app: integrate fsu

This removes the dependency on external user switchers like sudo/machinectl and decouples fortify user ids from the passwd database. Signed-off-by: Ophestra Umiker <cat@ophivana.moe>
2024-11-16 21:19:45 +09:00 · 2024-11-16 21:19:45 +09:00 · df33123bd7
parent 1a09b55bd4
commit df33123bd7
25 changed files with 450 additions and 378 deletions
--- a/.gitea/workflows/release.yml
+++ b/.gitea/workflows/release.yml
@ -33,9 +33,9 @@ jobs:
          go build -v -ldflags '-s -w
          -X git.ophivana.moe/security/fortify/internal.Version=${{ github.ref_name }}
          -X git.ophivana.moe/security/fortify/internal.Fsu=/usr/bin/fsu
-          -X git.ophivana.moe/security/fortify/internal.Fshim=/usr/libexec/fortify/fshim
          -X git.ophivana.moe/security/fortify/internal.Finit=/usr/libexec/fortify/finit
-          -X main.Fmain=/usr/bin/fortify'
+          -X main.Fmain=/usr/bin/fortify
+          -X main.Fshim=/usr/libexec/fortify/fshim'
          -o bin/ ./... &&
          (cd bin && sha512sum --tag -b * > sha512sums)

--- a/.gitea/workflows/test.yml
+++ b/.gitea/workflows/test.yml
@ -36,8 +36,8 @@ jobs:
          go build -v -ldflags '-s -w
          -X git.ophivana.moe/security/fortify/internal.Version=${{ github.ref_name }}
          -X git.ophivana.moe/security/fortify/internal.Fsu=/usr/bin/fsu
-          -X git.ophivana.moe/security/fortify/internal.Fshim=/usr/libexec/fortify/fshim
          -X git.ophivana.moe/security/fortify/internal.Finit=/usr/libexec/fortify/finit
-          -X main.Fmain=/usr/bin/fortify'
+          -X main.Fmain=/usr/bin/fortify
+          -X main.Fshim=/usr/libexec/fortify/fshim'
          -o bin/ ./... &&
          (cd bin && sha512sum --tag -b * > sha512sums)
--- a/cmd/finit/main.go
+++ b/cmd/finit/main.go
@ -103,7 +103,7 @@ func main() {
 	if err := cmd.Start(); err != nil {
 		fmsg.Fatalf("cannot start %q: %v", payload.Argv0, err)
 	}
-	fmsg.Withhold()
+	fmsg.Suspend()

 	// close setup pipe as setup is now complete
 	if err := setup.Close(); err != nil {
--- a/cmd/fshim/ipc/shim/shim.go
+++ b/cmd/fshim/ipc/shim/shim.go
@ -5,6 +5,7 @@ import (
 	"net"
 	"os"
 	"os/exec"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"syscall"
@ -12,6 +13,7 @@ import (

 	"git.ophivana.moe/security/fortify/acl"
 	shim0 "git.ophivana.moe/security/fortify/cmd/fshim/ipc"
+	"git.ophivana.moe/security/fortify/internal"
 	"git.ophivana.moe/security/fortify/internal/fmsg"
 )

@ -24,24 +26,26 @@ type Shim struct {
 	cmd *exec.Cmd
 	// uid of shim target user
 	uid uint32
-	// whether to check shim pid
-	checkPid bool
-	// user switcher executable path
-	executable string
+	// string representation of application id
+	aid string
+	// string representation of supplementary group ids
+	supp []string
 	// path to setup socket
 	socket string
 	// shim setup abort reason and completion
 	abort     chan error
 	abortErr  atomic.Pointer[error]
 	abortOnce sync.Once
+	// fallback exit notifier with error returned killing the process
+	killFallback chan error
 	// wayland mediation, nil if disabled
 	wl *shim0.Wayland
 	// shim setup payload
 	payload *shim0.Payload
 }

-func New(executable string, uid uint32, socket string, wl *shim0.Wayland, payload *shim0.Payload, checkPid bool) *Shim {
-	return &Shim{uid: uid, executable: executable, socket: socket, wl: wl, payload: payload, checkPid: checkPid}
+func New(uid uint32, aid string, supp []string, socket string, wl *shim0.Wayland, payload *shim0.Payload) *Shim {
+	return &Shim{uid: uid, aid: aid, supp: supp, socket: socket, wl: wl, payload: payload}
 }

 func (s *Shim) String() string {
@ -68,9 +72,11 @@ func (s *Shim) AbortWait(err error) {
 	<-s.abort
 }

