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>

cmd/derper/depaware.txt

@@ -2,11 +2,6 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
 
         filippo.io/edwards25519                                      from github.com/hdevalence/ed25519consensus
         filippo.io/edwards25519/field                                from filippo.io/edwards25519
-   W 💣 github.com/Microsoft/go-winio                                from tailscale.com/safesocket
-   W 💣 github.com/Microsoft/go-winio/internal/fs                    from github.com/Microsoft/go-winio
-   W 💣 github.com/Microsoft/go-winio/internal/socket                from github.com/Microsoft/go-winio
-   W    github.com/Microsoft/go-winio/internal/stringbuffer          from github.com/Microsoft/go-winio/internal/fs
-   W    github.com/Microsoft/go-winio/pkg/guid                       from github.com/Microsoft/go-winio+
    W 💣 github.com/alexbrainman/sspi                                 from github.com/alexbrainman/sspi/internal/common+
    W    github.com/alexbrainman/sspi/internal/common                 from github.com/alexbrainman/sspi/negotiate
    W 💣 github.com/alexbrainman/sspi/negotiate                       from tailscale.com/net/tshttpproxy
@@ -43,6 +38,11 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
   LD    github.com/prometheus/procfs                                 from github.com/prometheus/client_golang/prometheus
   LD    github.com/prometheus/procfs/internal/fs                     from github.com/prometheus/procfs
   LD    github.com/prometheus/procfs/internal/util                   from github.com/prometheus/procfs
+   W 💣 github.com/tailscale/go-winio                                from tailscale.com/safesocket
+   W 💣 github.com/tailscale/go-winio/internal/fs                    from github.com/tailscale/go-winio
+   W 💣 github.com/tailscale/go-winio/internal/socket                from github.com/tailscale/go-winio
+   W    github.com/tailscale/go-winio/internal/stringbuffer          from github.com/tailscale/go-winio/internal/fs
+   W    github.com/tailscale/go-winio/pkg/guid                       from github.com/tailscale/go-winio+
    L 💣 github.com/tailscale/netlink                                 from tailscale.com/util/linuxfw
    L 💣 github.com/vishvananda/netlink/nl                            from github.com/tailscale/netlink
    L    github.com/vishvananda/netns                                 from github.com/tailscale/netlink+
@@ -111,7 +111,7 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
      💣 tailscale.com/net/tshttpproxy                                from tailscale.com/derp/derphttp+
         tailscale.com/net/wsconn                                     from tailscale.com/cmd/derper+
         tailscale.com/paths                                          from tailscale.com/client/tailscale
-        tailscale.com/safesocket                                     from tailscale.com/client/tailscale
+     💣 tailscale.com/safesocket                                     from tailscale.com/client/tailscale
         tailscale.com/syncs                                          from tailscale.com/cmd/derper+
         tailscale.com/tailcfg                                        from tailscale.com/client/tailscale+
         tailscale.com/tka                                            from tailscale.com/client/tailscale+

