ssh/tailssh: terminate sessions when tailscaled shutsdown

Ideally we would re-establish these sessions when tailscaled comes back
up, however we do not do that yet so this is better than leaking the
sessions.

Updates #3802

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/ipnlocal/local.go

@@ -81,6 +81,9 @@ type SSHServer interface {
 	// so that existing sessions can be re-evaluated for validity
 	// and closed if they'd no longer be accepted.
 	OnPolicyChange()
+
+	// Shutdown is called when tailscaled is shutting down.
+	Shutdown()
 }
 
 type newSSHServerFunc func(logger.Logf, *LocalBackend) (SSHServer, error)
@@ -346,6 +349,9 @@ func (b *LocalBackend) Shutdown() {
 	b.mu.Lock()
 	b.shutdownCalled = true
 	cc := b.cc
+	if b.sshServer != nil {
+		b.sshServer.Shutdown()
+	}
 	b.closePeerAPIListenersLocked()
 	b.mu.Unlock()
 

ssh/tailssh/tailssh.go

@@ -58,11 +58,14 @@ type server struct {
 	pubKeyHTTPClient *http.Client     // or nil for http.DefaultClient
 	timeNow          func() time.Time // or nil for time.Now
 
+	sessionWaitGroup sync.WaitGroup
+
 	// mu protects the following
 	mu                      sync.Mutex
 	activeSessionByH        map[string]*sshSession      // ssh.SessionID (DH H) => session
 	activeSessionBySharedID map[string]*sshSession      // yyymmddThhmmss-XXXXX => session
 	fetchPublicKeysCache    map[string]pubKeyCacheEntry // by https URL
+	shutdownCalled          bool
 }
 
 func (srv *server) now() time.Time {
@@ -101,6 +104,20 @@ func (srv *server) HandleSSHConn(c net.Conn) error {
 	return nil
 }
 
+// Shutdown terminates all active sessions.
+func (srv *server) Shutdown() {
+	srv.mu.Lock()
+	srv.shutdownCalled = true
+	for _, s := range srv.activeSessionByH {
+		s.ctx.CloseWithError(userVisibleError{
+			fmt.Sprintf("Tailscale shutting down.\r\n"),
+			context.Canceled,
+		})
+	}
+	srv.mu.Unlock()
+	srv.sessionWaitGroup.Wait()
+}
+
 // OnPolicyChange terminates any active sessions that no longer match
 // the SSH access policy.
 func (srv *server) OnPolicyChange() {
@@ -227,6 +244,15 @@ func (c *conn) ServerConfig(ctx ssh.Context) *gossh.ServerConfig {
 }
 
 func (srv *server) newConn() (*conn, error) {
+	srv.mu.Lock()
+	shutdownCalled := srv.shutdownCalled
+	srv.mu.Unlock()
+	if shutdownCalled {
+		// Stop accepting new connections.
+		// Connections in the auth phase are handled in handleConnPostSSHAuth.
+		// Existing sessions are terminated by Shutdown.
+		return nil, gossh.ErrDenied
+	}
 	c := &conn{srv: srv, now: srv.now()}
 	c.Server = &ssh.Server{
 		Version:         "Tailscale",
@@ -756,10 +782,10 @@ func (srv *server) getSessionForContext(sctx ssh.Context) (ss *sshSession, ok bo
 	return
 }
 
-// startSession registers ss as an active session.
-func (srv *server) startSession(ss *sshSession) {
-	srv.mu.Lock()
-	defer srv.mu.Unlock()
+// startSessionLocked registers ss as an active session.
+// It must be called with srv.mu held.
+func (srv *server) startSessionLocked(ss *sshSession) {
+	srv.sessionWaitGroup.Add(1)
 	if ss.idH == "" {
 		panic("empty idH")
 	}
@@ -778,6 +804,7 @@ func (srv *server) startSession(ss *sshSession) {
 
 // endSession unregisters s from the list of active sessions.
 func (srv *server) endSession(ss *sshSession) {
+	defer srv.sessionWaitGroup.Done()
 	srv.mu.Lock()
 	defer srv.mu.Unlock()
 	delete(srv.activeSessionByH, ss.idH)
@@ -842,11 +869,21 @@ func (ss *sshSession) handleSSHAgentForwarding(s ssh.Session, lu *user.User) err
 // It handles ss once it's been accepted and determined
 // that it should run.
 func (ss *sshSession) run() {
-	srv := ss.conn.srv
-	srv.startSession(ss)
-	defer srv.endSession(ss)
-
 	defer ss.ctx.CloseWithError(errSessionDone)
+	srv := ss.conn.srv
+
+	srv.mu.Lock()
+	if srv.shutdownCalled {
+		srv.mu.Unlock()
+		// Do not start any new sessions.
+		fmt.Fprintf(ss, "Tailscale is shutting down\r\n")
+		ss.Exit(1)
+		return
+	}
+	srv.startSessionLocked(ss)
+	srv.mu.Unlock()
+
+	defer srv.endSession(ss)
 
 	if ss.action.SessionDuration != 0 {
 		t := time.AfterFunc(ss.action.SessionDuration, func() {
@@ -858,7 +895,7 @@ func (ss *sshSession) run() {
 		defer t.Stop()
 	}
 
-	logf := srv.logf
+	logf := ss.logf
 	lu := ss.conn.localUser
 	localUser := lu.Username