crypto/tls: Server can specify an unadvertised cipher suite

During the TLS handshake, check the cipher suite the server selects is
one of those offered in the ClientHello.  The code was checking it was
in the larger list that was sometimes whittled down for the ClientHello.

Fixes #13174

Change-Id: Iad8eebbcfa5027f30403b9700c43cfa949e135bb
Reviewed-on: https://go-review.googlesource.com/16698
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/crypto/tls/handshake_client_test.go

@@ -19,6 +19,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strconv"
+	"strings"
 	"testing"
 	"time"
 )
@@ -643,3 +644,53 @@ func TestNoIPAddressesInSNI(t *testing.T) {
 		}
 	}
 }
+
+func TestServerSelectingUnconfiguredCipherSuite(t *testing.T) {
+	// This checks that the server can't select a cipher suite that the
+	// client didn't offer. See #13174.
+
+	c, s := net.Pipe()
+	errChan := make(chan error, 1)
+
+	go func() {
+		client := Client(c, &Config{
+			ServerName:   "foo",
+			CipherSuites: []uint16{TLS_RSA_WITH_AES_128_GCM_SHA256},
+		})
+		errChan <- client.Handshake()
+	}()
+
+	var header [5]byte
+	if _, err := io.ReadFull(s, header[:]); err != nil {
+		t.Fatal(err)
+	}
+	recordLen := int(header[3])<<8 | int(header[4])
+
+	record := make([]byte, recordLen)
+	if _, err := io.ReadFull(s, record); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create a ServerHello that selects a different cipher suite than the
+	// sole one that the client offered.
+	serverHello := &serverHelloMsg{
+		vers:        VersionTLS12,
+		random:      make([]byte, 32),
+		cipherSuite: TLS_RSA_WITH_AES_256_GCM_SHA384,
+	}
+	serverHelloBytes := serverHello.marshal()
+
+	s.Write([]byte{
+		byte(recordTypeHandshake),
+		byte(VersionTLS12 >> 8),
+		byte(VersionTLS12 & 0xff),
+		byte(len(serverHelloBytes) >> 8),
+		byte(len(serverHelloBytes)),
+	})
+	s.Write(serverHelloBytes)
+	s.Close()
+
+	if err := <-errChan; !strings.Contains(err.Error(), "unconfigured cipher") {
+		t.Fatalf("Expected error about unconfigured cipher suite but got %q", err)
+	}
+}