cmd/tailscale/depaware.txt

@@ -2,11 +2,6 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
 
         filippo.io/edwards25519                                      from github.com/hdevalence/ed25519consensus
         filippo.io/edwards25519/field                                from filippo.io/edwards25519
-   W 💣 github.com/Microsoft/go-winio                                from tailscale.com/safesocket
-   W 💣 github.com/Microsoft/go-winio/internal/fs                    from github.com/Microsoft/go-winio
-   W 💣 github.com/Microsoft/go-winio/internal/socket                from github.com/Microsoft/go-winio
-   W    github.com/Microsoft/go-winio/internal/stringbuffer          from github.com/Microsoft/go-winio/internal/fs
-   W    github.com/Microsoft/go-winio/pkg/guid                       from github.com/Microsoft/go-winio+
    W 💣 github.com/alexbrainman/sspi                                 from github.com/alexbrainman/sspi/negotiate+
    W    github.com/alexbrainman/sspi/internal/common                 from github.com/alexbrainman/sspi/negotiate
    W 💣 github.com/alexbrainman/sspi/negotiate                       from tailscale.com/net/tshttpproxy
@@ -47,6 +42,11 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         github.com/skip2/go-qrcode                                   from tailscale.com/cmd/tailscale/cli
         github.com/skip2/go-qrcode/bitset                            from github.com/skip2/go-qrcode+
         github.com/skip2/go-qrcode/reedsolomon                       from github.com/skip2/go-qrcode
+   W 💣 github.com/tailscale/go-winio                                from tailscale.com/safesocket
+   W 💣 github.com/tailscale/go-winio/internal/fs                    from github.com/tailscale/go-winio
+   W 💣 github.com/tailscale/go-winio/internal/socket                from github.com/tailscale/go-winio
+   W    github.com/tailscale/go-winio/internal/stringbuffer          from github.com/tailscale/go-winio/internal/fs
+   W    github.com/tailscale/go-winio/pkg/guid                       from github.com/tailscale/go-winio+
         github.com/tailscale/goupnp                                  from github.com/tailscale/goupnp/dcps/internetgateway2+
         github.com/tailscale/goupnp/dcps/internetgateway2            from tailscale.com/net/portmapper
         github.com/tailscale/goupnp/httpu                            from github.com/tailscale/goupnp+
@@ -116,7 +116,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
      💣 tailscale.com/net/tshttpproxy                                from tailscale.com/derp/derphttp+
         tailscale.com/net/wsconn                                     from tailscale.com/control/controlhttp+
         tailscale.com/paths                                          from tailscale.com/cmd/tailscale/cli+
-        tailscale.com/safesocket                                     from tailscale.com/cmd/tailscale/cli+
+     💣 tailscale.com/safesocket                                     from tailscale.com/cmd/tailscale/cli+
         tailscale.com/syncs                                          from tailscale.com/net/netcheck+
         tailscale.com/tailcfg                                        from tailscale.com/cmd/tailscale/cli+
         tailscale.com/tka                                            from tailscale.com/client/tailscale+

cmd/tailscaled/depaware.txt

@@ -2,11 +2,6 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
 
         filippo.io/edwards25519                                      from github.com/hdevalence/ed25519consensus
         filippo.io/edwards25519/field                                from filippo.io/edwards25519
-   W 💣 github.com/Microsoft/go-winio                                from tailscale.com/safesocket
-   W 💣 github.com/Microsoft/go-winio/internal/fs                    from github.com/Microsoft/go-winio
-   W 💣 github.com/Microsoft/go-winio/internal/socket                from github.com/Microsoft/go-winio
-   W    github.com/Microsoft/go-winio/internal/stringbuffer          from github.com/Microsoft/go-winio/internal/fs
-   W    github.com/Microsoft/go-winio/pkg/guid                       from github.com/Microsoft/go-winio+
    W 💣 github.com/alexbrainman/sspi                                 from github.com/alexbrainman/sspi/internal/common+
    W    github.com/alexbrainman/sspi/internal/common                 from github.com/alexbrainman/sspi/negotiate
    W 💣 github.com/alexbrainman/sspi/negotiate                       from tailscale.com/net/tshttpproxy
@@ -136,6 +131,11 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
   LD    github.com/pkg/sftp                                          from tailscale.com/ssh/tailssh
   LD    github.com/pkg/sftp/internal/encoding/ssh/filexfer           from github.com/pkg/sftp
    W 💣 github.com/tailscale/certstore                               from tailscale.com/control/controlclient
+   W 💣 github.com/tailscale/go-winio                                from tailscale.com/safesocket
+   W 💣 github.com/tailscale/go-winio/internal/fs                    from github.com/tailscale/go-winio
+   W 💣 github.com/tailscale/go-winio/internal/socket                from github.com/tailscale/go-winio
+   W    github.com/tailscale/go-winio/internal/stringbuffer          from github.com/tailscale/go-winio/internal/fs
+   W    github.com/tailscale/go-winio/pkg/guid                       from github.com/tailscale/go-winio+
         github.com/tailscale/golang-x-crypto/acme                    from tailscale.com/ipn/ipnlocal
   LD    github.com/tailscale/golang-x-crypto/chacha20                from github.com/tailscale/golang-x-crypto/ssh
   LD 💣 github.com/tailscale/golang-x-crypto/internal/alias          from github.com/tailscale/golang-x-crypto/chacha20
@@ -297,7 +297,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
      💣 tailscale.com/portlist                                       from tailscale.com/ipn/ipnlocal
         tailscale.com/posture                                        from tailscale.com/ipn/ipnlocal
         tailscale.com/proxymap                                       from tailscale.com/tsd+
-        tailscale.com/safesocket                                     from tailscale.com/client/tailscale+
+     💣 tailscale.com/safesocket                                     from tailscale.com/client/tailscale+
         tailscale.com/smallzstd                                      from tailscale.com/control/controlclient+
   LD 💣 tailscale.com/ssh/tailssh                                    from tailscale.com/cmd/tailscaled
         tailscale.com/syncs                                          from tailscale.com/net/netcheck+
@@ -352,7 +352,6 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
      💣 tailscale.com/util/osdiag                                    from tailscale.com/cmd/tailscaled+
    W 💣 tailscale.com/util/osdiag/internal/wsc                       from tailscale.com/util/osdiag
         tailscale.com/util/osshare                                   from tailscale.com/ipn/ipnlocal+
-   W    tailscale.com/util/pidowner                                  from tailscale.com/ipn/ipnauth
         tailscale.com/util/race                                      from tailscale.com/net/dns/resolver
         tailscale.com/util/racebuild                                 from tailscale.com/logpolicy
         tailscale.com/util/rands                                     from tailscale.com/ipn/ipnlocal+

