desktop.go

/*
Copyright 2021 Gravitational, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package web

import (
	"bytes"
	"context"
	"crypto/sha1"
	"crypto/tls"
	"encoding/base64"
	"encoding/binary"
	"encoding/pem"
	"errors"
	"fmt"
	"io"
	"math/rand"
	"net"
	"net/http"
	"strconv"
	"strings"
	"sync"

	"github.com/gorilla/websocket"
	"github.com/gravitational/trace"
	"github.com/julienschmidt/httprouter"
	"github.com/sirupsen/logrus"

	"github.com/gravitational/teleport/api/client/proto"
	"github.com/gravitational/teleport/api/types"
	"github.com/gravitational/teleport/api/utils/keys"
	"github.com/gravitational/teleport/lib/auth"
	wantypes "github.com/gravitational/teleport/lib/auth/webauthntypes"
	"github.com/gravitational/teleport/lib/client"
	"github.com/gravitational/teleport/lib/defaults"
	"github.com/gravitational/teleport/lib/httplib"
	"github.com/gravitational/teleport/lib/multiplexer"
	"github.com/gravitational/teleport/lib/reversetunnel"
	"github.com/gravitational/teleport/lib/srv/desktop"
	"github.com/gravitational/teleport/lib/srv/desktop/tdp"
	"github.com/gravitational/teleport/lib/utils"
	"github.com/gravitational/teleport/lib/web/scripts"
)

// GET /webapi/sites/:site/desktops/:desktopName/connect?access_token=<bearer_token>&username=<username>&width=<width>&height=<height>
func (h *Handler) desktopConnectHandle(
	w http.ResponseWriter,
	r *http.Request,
	p httprouter.Params,
	sctx *SessionContext,
	site reversetunnel.RemoteSite,
) (interface{}, error) {
	desktopName := p.ByName("desktopName")
	if desktopName == "" {
		return nil, trace.BadParameter("missing desktopName in request URL")
	}

	log := sctx.cfg.Log.WithField("desktop-name", desktopName).WithField("cluster-name", site.GetName())
	log.Debug("New desktop access websocket connection")

	if err := h.createDesktopConnection(w, r, desktopName, site.GetName(), log, sctx, site); err != nil {
		// createDesktopConnection makes a best effort attempt to send an error to the user
		// (via websocket) before terminating the connection. We log the error here, but
		// return nil because our HTTP middleware will try to write the returned error in JSON
		// format, and this will fail since the HTTP connection has been upgraded to websockets.
		log.Error(err)
	}

	return nil, nil
}

const (
	// https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-rdpbcgr/cbe1ed0a-d320-4ea5-be5a-f2eb6e032853#Appendix_A_45
	maxRDPScreenWidth  = 8192
	maxRDPScreenHeight = 8192
)

func (h *Handler) createDesktopConnection(
	w http.ResponseWriter,
	r *http.Request,
	desktopName string,
	clusterName string,
	log *logrus.Entry,
	sctx *SessionContext,
	site reversetunnel.RemoteSite,
) error {
	upgrader := websocket.Upgrader{
		ReadBufferSize:  1024,
		WriteBufferSize: 1024,
	}
	ws, err := upgrader.Upgrade(w, r, nil)
	if err != nil {
		return trace.Wrap(err)
	}
	defer ws.Close()

	sendTDPError := func(err error) error {
		sendErr := sendTDPNotification(ws, err, tdp.SeverityError)
		if sendErr != nil {
			return sendErr
		}
		return err
	}

	q := r.URL.Query()
	username := q.Get("username")
	if username == "" {
		return sendTDPError(trace.BadParameter("missing username"))
	}
	width, err := strconv.Atoi(q.Get("width"))
	if err != nil {
		return sendTDPError(trace.BadParameter("width missing or invalid"))
	}
	height, err := strconv.Atoi(q.Get("height"))
	if err != nil {
		return sendTDPError(trace.BadParameter("height missing or invalid"))
	}

	if width > maxRDPScreenWidth || height > maxRDPScreenHeight {
		return sendTDPError(trace.BadParameter(
			"screen size of %d x %d is greater than the maximum allowed by RDP (%d x %d)",
			width, height, maxRDPScreenWidth, maxRDPScreenHeight,
		))
	}

	log.Debugf("Attempting to connect to desktop using username=%v, width=%v, height=%v\n", username, width, height)

