crypto/tls: reject CT extension with no SCTs included

When the CT extension is enabled but no SCTs are present, the existing
code calls "continue" which causes resizing the data byte slice to be
skipped. In fact, such extensions should be rejected.

Fixes #17958

Change-Id: Iad12da10d1ea72d04ae2e1012c28bb2636f06bcd
Reviewed-on: https://go-review.googlesource.com/33265
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

handshake_messages.go

@@ -802,12 +802,9 @@ func (m *serverHelloMsg) unmarshal(data []byte) bool {
 			}
 			l := int(d[0])<<8 | int(d[1])
 			d = d[2:]
-			if len(d) != l {
+			if len(d) != l || l == 0 {
 				return false
 			}
-			if l == 0 {
-				continue
-			}
 
 			m.scts = make([][]byte, 0, 3)
 			for len(d) != 0 {

handshake_messages_test.go

@@ -5,6 +5,7 @@
 package tls
 
 import (
+	"bytes"
 	"math/rand"
 	"reflect"
 	"testing"
@@ -260,3 +261,47 @@ func (*sessionState) Generate(rand *rand.Rand, size int) reflect.Value {
 	}
 	return reflect.ValueOf(s)
 }
+
+func TestRejectEmptySCTList(t *testing.T) {
+	// https://tools.ietf.org/html/rfc6962#section-3.3.1 specifies that
+	// empty SCT lists are invalid.
+
+	var random [32]byte
+	sct := []byte{0x42, 0x42, 0x42, 0x42}
+	serverHello := serverHelloMsg{
+		vers:   VersionTLS12,
+		random: random[:],
+		scts:   [][]byte{sct},
+	}
+	serverHelloBytes := serverHello.marshal()
+
+	var serverHelloCopy serverHelloMsg
+	if !serverHelloCopy.unmarshal(serverHelloBytes) {
+		t.Fatal("Failed to unmarshal initial message")
+	}
+
+	// Change serverHelloBytes so that the SCT list is empty
+	i := bytes.Index(serverHelloBytes, sct)
+	if i < 0 {
+		t.Fatal("Cannot find SCT in ServerHello")
+	}
+
+	var serverHelloEmptySCT []byte
+	serverHelloEmptySCT = append(serverHelloEmptySCT, serverHelloBytes[:i-6]...)
+	// Append the extension length and SCT list length for an empty list.
+	serverHelloEmptySCT = append(serverHelloEmptySCT, []byte{0, 2, 0, 0}...)
+	serverHelloEmptySCT = append(serverHelloEmptySCT, serverHelloBytes[i+4:]...)
+
+	// Update the handshake message length.
+	serverHelloEmptySCT[1] = byte((len(serverHelloEmptySCT) - 4) >> 16)
+	serverHelloEmptySCT[2] = byte((len(serverHelloEmptySCT) - 4) >> 8)
+	serverHelloEmptySCT[3] = byte(len(serverHelloEmptySCT) - 4)
+
+	// Update the extensions length
+	serverHelloEmptySCT[42] = byte((len(serverHelloEmptySCT) - 44) >> 8)
+	serverHelloEmptySCT[43] = byte((len(serverHelloEmptySCT) - 44))
+
+	if serverHelloCopy.unmarshal(serverHelloEmptySCT) {
+		t.Fatal("Unmarshaled ServerHello with empty SCT list")
+	}
+}