ipn, safesocket: use Windows token in LocalAPI

On Windows, the idiomatic way to check access on a named pipe is for
the server to impersonate the client on its current OS thread, perform
access checks using the client's access token, and then revert the OS
thread's access token back to its true self.

The access token is a better representation of the client's rights than just
a username/userid check, as it represents the client's effective rights
at connection time, which might differ from their normal rights.

This patch updates safesocket to do the aforementioned impersonation,
extract the token handle, and then revert the impersonation. We retain
the token handle for the remaining duration of the connection (the token
continues to be valid even after we have reverted back to self).

Since the token is a property of the connection, I changed ipnauth to wrap
the concrete net.Conn to include the token. I then plumbed that change
through ipnlocal, ipnserver, and localapi as necessary.

I also added a PermitLocalAdmin flag to the localapi Handler which I intend
to use for controlling access to a few new localapi endpoints intended
for configuring auto-update.

Updates https://github.com/tailscale/tailscale/issues/755

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

ipn/ipnauth/ipnauth.go

@@ -5,7 +5,9 @@
 package ipnauth
 
 import (
+	"errors"
 	"fmt"
+	"io"
 	"net"
 	"net/netip"
 	"os"
@@ -25,6 +27,35 @@ import (
 	"tailscale.com/version/distro"
 )
 
+// ErrNotImplemented is returned by ConnIdentity.WindowsToken when it is not
+// implemented for the current GOOS.
+var ErrNotImplemented = errors.New("not implemented for GOOS=" + runtime.GOOS)
+
+// WindowsToken represents the current security context of a Windows user.
+type WindowsToken interface {
+	io.Closer
+	// EqualUIDs reports whether other refers to the same user ID as the receiver.
+	EqualUIDs(other WindowsToken) bool
+	// IsAdministrator reports whether the receiver is a member of the built-in
+	// Administrators group, or else an error. Use IsElevated to determine whether
+	// the receiver is actually utilizing administrative rights.
+	IsAdministrator() (bool, error)
+	// IsUID reports whether the receiver's user ID matches uid.
+	IsUID(uid ipn.WindowsUserID) bool
+	// UID returns the ipn.WindowsUserID associated with the receiver, or else
+	// an error.
+	UID() (ipn.WindowsUserID, error)
+	// IsElevated reports whether the receiver is currently executing as an
+	// elevated administrative user.
+	IsElevated() bool
+	// UserDir returns the special directory identified by folderID as associated
+	// with the receiver. folderID must be one of the KNOWNFOLDERID values from
+	// the x/sys/windows package, serialized as a stringified GUID.
+	UserDir(folderID string) (string, error)
+	// Username returns the user name associated with the receiver.
+	Username() (string, error)
+}
+
 // ConnIdentity represents the owner of a localhost TCP or unix socket connection
 // connecting to the LocalAPI.
 type ConnIdentity struct {
@@ -38,9 +69,7 @@ type ConnIdentity struct {
 	// Used on Windows:
 	// TODO(bradfitz): merge these into the peercreds package and
 	// use that for all.
-	pid    int
-	userID ipn.WindowsUserID
-	user   *user.User
+	pid int
 }
 
 // WindowsUserID returns the local machine's userid of the connection
@@ -52,8 +81,11 @@ func (ci *ConnIdentity) WindowsUserID() ipn.WindowsUserID {
 	if envknob.GOOS() != "windows" {
 		return ""
 	}
-	if ci.userID != "" {
-		return ci.userID
+	if tok, err := ci.WindowsToken(); err == nil {
+		defer tok.Close()
+		if uid, err := tok.UID(); err == nil {
+			return uid
+		}
 	}
 	// For Linux tests running as Windows:
 	const isBroken = true // TODO(bradfitz,maisem): fix tests; this doesn't work yet
@@ -65,7 +97,6 @@ func (ci *ConnIdentity) WindowsUserID() ipn.WindowsUserID {
 	return ""
 }
 
-func (ci *ConnIdentity) User() *user.User       { return ci.user }
 func (ci *ConnIdentity) Pid() int               { return ci.pid }
 func (ci *ConnIdentity) IsUnixSock() bool       { return ci.isUnixSock }
 func (ci *ConnIdentity) Creds() *peercred.Creds { return ci.creds }

ipn/ipnauth/ipnauth_notwindows.go

@@ -21,3 +21,9 @@ func GetConnIdentity(_ logger.Logf, c net.Conn) (ci *ConnIdentity, err error) {
 	ci.creds, _ = peercred.Get(c)
 	return ci, nil
 }
+
+// WindowsToken is unsupported when GOOS != windows and always returns
+// ErrNotImplemented.
+func (ci *ConnIdentity) WindowsToken() (WindowsToken, error) {
+	return nil, ErrNotImplemented
+}

ipn/ipnauth/ipnauth_windows.go

@@ -6,53 +6,157 @@ package ipnauth
 import (
 	"fmt"
 	"net"
-	"syscall"
+	"runtime"
 	"unsafe"
 