go.mod

@@ -4,7 +4,6 @@ go 1.21
 
 require (
 	filippo.io/mkcert v1.4.4
-	github.com/Microsoft/go-winio v0.6.1
 	github.com/akutz/memconn v0.1.0
 	github.com/alexbrainman/sspi v0.0.0-20231016080023-1a75b4708caa
 	github.com/andybalholm/brotli v1.0.5
@@ -104,6 +103,7 @@ require (
 )
 
 require (
+	github.com/Microsoft/go-winio v0.6.1 // indirect
 	github.com/google/gnostic-models v0.6.9-0.20230804172637-c7be7c783f49 // indirect
 	github.com/gorilla/securecookie v1.1.1 // indirect
 )
@@ -321,6 +321,7 @@ require (
 	github.com/stretchr/testify v1.8.4 // indirect
 	github.com/subosito/gotenv v1.4.2 // indirect
 	github.com/t-yuki/gocover-cobertura v0.0.0-20180217150009-aaee18c8195c // indirect
+	github.com/tailscale/go-winio v0.0.0-20231025203758-c4f33415bf55
 	github.com/tdakkota/asciicheck v0.2.0 // indirect
 	github.com/tetafro/godot v1.4.11 // indirect
 	github.com/timakin/bodyclose v0.0.0-20230421092635-574207250966 // indirect

go.sum

@@ -868,6 +868,8 @@ github.com/tailscale/certstore v0.1.1-0.20231020161753-77811a65f4ff h1:vnxdYZUJb
 github.com/tailscale/certstore v0.1.1-0.20231020161753-77811a65f4ff/go.mod h1:XrBNfAFN+pwoWuksbFS9Ccxnopa15zJGgXRFN90l3K4=
 github.com/tailscale/depaware v0.0.0-20210622194025-720c4b409502 h1:34icjjmqJ2HPjrSuJYEkdZ+0ItmGQAQ75cRHIiftIyE=
 github.com/tailscale/depaware v0.0.0-20210622194025-720c4b409502/go.mod h1:p9lPsd+cx33L3H9nNoecRRxPssFKUwwI50I3pZ0yT+8=
+github.com/tailscale/go-winio v0.0.0-20231025203758-c4f33415bf55 h1:Gzfnfk2TWrk8Jj4P4c1a3CtQyMaTVCznlkLZI++hok4=
+github.com/tailscale/go-winio v0.0.0-20231025203758-c4f33415bf55/go.mod h1:4k4QO+dQ3R5FofL+SanAUZe+/QfeK0+OIuwDIRu2vSg=
 github.com/tailscale/goexpect v0.0.0-20210902213824-6e8c725cea41 h1:/V2rCMMWcsjYaYO2MeovLw+ClP63OtXgCF2Y1eb8+Ns=
 github.com/tailscale/goexpect v0.0.0-20210902213824-6e8c725cea41/go.mod h1:/roCdA6gg6lQyw/Oz6gIIGu3ggJKYhF+WC/AQReE5XQ=
 github.com/tailscale/golang-x-crypto v0.0.0-20230713185742-f0b76a10a08e h1:JyeJF/HuSwvxWtsR1c0oKX1lzaSH5Wh4aX+MgiStaGQ=

