net/tlsdial: add memory-optimized TLS cert verification path for iOS

Behind a build tag for now.

control/controlclient/direct.go

@@ -16,6 +16,7 @@
 	"io/ioutil"
 	"log"
 	"net/http"
+	"net/url"
 	"os"
 	"reflect"
 	"strconv"
@@ -114,6 +115,10 @@ func NewDirect(opts Options) (*Direct, error) {
 		return nil, errors.New("controlclient.New: no server URL specified")
 	}
 	opts.ServerURL = strings.TrimRight(opts.ServerURL, "/")
+	serverURL, err := url.Parse(opts.ServerURL)
+	if err != nil {
+		return nil, err
+	}
 	if opts.TimeNow == nil {
 		opts.TimeNow = time.Now
 	}
@@ -125,7 +130,7 @@ func NewDirect(opts Options) (*Direct, error) {
 
 	tr := http.DefaultTransport.(*http.Transport).Clone()
 	tr.ForceAttemptHTTP2 = true
-	tr.TLSClientConfig = tlsdial.Config("", tr.TLSClientConfig)
+	tr.TLSClientConfig = tlsdial.Config(serverURL.Host, tr.TLSClientConfig)
 	httpc := &http.Client{Transport: tr}
 
 	c := &Direct{

net/tlsdial/tlsdial.go

@@ -8,6 +8,8 @@
 
 import "crypto/tls"
 
+var platformModifyConf func(*tls.Config)
+
 // Config returns a tls.Config for dialing the given host.
 // If base is non-nil, it's cloned as the base config before
 // being configured and returned.
@@ -20,5 +22,9 @@ func Config(host string, base *tls.Config) *tls.Config {
 	}
 	conf.ServerName = host
 
+	if platformModifyConf != nil {
+		platformModifyConf(conf)
+	}
+
 	return conf
 }

net/tlsdial/verify_darwin_arm64.go

@@ -0,0 +1,58 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin,arm64,usex509fork
+
+package tlsdial
+
+import (
+	"crypto/tls"
+	"errors"
+	"time"
+
+	"crypto/x509"
+
+	x509fork "tailscale.com/tempfork/x509"
+)
+
+func init() {
+	platformModifyConf = useX509Fork
+}
+
+func useX509Fork(conf *tls.Config) {
+	// Modify conf to use our fork of crypto/x509 instead.
+
+	// This prevents crypto/tls from using the standard library's
+	// x509. We will then be responsible for the rest.
+	conf.InsecureSkipVerify = true
+
+	// Do what crypto/tls would've done for us:
+	conf.VerifyPeerCertificate = func(rawCerts [][]byte, _verifiedChains [][]*x509.Certificate) error {
+		if conf.ServerName == "" {
+			return errors.New("no tls.Config.ServerName set")
+		}
+		if len(rawCerts) == 0 {
+			// Shouldn't happen, but.
+			return errors.New("no rawCerts from server")
+		}
+		certs := make([]*x509fork.Certificate, len(rawCerts))
+		for i, asn1Data := range rawCerts {
+			cert, err := x509fork.ParseCertificate(asn1Data)
+			if err != nil {
+				return err
+			}
+			certs[i] = cert
+		}
+		opts := x509fork.VerifyOptions{
+			CurrentTime:   time.Now(),
+			DNSName:       conf.ServerName,
+			Intermediates: x509fork.NewCertPool(),
+		}
+		for _, cert := range certs[1:] {
+			opts.Intermediates.AddCert(cert)
+		}
+		_, err := certs[0].Verify(opts)
+		return err
+	}
+}