crypto/tls: enforce 1.3 record version semantics

1.3 expects the record version is always 1.2 (0x0303), this previously
wasn't enforced.

Change-Id: I8bc88f588e76f9b862b57601336bb5c5ff08b30e
Reviewed-on: https://go-review.googlesource.com/c/go/+/485876
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

conn.go

@@ -639,10 +639,16 @@ func (c *Conn) readRecordOrCCS(expectChangeCipherSpec bool) error {
 	}
 
 	vers := uint16(hdr[1])<<8 | uint16(hdr[2])
+	expectedVers := c.vers
+	if expectedVers == VersionTLS13 {
+		// All TLS 1.3 records are expected to have 0x0303 (1.2) after
+		// the initial hello (RFC 8446 Section 5.1).
+		expectedVers = VersionTLS12
+	}
 	n := int(hdr[3])<<8 | int(hdr[4])
-	if c.haveVers && c.vers != VersionTLS13 && vers != c.vers {
+	if c.haveVers && vers != expectedVers {
 		c.sendAlert(alertProtocolVersion)
-		msg := fmt.Sprintf("received record with version %x when expecting version %x", vers, c.vers)
+		msg := fmt.Sprintf("received record with version %x when expecting version %x", vers, expectedVers)
 		return c.in.setErrorLocked(c.newRecordHeaderError(nil, msg))
 	}
 	if !c.haveVers {

conn_test.go

@@ -285,3 +285,35 @@ func TestHairpinInClose(t *testing.T) {
 	// This call should not deadlock.
 	tlsConn.Close()
 }
+
+func TestRecordBadVersionTLS13(t *testing.T) {
+	client, server := localPipe(t)
+	defer server.Close()
+	defer client.Close()
+
+	config := testConfig.Clone()
+	config.MinVersion, config.MaxVersion = VersionTLS13, VersionTLS13
+
+	go func() {
+		tlsConn := Client(client, config)
+		if err := tlsConn.Handshake(); err != nil {
+			t.Errorf("Error from client handshake: %v", err)
+			return
+		}
+		tlsConn.vers = 0x1111
+		tlsConn.Write([]byte{1})
+	}()
+
+	tlsConn := Server(server, config)
+	if err := tlsConn.Handshake(); err != nil {
+		t.Errorf("Error from client handshake: %v", err)
+		return
+	}
+
+	expectedErr := "tls: received record with version 1111 when expecting version 303"
+
+	_, err := tlsConn.Read(make([]byte, 10))
+	if err.Error() != expectedErr {
+		t.Fatalf("unexpected error: got %q, want %q", err, expectedErr)
+	}
+}