ipn: add Login backend command for sign-in with token

The StartLoginInteractive command is for delegating the sign-in flow
to a browser. The Android Gooogle Sign-In SDK inverts the flow by
giving the client ID tokens.

Add a new backend command for accepting such tokens by exposing the existing
controlclient.Client.Login support for OAuth2 tokens. Introduce a custom
TokenType to distinguish ID tokens from other OAuth2 tokens.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ipn/backend.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"time"
 
+	"golang.org/x/oauth2"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/tailcfg"
@@ -27,6 +28,10 @@ const (
 	Running
 )
 
+// GoogleIDToken Type is the oauth2.Token.TokenType for the Google
+// ID tokens used by the Android client.
+const GoogleIDTokenType = "ts_android_google_login"
+
 func (s State) String() string {
 	return [...]string{"NoState", "NeedsLogin", "NeedsMachineAuth",
 		"Stopped", "Starting", "Running"}[s]
@@ -129,6 +134,8 @@ type Backend interface {
 	// flow. This should trigger a new BrowseToURL notification
 	// eventually.
 	StartLoginInteractive()
+	// Login logs in with an OAuth2 token.
+	Login(token *oauth2.Token)
 	// Logout terminates the current login session and stops the
 	// wireguard engine.
 	Logout()

ipn/fake_test.go

@@ -8,6 +8,7 @@ import (
 	"log"
 	"time"
 
+	"golang.org/x/oauth2"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/ipn/ipnstate"
 )
@@ -42,6 +43,14 @@ func (b *FakeBackend) newState(s State) {
 func (b *FakeBackend) StartLoginInteractive() {
 	u := b.serverURL + "/this/is/fake"
 	b.notify(Notify{BrowseToURL: &u})
+	b.login()
+}
+
+func (b *FakeBackend) Login(token *oauth2.Token) {
+	b.login()
+}
+
+func (b *FakeBackend) login() {
 	b.newState(NeedsMachineAuth)
 	b.newState(Stopped)
 	// TODO(apenwarr): Fill in a more interesting netmap here.

ipn/handle.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/tailscale/wireguard-go/wgcfg"
+	"golang.org/x/oauth2"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/types/logger"
 )
@@ -154,6 +155,10 @@ func (h *Handle) StartLoginInteractive() {
 	h.b.StartLoginInteractive()
 }
 
+func (h *Handle) Login(token *oauth2.Token) {
+	h.b.Login(token)
+}
+
 func (h *Handle) Logout() {
 	h.b.Logout()
 }

ipn/local.go

@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"github.com/tailscale/wireguard-go/wgcfg"
+	"golang.org/x/oauth2"
 	"inet.af/netaddr"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/ipn/ipnstate"
@@ -612,6 +613,16 @@ func (b *LocalBackend) getEngineStatus() EngineStatus {
 	return b.engineStatus
 }
 
+// Login implements Backend.
+func (b *LocalBackend) Login(token *oauth2.Token) {
+	b.mu.Lock()
+	b.assertClientLocked()
+	c := b.c
+	b.mu.Unlock()
+
+	c.Login(token, controlclient.LoginInteractive)
+}
+
 // StartLoginInteractive implements Backend. It requests a new
 // interactive login from controlclient, unless such a flow is already
 // in progress, in which case StartLoginInteractive attempts to pick

ipn/message.go

@@ -13,6 +13,7 @@ import (
 	"log"
 	"time"
 
+	"golang.org/x/oauth2"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/structs"
 	"tailscale.com/version"
@@ -49,6 +50,7 @@ type Command struct {
 	Quit                  *NoArgs
 	Start                 *StartArgs
 	StartLoginInteractive *NoArgs
+	Login                 *oauth2.Token
 	Logout                *NoArgs
 	SetPrefs              *SetPrefsArgs
 	RequestEngineStatus   *NoArgs
@@ -128,6 +130,9 @@ func (bs *BackendServer) GotCommand(cmd *Command) error {
 	} else if c := cmd.StartLoginInteractive; c != nil {
 		bs.b.StartLoginInteractive()
 		return nil
+	} else if c := cmd.Login; c != nil {
+		bs.b.Login(c)
+		return nil
 	} else if c := cmd.Logout; c != nil {
 		bs.b.Logout()
 		return nil
@@ -225,6 +230,10 @@ func (bc *BackendClient) StartLoginInteractive() {
 	bc.send(Command{StartLoginInteractive: &NoArgs{}})
 }
 
+func (bc *BackendClient) Login(token *oauth2.Token) {
+	bc.send(Command{Login: token})
+}
+
 func (bc *BackendClient) Logout() {
 	bc.send(Command{Logout: &NoArgs{}})
 }

ipn/message_test.go

@@ -9,6 +9,7 @@ import (
 	"testing"
 	"time"
 
+	"golang.org/x/oauth2"
 	"tailscale.com/tstest"
 )
 
@@ -177,4 +178,10 @@ func TestClientServer(t *testing.T) {
 
 	h.Logout()
 	flushUntil(NeedsLogin)
+
+	h.Login(&oauth2.Token{
+		AccessToken: "google_id_token",
+		TokenType:   GoogleIDTokenType,
+	})
+	flushUntil(Running)
 }