-type CommandBuilder func(shimEnv string) (args []string)
+func (s *Shim) WaitFallback() chan error {
+	return s.killFallback
+}

-func (s *Shim) Start(f CommandBuilder) (*time.Time, error) {
+func (s *Shim) Start() (*time.Time, error) {
 	var (
 		cf     chan *net.UnixConn
 		accept func()
@ -87,22 +93,37 @@ func (s *Shim) Start(f CommandBuilder) (*time.Time, error) {
 	}

 	// start user switcher process and save time
-	s.cmd = exec.Command(s.executable, f(shim0.Env+"="+s.socket)...)
-	s.cmd.Env = []string{}
+	var fsu string
+	if p, ok := internal.Check(internal.Fsu); !ok {
+		fmsg.Fatal("invalid fsu path, this copy of fshim is not compiled correctly")
+		panic("unreachable")
+	} else {
+		fsu = p
+	}
+	s.cmd = exec.Command(fsu)
+	s.cmd.Env = []string{
+		shim0.Env + "=" + s.socket,
+		"FORTIFY_APP_ID=" + s.aid,
+	}
+	if len(s.supp) > 0 {
+		fmsg.VPrintf("attaching supplementary group ids %s", s.supp)
+		s.cmd.Env = append(s.cmd.Env, "FORTIFY_GROUPS="+strings.Join(s.supp, " "))
+	}
 	s.cmd.Stdin, s.cmd.Stdout, s.cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
 	s.cmd.Dir = "/"
-	fmsg.VPrintln("starting shim via user switcher:", s.cmd)
-	fmsg.Withhold() // withhold messages to stderr
+	fmsg.VPrintln("starting shim via fsu:", s.cmd)
+	fmsg.Suspend() // withhold messages to stderr
 	if err := s.cmd.Start(); err != nil {
 		return nil, fmsg.WrapErrorSuffix(err,
-			"cannot start user switcher:")
+			"cannot start fsu:")
 	}
 	startTime := time.Now().UTC()

 	// kill shim if something goes wrong and an error is returned
+	s.killFallback = make(chan error, 1)
 	killShim := func() {
 		if err := s.cmd.Process.Signal(os.Interrupt); err != nil {
-			fmsg.Println("cannot terminate shim on faulted setup:", err)
+			s.killFallback <- err
 		}
 	}
 	defer func() { killShim() }()
@ -132,7 +153,7 @@ func (s *Shim) Start(f CommandBuilder) (*time.Time, error) {
 		err = errors.New("compromised fortify build")
 		s.Abort(err)
 		return &startTime, err
-	} else if s.checkPid && cred.Pid != int32(s.cmd.Process.Pid) {
+	} else if cred.Pid != int32(s.cmd.Process.Pid) {
 		fmsg.Printf("process %d tried to connect to shim setup socket, expecting shim %d",
 			cred.Pid, s.cmd.Process.Pid)
 		err = errors.New("compromised target user")
--- a/cmd/fshim/main.go
+++ b/cmd/fshim/main.go
@ -58,7 +58,7 @@ func main() {
 	// dial setup socket
 	var conn *net.UnixConn
 	if c, err := net.DialUnix("unix", nil, &net.UnixAddr{Name: socketPath, Net: "unix"}); err != nil {
-		fmsg.Fatal("cannot dial setup socket:", err)
+		fmsg.Fatal(err.Error())
 		panic("unreachable")
 	} else {
 		conn = c
@ -67,7 +67,7 @@ func main() {
 	// decode payload gob stream
 	var payload shim.Payload
 	if err := gob.NewDecoder(conn).Decode(&payload); err != nil {
-		fmsg.Fatal("cannot decode shim payload:", err)
+		fmsg.Fatalf("cannot decode shim payload: %v", err)
 	} else {
 		fmsg.SetVerbose(payload.Verbose)
 	}
@ -80,7 +80,7 @@ func main() {
 	wfd := -1
 	if payload.WL {
 		if fd, err := receiveWLfd(conn); err != nil {
-			fmsg.Fatal("cannot receive wayland fd:", err)
+			fmsg.Fatalf("cannot receive wayland fd: %v", err)
 		} else {
 			wfd = fd
 		}
@ -102,7 +102,10 @@ func main() {
 	} else {
 		// no argv, look up shell instead
 		var ok bool
-		if ic.Argv0, ok = os.LookupEnv("SHELL"); !ok {
+		if payload.Bwrap.SetEnv == nil {
+			fmsg.Fatal("no command was specified and environment is unset")
+		}
+		if ic.Argv0, ok = payload.Bwrap.SetEnv["SHELL"]; !ok {
 			fmsg.Fatal("no command was specified and $SHELL was unset")
 		}