ipn/ipnauth/ipnauth.go

@@ -5,7 +5,9 @@
 package ipnauth
 
 import (
+	"errors"
 	"fmt"
+	"io"
 	"net"
 	"net/netip"
 	"os"
@@ -25,6 +27,35 @@
 	"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 {
@@ -39,8 +70,6 @@ type ConnIdentity struct {
 	// TODO(bradfitz): merge these into the peercreds package and
 	// use that for all.
 	pid int
-	userID ipn.WindowsUserID
-	user   *user.User
 }
 
 // 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 != "" {
+	if tok, err := ci.WindowsToken(); err == nil {
-		return ci.userID
+		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 @@
 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 {
+	wcc, ok := c.(*safesocket.WindowsClientConn)
-		Fd() uintptr
-	})
 	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.
+// On non-multi-user systems, the token should be set to nil.
-func (b *LocalBackend) SetCurrentUserID(uid ipn.WindowsUserID) {
+//
+// 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() {
+		if active != nil {
-			return inUseOtherUserError{fmt.Errorf("Tailscale already in use by %s, pid %d", active.User().Username, active.Pid())}
+			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.
@@ -372,9 +420,19 @@ 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 {
+	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.
-		lb.SetCurrentUserID(uid)
+			uid, err := lb.SetCurrentUser(token)
+			if err != nil {
+				token.Close()
+				return nil, err
+			}
 			if s.lastUserID != uid {
 				if s.lastUserID != "" {
 					doReset = true
@@ -382,6 +440,7 @@ func (s *Server) addActiveHTTPRequest(req *http.Request, ci *ipnauth.ConnIdentit
 				s.lastUserID = uid
 			}
 		}
+	}
 
 	onDone = func() {
 		s.mu.Lock()

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

safesocket/pipe_windows.go

@@ -3,16 +3,27 @@
 
 package safesocket
 
+//go:generate go run golang.org/x/sys/windows/mkwinsyscall -output zsyscall_windows.go pipe_windows.go
+
 import (
+	"context"
 	"fmt"
 	"net"
+	"runtime"
 	"syscall"
+	"time"
 
-	"github.com/Microsoft/go-winio"
+	"github.com/tailscale/go-winio"
+	"golang.org/x/sys/windows"
 )
 
 func connect(s *ConnectionStrategy) (net.Conn, error) {
-	return winio.DialPipe(s.path, nil)
+	dl := time.Now().Add(20 * time.Second)
+	ctx, cancel := context.WithDeadline(context.Background(), dl)
+	defer cancel()
+	// We use the identification impersonation level so that tailscaled may
+	// obtain information about our token for access control purposes.
+	return winio.DialPipeAccessImpLevel(ctx, s.path, windows.GENERIC_READ|windows.GENERIC_WRITE, winio.PipeImpLevelIdentification)
 }
 