	// Pick a random Windows desktop service as our gateway.
	// When agent mode is implemented in the service, we'll have to filter out
	// the services in agent mode.
	//
	// In the future, we may want to do something smarter like latency-based
	// routing.
	clt, err := sctx.GetUserClient(r.Context(), site)
	if err != nil {
		return sendTDPError(trace.Wrap(err))
	}
	winDesktops, err := clt.GetWindowsDesktops(r.Context(), types.WindowsDesktopFilter{Name: desktopName})
	if err != nil {
		return sendTDPError(trace.Wrap(err, "cannot get Windows desktops"))
	}
	if len(winDesktops) == 0 {
		return sendTDPError(trace.NotFound("no Windows desktops were found"))
	}
	var validServiceIDs []string
	for _, desktop := range winDesktops {
		if desktop.GetHostID() == "" {
			// desktops with empty host ids are invalid and should
			// only occur when migrating from an old version of teleport
			continue
		}
		validServiceIDs = append(validServiceIDs, desktop.GetHostID())
	}
	rand.Shuffle(len(validServiceIDs), func(i, j int) {
		validServiceIDs[i], validServiceIDs[j] = validServiceIDs[j], validServiceIDs[i]
	})

	pc, err := proxyClient(r.Context(), sctx, h.ProxyHostPort(), username, h.cfg.PROXYSigner)
	if err != nil {
		return sendTDPError(trace.Wrap(err))
	}
	defer pc.Close()

	// Issue certificate for TLS config and pass MFA check if required.
	tlsConfig, err := desktopTLSConfig(r.Context(), ws, pc, sctx, desktopName, username, site.GetName())
	if err != nil {
		return sendTDPError(err)
	}

	clientSrcAddr, clientDstAddr := utils.ClientAddrFromContext(r.Context())

	c := &connector{
		log:           log,
		clt:           clt,
		site:          site,
		clientSrcAddr: clientSrcAddr,
		clientDstAddr: clientDstAddr,
	}
	serviceConn, err := c.connectToWindowsService(clusterName, validServiceIDs)
	if err != nil {
		return sendTDPError(trace.Wrap(err, "cannot connect to Windows Desktop Service"))
	}
	defer serviceConn.Close()

	serviceConnTLS := tls.Client(serviceConn, tlsConfig)

	if err := serviceConnTLS.HandshakeContext(r.Context()); err != nil {
		return sendTDPError(err)
	}
	log.Debug("Connected to windows_desktop_service")

	tdpConn := tdp.NewConn(serviceConnTLS)
	err = tdpConn.WriteMessage(tdp.ClientUsername{Username: username})
	if err != nil {
		return sendTDPError(err)
	}
	err = tdpConn.WriteMessage(tdp.ClientScreenSpec{Width: uint32(width), Height: uint32(height)})
	if err != nil {
		return sendTDPError(err)
	}

	// proxyWebsocketConn hangs here until connection is closed
	handleProxyWebsocketConnErr(
		proxyWebsocketConn(ws, serviceConnTLS), log)

	return nil
}

func proxyClient(ctx context.Context, sessCtx *SessionContext, addr, windowsUser string, proxySigner multiplexer.PROXYHeaderSigner) (*client.ProxyClient, error) {
	cfg, err := makeTeleportClientConfig(ctx, sessCtx)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	// Set HostLogin to avoid the default behavior of looking up the
	// Unix user Teleport is running as (which doesn't work in containerized
	// environments where we're running as an arbitrary UID)
	cfg.HostLogin = windowsUser

	cfg.PROXYSigner = proxySigner

	if err := cfg.ParseProxyHost(addr); err != nil {
		return nil, trace.Wrap(err)
	}
	tc, err := client.NewClient(cfg)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	pc, err := tc.ConnectToProxy(ctx)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	return pc, nil
}

