
-        sender = s->method->ssl3_enc->server_finished_label;
-        slen = s->method->ssl3_enc->server_finished_label_len;
-    } else {
-        sender = s->method->ssl3_enc->client_finished_label;
-        slen = s->method->ssl3_enc->client_finished_label_len;
-    }
-
-    finish_md_len = s->method->ssl3_enc->final_finish_mac(s, sender, slen,
-                                            s->s3->tmp.peer_finish_md);
-    if (finish_md_len == 0) {
-        SSLerr(SSL_F_SSL3_DO_CHANGE_CIPHER_SPEC, ERR_R_INTERNAL_ERROR);
-        return 0;
-    }
-    s->s3->tmp.peer_finish_md_len = finish_md_len;
-
     return 1;
 }
 

ssl/statem/statem_dtls.c

@@ -376,6 +376,15 @@ int dtls_get_message(SSL *s, int *mt, size_t *len)
         msg_len += DTLS1_HM_HEADER_LENGTH;
     }
 
+    /*
+     * If receiving Finished, record MAC of prior handshake messages for
+     * Finished verification.
+     */
+    if (*mt == SSL3_MT_FINISHED && !ssl3_take_mac(s)) {
+        /* SSLfatal() already called */
+        return 0;
+    }
+
     if (!ssl3_finish_mac(s, p, msg_len))
         return 0;
     if (s->msg_callback)

ssl/statem/statem_lib.c

@@ -646,21 +646,15 @@ MSG_PROCESS_RETURN tls_process_key_update(SSL *s, PACKET *pkt)
     return MSG_PROCESS_FINISHED_READING;
 }
 
-#ifndef OPENSSL_NO_NEXTPROTONEG
 /*
  * ssl3_take_mac calculates the Finished MAC for the handshakes messages seen
  * to far.
  */
-static void ssl3_take_mac(SSL *s)
+int ssl3_take_mac(SSL *s)
 {
     const char *sender;
     size_t slen;
-    /*
-     * If no new cipher setup return immediately: other functions will set
-     * the appropriate error.
-     */
-    if (s->s3->tmp.new_cipher == NULL)
-        return;
+
     if (!s->server) {
         sender = s->method->ssl3_enc->server_finished_label;
         slen = s->method->ssl3_enc->server_finished_label_len;
@@ -669,12 +663,17 @@ static void ssl3_take_mac(SSL *s)
         slen = s->method->ssl3_enc->client_finished_label_len;
     }
 
-    s->s3->tmp.peer_finish_md_len = s->method->ssl3_enc->final_finish_mac(s,
-                                                                          sender,
-                                                                          slen,
-                                                                          s->s3->tmp.peer_finish_md);
+    s->s3->tmp.peer_finish_md_len =
+        s->method->ssl3_enc->final_finish_mac(s, sender, slen,
+                                              s->s3->tmp.peer_finish_md);
+
+    if (s->s3->tmp.peer_finish_md_len == 0) {
+        /* SSLfatal() already called */
+        return 0;
+    }
+
+    return 1;
 }
-#endif
 
 MSG_PROCESS_RETURN tls_process_change_cipher_spec(SSL *s, PACKET *pkt)
 {
@@ -1227,14 +1226,15 @@ int tls_get_message_body(SSL *s, size_t *len)
         n -= readbytes;
     }
 
-#ifndef OPENSSL_NO_NEXTPROTONEG
     /*
      * If receiving Finished, record MAC of prior handshake messages for
      * Finished verification.
      */
-    if (*s->init_buf->data == SSL3_MT_FINISHED)
-        ssl3_take_mac(s);
-#endif
+    if (*(s->init_buf->data) == SSL3_MT_FINISHED && !ssl3_take_mac(s)) {
+        /* SSLfatal() already called */
+        *len = 0;
+        return 0;
+    }
 
     /* Feed this message into MAC computation. */
     if (RECORD_LAYER_is_sslv2_record(&s->rlayer)) {

ssl/statem/statem_locl.h

@@ -54,6 +54,7 @@ typedef enum {
 
 typedef int (*confunc_f) (SSL *s, WPACKET *pkt);
 
+int ssl3_take_mac(SSL *s);
 int check_in_list(SSL *s, uint16_t group_id, const uint16_t *groups,
                   size_t num_groups, int checkallow);
 int create_synthetic_message_hash(SSL *s, const unsigned char *hashval,