 func setFlags(network, address string, c syscall.RawConn) error {
@@ -39,5 +50,109 @@ func listen(path string) (net.Listener, error) {
 	if err != nil {
 		return nil, fmt.Errorf("namedpipe.Listen: %w", err)
 	}
-	return lc, nil
+	return &winIOPipeListener{Listener: lc}, nil
 }
+
+// WindowsClientConn is an implementation of net.Conn that permits retrieval of
+// the Windows access token associated with the connection's client. The
+// embedded net.Conn must be a go-winio PipeConn.
+type WindowsClientConn struct {
+	net.Conn
+	token windows.Token
+}
+
+// winioPipeHandle is fulfilled by the underlying code implementing go-winio's
+// PipeConn interface.
+type winioPipeHandle interface {
+	// Fd returns the Windows handle associated with the connection.
+	Fd() uintptr
+}
+
+func resolvePipeHandle(c net.Conn) windows.Handle {
+	wph, ok := c.(winioPipeHandle)
+	if !ok {
+		return 0
+	}
+	return windows.Handle(wph.Fd())
+}
+
+func (conn *WindowsClientConn) handle() windows.Handle {
+	return resolvePipeHandle(conn.Conn)
+}
+
+// ClientPID returns the pid of conn's client, or else an error.
+func (conn *WindowsClientConn) ClientPID() (int, error) {
+	var pid uint32
+	if err := getNamedPipeClientProcessId(conn.handle(), &pid); err != nil {
+		return -1, fmt.Errorf("GetNamedPipeClientProcessId: %w", err)
+	}
+	return int(pid), nil
+}
+
+// Token returns the Windows access token of the client user.
+func (conn *WindowsClientConn) Token() windows.Token {
+	return conn.token
+}
+
+func (conn *WindowsClientConn) Close() error {
+	if conn.token != 0 {
+		conn.token.Close()
+		conn.token = 0
+	}
+	return conn.Conn.Close()
+}
+
+type winIOPipeListener struct {
+	net.Listener
+}
+
+func (lw *winIOPipeListener) Accept() (net.Conn, error) {
+	conn, err := lw.Listener.Accept()
+	if err != nil {
+		return nil, err
+	}
+
+	token, err := clientUserAccessToken(conn)
+	if err != nil {
+		conn.Close()
+		return nil, err
+	}
+
+	return &WindowsClientConn{
+		Conn:  conn,
+		token: token,
+	}, nil
+}
+
+func clientUserAccessToken(c net.Conn) (windows.Token, error) {
+	h := resolvePipeHandle(c)
+	if h == 0 {
+		return 0, fmt.Errorf("not a windows handle: %T", c)
+	}
+
+	// Impersonation touches thread-local state, so we need to lock until the
+	// client access token has been extracted.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	if err := impersonateNamedPipeClient(h); err != nil {
+		return 0, err
+	}
+	defer func() {
+		// Revert the current thread's impersonation.
+		if err := windows.RevertToSelf(); err != nil {
+			panic(fmt.Errorf("could not revert impersonation: %w", err))
+		}
+	}()
+
+	// Extract the client's access token from the thread-local state.
+	var token windows.Token
+	if err := windows.OpenThreadToken(windows.CurrentThread(), windows.TOKEN_DUPLICATE|windows.TOKEN_QUERY, true, &token); err != nil {
+		return 0, err
+	}
+
+	return token, nil
+}
+
+//sys getNamedPipeClientProcessId(h windows.Handle, clientPid *uint32) (err error) [int32(failretval)==0] = kernel32.GetNamedPipeClientProcessId
+//sys impersonateNamedPipeClient(h windows.Handle) (err error) [int32(failretval)==0] = advapi32.ImpersonateNamedPipeClient

safesocket/zsyscall_windows.go

@@ -0,0 +1,62 @@
+// Code generated by 'go generate'; DO NOT EDIT.
+
+package safesocket
+
+import (
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+var _ unsafe.Pointer
+
+// Do the interface allocations only once for common
+// Errno values.
+const (
+	errnoERROR_IO_PENDING = 997
+)
+
+var (
+	errERROR_IO_PENDING error = syscall.Errno(errnoERROR_IO_PENDING)
+	errERROR_EINVAL     error = syscall.EINVAL
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e syscall.Errno) error {
+	switch e {
+	case 0:
+		return errERROR_EINVAL
+	case errnoERROR_IO_PENDING:
+		return errERROR_IO_PENDING
+	}
+	// TODO: add more here, after collecting data on the common
+	// error values see on Windows. (perhaps when running
+	// all.bat?)
+	return e
+}
+
+var (
+	modadvapi32 = windows.NewLazySystemDLL("advapi32.dll")
+	modkernel32 = windows.NewLazySystemDLL("kernel32.dll")
+
+	procImpersonateNamedPipeClient  = modadvapi32.NewProc("ImpersonateNamedPipeClient")
+	procGetNamedPipeClientProcessId = modkernel32.NewProc("GetNamedPipeClientProcessId")
+)
+
+func impersonateNamedPipeClient(h windows.Handle) (err error) {
+	r1, _, e1 := syscall.Syscall(procImpersonateNamedPipeClient.Addr(), 1, uintptr(h), 0, 0)
+	if int32(r1) == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func getNamedPipeClientProcessId(h windows.Handle, clientPid *uint32) (err error) {
+	r1, _, e1 := syscall.Syscall(procGetNamedPipeClientProcessId.Addr(), 2, uintptr(h), uintptr(unsafe.Pointer(clientPid)), 0)
+	if int32(r1) == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}