func desktopTLSConfig(ctx context.Context, ws *websocket.Conn, pc *client.ProxyClient, sessCtx *SessionContext, desktopName, username, siteName string) (*tls.Config, error) {
	pk, err := keys.ParsePrivateKey(sessCtx.cfg.Session.GetPriv())
	if err != nil {
		return nil, trace.Wrap(err)
	}

	codec := tdpMFACodec{}

	key, err := pc.IssueUserCertsWithMFA(ctx, client.ReissueParams{
		RouteToWindowsDesktop: proto.RouteToWindowsDesktop{
			WindowsDesktop: desktopName,
			Login:          username,
		},
		RouteToCluster: siteName,
		ExistingCreds: &client.Key{
			PrivateKey:          pk,
			Cert:                sessCtx.cfg.Session.GetPub(),
			TLSCert:             sessCtx.cfg.Session.GetTLSCert(),
			WindowsDesktopCerts: make(map[string][]byte),
		},
	}, func(ctx context.Context, c *proto.MFAAuthenticateChallenge) (*proto.MFAAuthenticateResponse, error) {
		challenge := &client.MFAAuthenticateChallenge{
			WebauthnChallenge: wantypes.CredentialAssertionFromProto(c.WebauthnChallenge),
		}

		// Send the challenge over the socket.
		msg, err := codec.encode(challenge, defaults.WebsocketWebauthnChallenge)
		if err != nil {
			return nil, trace.Wrap(err)
		}

		if err := ws.WriteMessage(websocket.BinaryMessage, msg); err != nil {
			return nil, trace.Wrap(err)
		}

		ty, buf, err := ws.ReadMessage()
		if err != nil {
			return nil, trace.Wrap(err)
		}

		if ty != websocket.BinaryMessage {
			return nil, trace.BadParameter("received unexpected web socket message type %d", ty)
		}

		resp, err := codec.decodeResponse(buf, defaults.WebsocketWebauthnChallenge)
		return resp, trace.Wrap(err)
	})
	if err != nil {
		return nil, trace.Wrap(err)
	}
	windowsDesktopCerts, ok := key.WindowsDesktopCerts[desktopName]
	if !ok {
		return nil, trace.NotFound("failed to find windows desktop certificates for %q", desktopName)
	}
	certConf, err := pk.TLSCertificate(windowsDesktopCerts)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	tlsConfig, err := sessCtx.ClientTLSConfig(ctx)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	tlsConfig.Certificates = []tls.Certificate{certConf}
	// Pass target desktop name via SNI.
	tlsConfig.ServerName = desktopName + desktop.SNISuffix
	return tlsConfig, nil
}

type connector struct {
	log           *logrus.Entry
	clt           auth.ClientI
	site          reversetunnel.RemoteSite
	clientSrcAddr net.Addr
	clientDstAddr net.Addr
}

// connectToWindowsService tries to make a connection to a Windows Desktop Service
// by trying each of the services provided. It returns an error if it could not connect
// to any of the services or if it encounters an error that is not a connection problem.
func (c *connector) connectToWindowsService(clusterName string, desktopServiceIDs []string) (net.Conn, error) {
	for _, id := range desktopServiceIDs {
		conn, err := c.tryConnect(clusterName, id)
		if err != nil && !trace.IsConnectionProblem(err) {
			return nil, trace.WrapWithMessage(err,
				"error connecting to windows_desktop_service %q", id)
		}
		if trace.IsConnectionProblem(err) {
			c.log.Warnf("failed to connect to windows_desktop_service %q: %v", id, err)
			continue
		}
		if err == nil {
			return conn, err
		}
	}
	return nil, trace.Errorf("failed to connect to any windows_desktop_service")
}

func (c *connector) tryConnect(clusterName, desktopServiceID string) (net.Conn, error) {
	service, err := c.clt.GetWindowsDesktopService(context.Background(), desktopServiceID)
	if err != nil {
		log.Errorf("Error finding service with id %s", desktopServiceID)
		return nil, trace.NotFound("could not find windows desktop service %s: %v", desktopServiceID, err)
	}