 	"golang.org/x/sys/windows"
 	"tailscale.com/ipn"
+	"tailscale.com/safesocket"
 	"tailscale.com/types/logger"
-	"tailscale.com/util/pidowner"
 )
 
-var (
-	kernel32                        = syscall.NewLazyDLL("kernel32.dll")
-	procGetNamedPipeClientProcessId = kernel32.NewProc("GetNamedPipeClientProcessId")
-)
-
-func getNamedPipeClientProcessId(h windows.Handle) (pid uint32, err error) {
-	r1, _, err := procGetNamedPipeClientProcessId.Call(uintptr(h), uintptr(unsafe.Pointer(&pid)))
-	if r1 > 0 {
-		return pid, nil
-	}
-	return 0, err
-}
-
 // GetConnIdentity extracts the identity information from the connection
 // based on the user who owns the other end of the connection.
 // If c is not backed by a named pipe, an error is returned.
 func GetConnIdentity(logf logger.Logf, c net.Conn) (ci *ConnIdentity, err error) {
 	ci = &ConnIdentity{conn: c}
-	h, ok := c.(interface {
-		Fd() uintptr
-	})
+	wcc, ok := c.(*safesocket.WindowsClientConn)
 	if !ok {
-		return ci, fmt.Errorf("not a windows handle: %T", c)
+		return nil, fmt.Errorf("not a WindowsClientConn: %T", c)
 	}
-	pid, err := getNamedPipeClientProcessId(windows.Handle(h.Fd()))
+	ci.pid, err = wcc.ClientPID()
 	if err != nil {
-		return ci, fmt.Errorf("getNamedPipeClientProcessId: %v", err)
+		return nil, err
 	}
-	ci.pid = int(pid)
-	uid, err := pidowner.OwnerOfPID(ci.pid)
-	if err != nil {
-		return ci, fmt.Errorf("failed to map connection's pid to a user (WSL?): %w", err)
-	}
-	ci.userID = ipn.WindowsUserID(uid)
-	u, err := LookupUserFromID(logf, uid)
-	if err != nil {
-		return ci, fmt.Errorf("failed to look up user from userid: %w", err)
-	}
-	ci.user = u
 	return ci, nil
 }
+
+type token struct {
+	t windows.Token
+}
+
+func (t *token) UID() (ipn.WindowsUserID, error) {
+	sid, err := t.uid()
+	if err != nil {
+		return "", fmt.Errorf("failed to look up user from token: %w", err)
+	}
+
+	return ipn.WindowsUserID(sid.String()), nil
+}
+
+func (t *token) Username() (string, error) {
+	sid, err := t.uid()
+	if err != nil {
+		return "", fmt.Errorf("failed to look up user from token: %w", err)
+	}
+
+	username, domain, _, err := sid.LookupAccount("")
+	if err != nil {
+		return "", fmt.Errorf("failed to look up username from SID: %w", err)
+	}
+
+	return fmt.Sprintf(`%s\%s`, domain, username), nil
+}
+
+func (t *token) IsAdministrator() (bool, error) {
+	baSID, err := windows.CreateWellKnownSid(windows.WinBuiltinAdministratorsSid)
+	if err != nil {
+		return false, err
+	}
+
+	return t.t.IsMember(baSID)
+}
+
+func (t *token) IsElevated() bool {
+	return t.t.IsElevated()
+}
+
+func (t *token) UserDir(folderID string) (string, error) {
+	guid, err := windows.GUIDFromString(folderID)
+	if err != nil {
+		return "", err
+	}
+
+	return t.t.KnownFolderPath((*windows.KNOWNFOLDERID)(unsafe.Pointer(&guid)), 0)
+}
+
+func (t *token) Close() error {
+	if t.t == 0 {
+		return nil
+	}
+	if err := t.t.Close(); err != nil {
+		return err
+	}
+	t.t = 0
+	runtime.SetFinalizer(t, nil)
+	return nil
+}
+
+func (t *token) EqualUIDs(other WindowsToken) bool {
+	if t != nil && other == nil || t == nil && other != nil {
+		return false
+	}
+	ot, ok := other.(*token)
+	if !ok {
+		return false
+	}
+	if t == ot {
+		return true
+	}
+	uid, err := t.uid()
+	if err != nil {
+		return false
+	}
+	oUID, err := ot.uid()
+	if err != nil {
+		return false
+	}
+	return uid.Equals(oUID)
+}
+
+func (t *token) uid() (*windows.SID, error) {
+	tu, err := t.t.GetTokenUser()
+	if err != nil {
+		return nil, err
+	}
+
+	return tu.User.Sid, nil
+}
+
+func (t *token) IsUID(uid ipn.WindowsUserID) bool {
+	tUID, err := t.UID()
+	if err != nil {
+		return false
+	}
+
+	return tUID == uid
+}
+
+// WindowsToken returns the WindowsToken representing the security context
+// of the connection's client.
+func (ci *ConnIdentity) WindowsToken() (WindowsToken, error) {
+	var wcc *safesocket.WindowsClientConn
+	var ok bool
+	if wcc, ok = ci.conn.(*safesocket.WindowsClientConn); !ok {
+		return nil, fmt.Errorf("not a WindowsClientConn: %T", ci.conn)
+	}
+
+	// We duplicate the token's handle so that the WindowsToken we return may have
+	// a lifetime independent from the original connection.
+	var h windows.Handle
+	if err := windows.DuplicateHandle(
+		windows.CurrentProcess(),
+		windows.Handle(wcc.Token()),
+		windows.CurrentProcess(),
+		&h,
+		0,
+		false,
+		windows.DUPLICATE_SAME_ACCESS,
+	); err != nil {
+		return nil, err
+	}
+
+	result := &token{t: windows.Token(h)}
+	runtime.SetFinalizer(result, func(t *token) { t.Close() })
+	return result, nil
+}

