crypto/tls: check cert chain during VerifyHostname

Fixes #9063.

Change-Id: I536ef1f0b30c94c1ebf7922d84cb2f701b7d8a1a
Reviewed-on: https://go-review.googlesource.com/12526Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/tls/conn.go

@@ -1025,5 +1025,8 @@ func (c *Conn) VerifyHostname(host string) error {
 	if !c.handshakeComplete {
 		return errors.New("tls: handshake has not yet been performed")
 	}
+	if len(c.verifiedChains) == 0 {
+		return errors.New("tls: handshake did not verify certificate chain")
+	}
 	return c.peerCertificates[0].VerifyHostname(host)
 }

src/crypto/tls/tls_test.go

@@ -7,6 +7,7 @@ package tls
 import (
 	"bytes"
 	"fmt"
+	"internal/testenv"
 	"io"
 	"net"
 	"strings"
@@ -280,3 +281,29 @@ func TestTLSUniqueMatches(t *testing.T) {
 		t.Error("client and server channel bindings differ when session resumption is used")
 	}
 }
+
+func TestVerifyHostname(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+
+	c, err := Dial("tcp", "www.google.com:https", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := c.VerifyHostname("www.google.com"); err != nil {
+		t.Fatalf("verify www.google.com: %v", err)
+	}
+	if err := c.VerifyHostname("www.yahoo.com"); err == nil {
+		t.Fatalf("verify www.yahoo.com succeeded")
+	}
+
+	c, err = Dial("tcp", "www.google.com:https", &Config{InsecureSkipVerify: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := c.VerifyHostname("www.google.com"); err == nil {
+		t.Fatalf("verify www.google.com succeeded with InsecureSkipVerify=true")
+	}
+	if err := c.VerifyHostname("www.yahoo.com"); err == nil {
+		t.Fatalf("verify www.google.com succeeded with InsecureSkipVerify=true")
+	}
+}