	*c.log = *c.log.WithField("windows-service-uuid", service.GetName())
	*c.log = *c.log.WithField("windows-service-addr", service.GetAddr())
	return c.site.DialTCP(reversetunnel.DialParams{
		From:                  c.clientSrcAddr,
		To:                    &utils.NetAddr{AddrNetwork: "tcp", Addr: service.GetAddr()},
		ConnType:              types.WindowsDesktopTunnel,
		ServerID:              service.GetName() + "." + clusterName,
		ProxyIDs:              service.GetProxyIDs(),
		OriginalClientDstAddr: c.clientDstAddr,
	})
}

// proxyWebsocketConn does a bidrectional copy between the websocket
// connection to the browser (ws) and the mTLS connection to Windows
// Desktop Serivce (wds)
func proxyWebsocketConn(ws *websocket.Conn, wds net.Conn) error {
	var closeOnce sync.Once
	close := func() {
		ws.Close()
		wds.Close()
	}

	errs := make(chan error, 2)

	go func() {
		defer closeOnce.Do(close)

		// we avoid using io.Copy here, as we want to make sure
		// each TDP message is sent as a unit so that a single
		// 'message' event is emitted in the browser
		// (io.Copy's internal buffer could split one message
		// into multiple ws.WriteMessage calls)
		tc := tdp.NewConn(wds)

		// we don't care about the content of the message, we just
		// need to split the stream into individual messages and
		// write them to the websocket
		for {
			msg, err := tc.ReadMessage()
			if utils.IsOKNetworkError(err) {
				errs <- nil
				return
			} else if err != nil {
				isFatal := tdp.IsFatalErr(err)
				severity := tdp.SeverityError
				if !isFatal {
					severity = tdp.SeverityWarning
				}
				sendErr := sendTDPNotification(ws, err, severity)

				// If the error wasn't fatal and we successfully
				// sent it back to the client, continue.
				if !isFatal && sendErr == nil {
					continue
				}

				// If the error was fatal or we failed to send it back
				// to the client, send it to the errs channel and end
				// the session.
				if sendErr != nil {
					err = sendErr
				}
				errs <- err
				return
			}
			encoded, err := msg.Encode()
			if err != nil {
				errs <- err
				return
			}
			err = ws.WriteMessage(websocket.BinaryMessage, encoded)
			if utils.IsOKNetworkError(err) {
				errs <- nil
				return
			}
			if err != nil {
				errs <- err
				return
			}
		}
	}()

	go func() {
		defer closeOnce.Do(close)

		// io.Copy is fine here, as the Windows Desktop Service
		// operates on a stream and doesn't care if TPD messages
		// are fragmented
		stream := &WebsocketIO{Conn: ws}
		_, err := io.Copy(wds, stream)
		if utils.IsOKNetworkError(err) {
			err = nil
		}
		errs <- err
	}()

	var retErrs []error
	for i := 0; i < 2; i++ {
		retErrs = append(retErrs, <-errs)
	}
	return trace.NewAggregate(retErrs...)
}

// handleProxyWebsocketConnErr handles the error returned by proxyWebsocketConn by
// unwrapping it and determining whether to log an error.
func handleProxyWebsocketConnErr(proxyWsConnErr error, log *logrus.Entry) {
	if proxyWsConnErr == nil {
		log.Debug("proxyWebsocketConn returned with no error")
		return
	}

	errs := []error{proxyWsConnErr}
	for len(errs) > 0 {
		err := errs[0] // pop first error
		errs = errs[1:]

		switch err := err.(type) {
		case trace.Aggregate:
			errs = append(errs, err.Errors()...)
		case *websocket.CloseError:
			switch err.Code {
			case websocket.CloseNormalClosure, // when the user hits "disconnect" from the menu
				websocket.CloseGoingAway: // when the user closes the tab
				log.Debugf("Web socket closed by client with code: %v", err.Code)
				return
			}
			return
		default:
			if wrapped := errors.Unwrap(err); wrapped != nil {
				errs = append(errs, wrapped)
			}
		}
	}

	log.WithError(proxyWsConnErr).Warning("Error proxying a desktop protocol websocket to windows_desktop_service")
}