ipn/ipnlocal/local.go

@@ -261,6 +261,7 @@ type LocalBackend struct {
 	componentLogUntil       map[string]componentLogState
 	// c2nUpdateStatus is the status of c2n-triggered client update.
 	c2nUpdateStatus updateStatus
+	currentUser     ipnauth.WindowsToken
 
 	// ServeConfig fields. (also guarded by mu)
 	lastServeConfJSON   mem.RO              // last JSON that was parsed into serveConfig
@@ -2722,7 +2723,7 @@ func (b *LocalBackend) shouldUploadServices() bool {
 	return !p.ShieldsUp() && b.netMap.CollectServices
 }
 
-// SetCurrentUserID is used to implement support for multi-user systems (only
+// SetCurrentUser is used to implement support for multi-user systems (only
 // Windows 2022-11-25). On such systems, the uid is used to determine which
 // user's state should be used. The current user is maintained by active
 // connections open to the backend.
@@ -2737,18 +2738,35 @@ func (b *LocalBackend) shouldUploadServices() bool {
 // unattended mode. The user must disable unattended mode before the user can be
 // changed.
 //
-// On non-multi-user systems, the uid should be set to empty string.
-func (b *LocalBackend) SetCurrentUserID(uid ipn.WindowsUserID) {
+// On non-multi-user systems, the token should be set to nil.
+//
+// SetCurrentUser returns the ipn.WindowsUserID associated with token
+// when successful.
+func (b *LocalBackend) SetCurrentUser(token ipnauth.WindowsToken) (ipn.WindowsUserID, error) {
+	var uid ipn.WindowsUserID
+	if token != nil {
+		var err error
+		uid, err = token.UID()
+		if err != nil {
+			return "", err
+		}
+	}
+
 	b.mu.Lock()
 	if b.pm.CurrentUserID() == uid {
 		b.mu.Unlock()
-		return
+		return uid, nil
 	}
 	if err := b.pm.SetCurrentUserID(uid); err != nil {
 		b.mu.Unlock()
-		return
+		return uid, nil
 	}
+	if b.currentUser != nil {
+		b.currentUser.Close()
+	}
+	b.currentUser = token
 	b.resetForProfileChangeLockedOnEntry()
+	return uid, nil
 }
 
 func (b *LocalBackend) CheckPrefs(p *ipn.Prefs) error {
@@ -4112,6 +4130,10 @@ func (b *LocalBackend) ResetForClientDisconnect() {
 
 	b.setNetMapLocked(nil)
 	b.pm.Reset()
+	if b.currentUser != nil {
+		b.currentUser.Close()
+		b.currentUser = nil
+	}
 	b.keyExpired = false
 	b.authURL = ""
 	b.authURLSticky = ""