@ -125,7 +128,7 @@ func main() {

 	// share config pipe
 	if r, w, err := os.Pipe(); err != nil {
-		fmsg.Fatal("cannot pipe:", err)
+		fmsg.Fatalf("cannot pipe: %v", err)
 	} else {
 		conf.SetEnv[init0.Env] = strconv.Itoa(3 + len(extraFiles))
 		extraFiles = append(extraFiles, r)
@ -134,7 +137,7 @@ func main() {
 		go func() {
 			// stream config to pipe
 			if err = gob.NewEncoder(w).Encode(&ic); err != nil {
-				fmsg.Fatal("cannot transmit init config:", err)
+				fmsg.Fatalf("cannot transmit init config: %v", err)
 			}
 		}()
 	}
@ -142,7 +145,7 @@ func main() {
 	helper.BubblewrapName = payload.Exec[0] // resolved bwrap path by parent
 	if b, err := helper.NewBwrap(conf, nil, finitPath,
 		func(int, int) []string { return make([]string, 0) }); err != nil {
-		fmsg.Fatal("malformed sandbox config:", err)
+		fmsg.Fatalf("malformed sandbox config: %v", err)
 	} else {
 		cmd := b.Unwrap()
 		cmd.Stdin, cmd.Stdout, cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
@ -154,7 +157,7 @@ func main() {

 		// run and pass through exit code
 		if err = b.Start(); err != nil {
-			fmsg.Fatal("cannot start target process:", err)
+			fmsg.Fatalf("cannot start target process: %v", err)
 		} else if err = b.Wait(); err != nil {
 			fmsg.VPrintln("wait:", err)
 		}
--- a/cmd/fsu/main.go
+++ b/cmd/fsu/main.go
@ -1,10 +1,12 @@
 package main

 import (
-	"bufio"
+	"bytes"
+	"fmt"
 	"log"
 	"os"
 	"path"
+	"slices"
 	"strconv"
 	"strings"
 	"syscall"
@ -15,11 +17,15 @@ const (
 	fsuConfFile = "/etc/fsurc"
 	envShim     = "FORTIFY_SHIM"
 	envAID      = "FORTIFY_APP_ID"
+	envGroups   = "FORTIFY_GROUPS"

 	PR_SET_NO_NEW_PRIVS = 0x26
 )

-var Fmain = compPoison
+var (
+	Fmain = compPoison
+	Fshim = compPoison
+)

 func main() {
 	log.SetFlags(0)
@ -35,12 +41,17 @@ func main() {
 		log.Fatal("this program must not be started by root")
 	}

-	var fmain string
+	var fmain, fshim string
 	if p, ok := checkPath(Fmain); !ok {
 		log.Fatal("invalid fortify path, this copy of fsu is not compiled correctly")
 	} else {
 		fmain = p
 	}
+	if p, ok := checkPath(Fshim); !ok {
+		log.Fatal("invalid fshim path, this copy of fsu is not compiled correctly")
+	} else {
+		fshim = p
+	}

 	pexe := path.Join("/proc", strconv.Itoa(os.Getppid()), "exe")
 	if p, err := os.Readlink(pexe); err != nil {
@ -63,87 +74,76 @@ func main() {
 		uid += fid * 10000
 	}

+	// allowed aid range 0 to 9999
+	if as, ok := os.LookupEnv(envAID); !ok {
+		log.Fatal("FORTIFY_APP_ID not set")
+	} else if aid, err := parseUint32Fast(as); err != nil || aid < 0 || aid > 9999 {
+		log.Fatal("invalid aid")
+	} else {
+		uid += aid
+	}
+
 	// pass through setup path to shim
 	var shimSetupPath string
 	if s, ok := os.LookupEnv(envShim); !ok {
-		log.Fatal("FORTIFY_SHIM not set")
+		// fortify requests target uid
+		// print resolved uid and exit
+		fmt.Print(uid)
+		os.Exit(0)
 	} else if !path.IsAbs(s) {
 		log.Fatal("FORTIFY_SHIM is not absolute")
 	} else {
 		shimSetupPath = s
 	}

