Fix permissions checks on windows

pkg/cli/agent/agent.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"runtime"
 
 	"github.com/gorilla/mux"
 	"github.com/k3s-io/k3s/pkg/agent"
@@ -19,8 +18,10 @@ import (
 	"github.com/k3s-io/k3s/pkg/profile"
 	"github.com/k3s-io/k3s/pkg/spegel"
 	"github.com/k3s-io/k3s/pkg/util"
+	"github.com/k3s-io/k3s/pkg/util/permissions"
 	"github.com/k3s-io/k3s/pkg/version"
 	"github.com/k3s-io/k3s/pkg/vpn"
+	"github.com/pkg/errors"
 	"github.com/rancher/wrangler/v3/pkg/signals"
 	"github.com/sirupsen/logrus"
 	"github.com/urfave/cli"
@@ -45,8 +46,10 @@ func Run(ctx *cli.Context) error {
 		return err
 	}
 
-	if runtime.GOOS != "windows" && os.Getuid() != 0 && !cmds.AgentConfig.Rootless {
-		return fmt.Errorf("agent must be run as root, or with --rootless")
+	if !cmds.AgentConfig.Rootless {
+		if err := permissions.IsPrivileged(); err != nil {
+			return errors.Wrap(err, "agent requires additional privilege if not run with --rootless")
+		}
 	}
 
 	if cmds.AgentConfig.TokenFile != "" {

pkg/cli/server/server.go

@@ -26,6 +26,7 @@ import (
 	"github.com/k3s-io/k3s/pkg/server"
 	"github.com/k3s-io/k3s/pkg/spegel"
 	"github.com/k3s-io/k3s/pkg/util"
+	"github.com/k3s-io/k3s/pkg/util/permissions"
 	"github.com/k3s-io/k3s/pkg/version"
 	"github.com/k3s-io/k3s/pkg/vpn"
 	"github.com/pkg/errors"
@@ -72,8 +73,10 @@ func run(app *cli.Context, cfg *cmds.Server, leaderControllers server.CustomCont
 		return err
 	}
 
-	if !cfg.DisableAgent && os.Getuid() != 0 && !cfg.Rootless {
-		return fmt.Errorf("server must run as root, or with --rootless and/or --disable-agent")
+	if !cfg.DisableAgent && !cfg.Rootless {
+		if err := permissions.IsPrivileged(); err != nil {
+			return errors.Wrap(err, "server requires additional privilege when not run with --rootless and/or --disable-agent")
+		}
 	}
 
 	if cfg.Rootless {

pkg/datadir/datadir.go

 package datadir
 
 import (
-	"os"
 	"path/filepath"
 
+	"github.com/k3s-io/k3s/pkg/util/permissions"
 	"github.com/k3s-io/k3s/pkg/version"
 	"github.com/pkg/errors"
 	"github.com/rancher/wrangler/v3/pkg/resolvehome"
@@ -22,7 +22,7 @@ func Resolve(dataDir string) (string, error) {
 
 func LocalHome(dataDir string, forceLocal bool) (string, error) {
 	if dataDir == "" {
-		if os.Getuid() == 0 && !forceLocal {
+		if permissions.IsPrivileged() == nil && !forceLocal {
 			dataDir = DefaultDataDir
 		} else {
 			dataDir = DefaultHomeDataDir

pkg/server/server.go

@@ -26,6 +26,7 @@ import (
 	"github.com/k3s-io/k3s/pkg/server/handlers"
 	"github.com/k3s-io/k3s/pkg/static"
 	"github.com/k3s-io/k3s/pkg/util"
+	"github.com/k3s-io/k3s/pkg/util/permissions"
 	"github.com/k3s-io/k3s/pkg/version"
 	"github.com/pkg/errors"
 	"github.com/rancher/wrangler/v3/pkg/apply"
@@ -330,7 +331,7 @@ func addrTypesPrioTemplate(flannelExternal bool) string {
 
 func HomeKubeConfig(write, rootless bool) (string, error) {
 	if write {
-		if os.Getuid() == 0 && !rootless {
+		if permissions.IsPrivileged() == nil && !rootless {
 			return datadir.GlobalConfig, nil
 		}
 		return resolvehome.Resolve(datadir.HomeConfig)

pkg/util/permissions/permissions_others.go

+//go:build !windows
+// +build !windows
+
+package permissions
+
+import (
+	"fmt"
+	"os"
+)
+
+// IsPrivileged returns an error if the process is not running as root.
+// Ref: https://github.com/kubernetes/kubernetes/pull/96616
+func IsPrivileged() error {
+	if os.Getuid() != 0 {
+		return fmt.Errorf("not running as root")
+	}
+	return nil
+}

pkg/util/permissions/permissions_windows.go

+//go:build windows
+// +build windows
+
+package permissions
+
+import (
+	"fmt"
+
+	"github.com/pkg/errors"
+	"golang.org/x/sys/windows"
+)
+
+// IsPrivileged returns an error if the the process is not running as a member of the BUILTIN\Administrators group.
+// Ref: https://github.com/kubernetes/kubernetes/pull/96616
+func IsPrivileged() error {
+	var sid *windows.SID
+
+	// Although this looks scary, it is directly copied from the
+	// official windows documentation. The Go API for this is a
+	// direct wrap around the official C++ API.
+	// Ref: https://docs.microsoft.com/en-us/windows/desktop/api/securitybaseapi/nf-securitybaseapi-checktokenmembership
+	err := windows.AllocateAndInitializeSid(
+		&windows.SECURITY_NT_AUTHORITY,
+		2,
+		windows.SECURITY_BUILTIN_DOMAIN_RID,
+		windows.DOMAIN_ALIAS_RID_ADMINS,
+		0, 0, 0, 0, 0, 0,
+		&sid)
+	if err != nil {
+		return errors.Wrap(err, "failed to create Windows SID")
+	}
+	defer windows.FreeSid(sid)
+
+	// Ref: https://github.com/golang/go/issues/28804#issuecomment-438838144
+	token := windows.Token(0)
+
+	member, err := token.IsMember(sid)
+	if err != nil {
+		return errors.Wrap(err, "failed to check group membership")
+	}
+
+	if !member {
+		return fmt.Errorf("not running as member of BUILTIN\\Administrators group")
+	}
+
+	return nil
+}