// createCertificateBlob creates Certificate BLOB
// It has following structure:
//
//	CertificateBlob {
//		PropertyID: u32, little endian,
//		Reserved: u32, little endian, must be set to 0x01 0x00 0x00 0x00
//		Length: u32, little endian
//		Value: certificate data
//	}
func createCertificateBlob(certData []byte) []byte {
	buf := new(bytes.Buffer)
	buf.Grow(len(certData) + 12)
	// PropertyID for certificate is 32
	binary.Write(buf, binary.LittleEndian, int32(32))
	binary.Write(buf, binary.LittleEndian, int32(1))
	binary.Write(buf, binary.LittleEndian, int32(len(certData)))
	buf.Write(certData)

	return buf.Bytes()
}

func (h *Handler) desktopAccessScriptConfigureHandle(w http.ResponseWriter, r *http.Request, p httprouter.Params) (interface{}, error) {
	tokenStr := p.ByName("token")
	if tokenStr == "" {
		return "", trace.BadParameter("invalid token")
	}

	// verify that the token exists
	token, err := h.GetProxyClient().GetToken(r.Context(), tokenStr)
	if err != nil {
		return "", trace.BadParameter("invalid token")
	}

	proxyServers, err := h.GetProxyClient().GetProxies()
	if err != nil {
		return "", trace.Wrap(err)
	}

	if len(proxyServers) == 0 {
		return "", trace.NotFound("no proxy servers found")
	}

	clusterName, err := h.GetProxyClient().GetDomainName(r.Context())
	if err != nil {
		return nil, trace.Wrap(err)
	}

	certAuthority, err := h.GetProxyClient().GetCertAuthority(
		r.Context(),
		types.CertAuthID{Type: types.UserCA, DomainName: clusterName},
		false,
	)

	if err != nil {
		return nil, trace.Wrap(err)
	}

	if len(certAuthority.GetActiveKeys().TLS) != 1 {
		return nil, trace.BadParameter("expected one TLS key pair, got %v", len(certAuthority.GetActiveKeys().TLS))
	}

	var internalResourceID string
	for labelKey, labelValues := range token.GetSuggestedLabels() {
		if labelKey == types.InternalResourceIDLabel {
			internalResourceID = strings.Join(labelValues, " ")
			break
		}
	}

	keyPair := certAuthority.GetActiveKeys().TLS[0]
	block, _ := pem.Decode(keyPair.Cert)
	if block == nil {
		return nil, trace.BadParameter("no PEM data in CA data")
	}

	httplib.SetScriptHeaders(w.Header())
	w.WriteHeader(http.StatusOK)
	err = scripts.DesktopAccessScriptConfigure.Execute(w, map[string]string{
		"caCertPEM":          string(keyPair.Cert),
		"caCertSHA1":         fmt.Sprintf("%X", sha1.Sum(block.Bytes)),
		"caCertBase64":       base64.StdEncoding.EncodeToString(createCertificateBlob(block.Bytes)),
		"proxyPublicAddr":    proxyServers[0].GetPublicAddr(),
		"provisionToken":     tokenStr,
		"internalResourceID": internalResourceID,
	})

	return nil, trace.Wrap(err)

}

func (h *Handler) desktopAccessScriptInstallADDSHandle(w http.ResponseWriter, r *http.Request, p httprouter.Params) (interface{}, error) {
	httplib.SetScriptHeaders(w.Header())
	w.WriteHeader(http.StatusOK)
	_, err := io.WriteString(w, scripts.DesktopAccessScriptInstallADDS)
	return nil, trace.Wrap(err)
}

func (h *Handler) desktopAccessScriptInstallADCSHandle(w http.ResponseWriter, r *http.Request, p httprouter.Params) (interface{}, error) {
	httplib.SetScriptHeaders(w.Header())
	w.WriteHeader(http.StatusOK)
	_, err := io.WriteString(w, scripts.DesktopAccessScriptInstallADCS)
	return nil, trace.Wrap(err)
}

// sendTDPNotification sends a tdp Notification over the supplied websocket with the
// error message of err.
func sendTDPNotification(ws *websocket.Conn, err error, severity tdp.Severity) error {
	msg := tdp.Notification{Message: err.Error(), Severity: severity}
	b, err := msg.Encode()
	if err != nil {
		return trace.Wrap(err)
	}
	return ws.WriteMessage(websocket.BinaryMessage, b)
}