ipn/ipnserver/server.go

@@ -202,6 +202,7 @@ func (s *Server) serveHTTP(w http.ResponseWriter, r *http.Request) {
 		lah := localapi.NewHandler(lb, s.logf, s.netMon, s.backendLogID)
 		lah.PermitRead, lah.PermitWrite = s.localAPIPermissions(ci)
 		lah.PermitCert = s.connCanFetchCerts(ci)
+		lah.CallerIsLocalAdmin = s.connIsLocalAdmin(ci)
 		lah.ServeHTTP(w, r)
 		return
 	}
@@ -242,8 +243,30 @@ func (s *Server) checkConnIdentityLocked(ci *ipnauth.ConnIdentity) error {
 		for _, active = range s.activeReqs {
 			break
 		}
-		if active != nil && ci.WindowsUserID() != active.WindowsUserID() {
-			return inUseOtherUserError{fmt.Errorf("Tailscale already in use by %s, pid %d", active.User().Username, active.Pid())}
+		if active != nil {
+			chkTok, err := ci.WindowsToken()
+			if err == nil {
+				defer chkTok.Close()
+			} else if !errors.Is(err, ipnauth.ErrNotImplemented) {
+				return err
+			}
+
+			activeTok, err := active.WindowsToken()
+			if err == nil {
+				defer activeTok.Close()
+			} else if !errors.Is(err, ipnauth.ErrNotImplemented) {
+				return err
+			}
+
+			if chkTok != nil && !chkTok.EqualUIDs(activeTok) {
+				var b strings.Builder
+				b.WriteString("Tailscale already in use")
+				if username, err := activeTok.Username(); err == nil {
+					fmt.Fprintf(&b, " by %s", username)
+				}
+				fmt.Fprintf(&b, ", pid %d", active.Pid())
+				return inUseOtherUserError{errors.New(b.String())}
+			}
 		}
 	}
 	if err := s.mustBackend().CheckIPNConnectionAllowed(ci); err != nil {
@@ -341,6 +364,31 @@ func (s *Server) connCanFetchCerts(ci *ipnauth.ConnIdentity) bool {
 	return false
 }
 
+// connIsLocalAdmin reports whether ci has administrative access to the local
+// machine, for whatever that means with respect to the current OS.
+//
+// This returns true only on Windows machines when the client user is a
+// member of the built-in Administrators group (but not necessarily elevated).
+// This is useful because, on Windows, tailscaled itself always runs with
+// elevated rights: we want to avoid privilege escalation for certain mutative operations.
+func (s *Server) connIsLocalAdmin(ci *ipnauth.ConnIdentity) bool {
+	tok, err := ci.WindowsToken()
+	if err != nil {
+		if !errors.Is(err, ipnauth.ErrNotImplemented) {
+			s.logf("ipnauth.ConnIdentity.WindowsToken() error: %v", err)
+		}
+		return false
+	}
+	defer tok.Close()
+
+	isAdmin, err := tok.IsAdministrator()
+	if err != nil {
+		s.logf("ipnauth.WindowsToken.IsAdministrator() error: %v", err)
+		return false
+	}
+	return isAdmin
+}
+
 // addActiveHTTPRequest adds c to the server's list of active HTTP requests.
 //
 // If the returned error may be of type inUseOtherUserError.
@@ -372,14 +420,25 @@ func (s *Server) addActiveHTTPRequest(req *http.Request, ci *ipnauth.ConnIdentit
 
 	mak.Set(&s.activeReqs, req, ci)
 
-	if uid := ci.WindowsUserID(); uid != "" && len(s.activeReqs) == 1 {
-		// Tell the LocalBackend about the identity we're now running as.
-		lb.SetCurrentUserID(uid)
-		if s.lastUserID != uid {
-			if s.lastUserID != "" {
-				doReset = true
+	if len(s.activeReqs) == 1 {
+		token, err := ci.WindowsToken()
+		if err != nil {
+			if !errors.Is(err, ipnauth.ErrNotImplemented) {
+				s.logf("error obtaining access token: %v", err)
+			}
+		} else {
+			// Tell the LocalBackend about the identity we're now running as.
+			uid, err := lb.SetCurrentUser(token)
+			if err != nil {
+				token.Close()
+				return nil, err
+			}
+			if s.lastUserID != uid {
+				if s.lastUserID != "" {
+					doReset = true
+				}
+				s.lastUserID = uid
 			}
-			s.lastUserID = uid
 		}
 	}
 

ipn/localapi/localapi.go

@@ -157,6 +157,17 @@ type Handler struct {
 	// cert fetching access.
 	PermitCert bool
 
+	// CallerIsLocalAdmin is whether the this handler is being invoked as a
+	// result of a LocalAPI call from a user who is a local admin of the current
+	// machine.
+	//
+	// As of 2023-10-26 it is only populated on Windows.
+	//
+	// It can be used to to restrict some LocalAPI operations which should only
+	// be run by an admin and not unprivileged users in a computing environment
+	// managed by IT admins.
+	CallerIsLocalAdmin bool
+
 	b            *ipnlocal.LocalBackend
 	logf         logger.Logf
 	netMon       *netmon.Monitor // optional; nil means interfaces will be looked up on-demand