-	// allowed aid range 0 to 9999
-	if as, ok := os.LookupEnv(envAID); !ok {
-		log.Fatal("FORTIFY_APP_ID not set")
-	} else if aid, err := strconv.Atoi(as); err != nil || aid < 0 || aid > 9999 {
-		log.Fatal("invalid aid")
+	// supplementary groups
+	var suppGroups, suppCurrent []int
+
+	if gs, ok := os.LookupEnv(envGroups); ok {
+		if cur, err := os.Getgroups(); err != nil {
+			log.Fatalf("cannot get groups: %v", err)
+		} else {
+			suppCurrent = cur
+		}
+
+		// parse space-separated list of group ids
+		gss := bytes.Split([]byte(gs), []byte{' '})
+		suppGroups = make([]int, len(gss)+1)
+		for i, s := range gss {
+			if gid, err := strconv.Atoi(string(s)); err != nil {
+				log.Fatalf("cannot parse %q: %v", string(s), err)
+			} else if gid > 0 && gid != uid && gid != os.Getgid() && slices.Contains(suppCurrent, gid) {
+				suppGroups[i] = gid
+			} else {
+				log.Fatalf("invalid gid %d", gid)
+			}
+		}
+		suppGroups[len(suppGroups)-1] = uid
 	} else {
-		uid += aid
+		suppGroups = []int{uid}
 	}

+	// careful! users in the allowlist is effectively allowed to drop groups via fsu
+
 	if err := syscall.Setresgid(uid, uid, uid); err != nil {
 		log.Fatalf("cannot set gid: %v", err)
 	}
+	if err := syscall.Setgroups(suppGroups); err != nil {
+		log.Fatalf("cannot set supplementary groups: %v", err)
+	}
 	if err := syscall.Setresuid(uid, uid, uid); err != nil {
 		log.Fatalf("cannot set uid: %v", err)
 	}
 	if _, _, errno := syscall.AllThreadsSyscall(syscall.SYS_PRCTL, PR_SET_NO_NEW_PRIVS, 1, 0); errno != 0 {
 		log.Fatalf("cannot set no_new_privs flag: %s", errno.Error())
 	}
-	if err := syscall.Exec(fmain, []string{"fortify", "shim"}, []string{envShim + "=" + shimSetupPath}); err != nil {
+	if err := syscall.Exec(fshim, []string{"fshim"}, []string{envShim + "=" + shimSetupPath}); err != nil {
 		log.Fatalf("cannot start shim: %v", err)
 	}

 	panic("unreachable")
 }

-func parseConfig(p string, puid int) (fid int, ok bool) {
-	// refuse to run if fsurc is not protected correctly
-	if s, err := os.Stat(p); err != nil {
-		log.Fatal(err)
-	} else if s.Mode().Perm() != 0400 {
-		log.Fatal("bad fsurc perm")
-	} else if st := s.Sys().(*syscall.Stat_t); st.Uid != 0 || st.Gid != 0 {
-		log.Fatal("fsurc must be owned by uid 0")
-	}
-
-	if r, err := os.Open(p); err != nil {
-		log.Fatal(err)
-		return -1, false
-	} else {
-		s := bufio.NewScanner(r)
-		var line int
-		for s.Scan() {
-			line++
-
-			// <puid> <fid>
-			lf := strings.SplitN(s.Text(), " ", 2)
-			if len(lf) != 2 {
-				log.Fatalf("invalid entry on line %d", line)
-			}
-
-			var puid0 int
-			if puid0, err = strconv.Atoi(lf[0]); err != nil || puid0 < 1 {
-				log.Fatalf("invalid parent uid on line %d", line)
-			}
-
-			ok = puid0 == puid
-			if ok {
-				// allowed fid range 0 to 99
-				if fid, err = strconv.Atoi(lf[1]); err != nil || fid < 0 || fid > 99 {
-					log.Fatalf("invalid fortify uid on line %d", line)
-				}
-				return
-			}
-		}
-		if err = s.Err(); err != nil {
-			log.Fatalf("cannot read fsurc: %v", err)
-		}
-		return -1, false
-	}
-}
-
 func checkPath(p string) (string, bool) {
 	return p, p != compPoison && p != "" && path.IsAbs(p)
 }
--- a/cmd/fsu/parse.go
+++ b/cmd/fsu/parse.go
@ -0,0 +1,77 @@
+package main
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"log"
+	"os"
+	"strings"
+	"syscall"
+)
+
+func parseUint32Fast(s string) (int, error) {
+	sLen := len(s)
+	if sLen < 1 {
+		return -1, errors.New("zero length string")
+	}
+	if sLen > 10 {
+		return -1, errors.New("string too long")
+	}
+
+	n := 0
+	for i, ch := range []byte(s) {
+		ch -= '0'
+		if ch > 9 {
+			return -1, fmt.Errorf("invalid character '%s' at index %d", string([]byte{ch}), i)
+		}
+		n = n*10 + int(ch)
+	}
+	return n, nil
+}
+
+func parseConfig(p string, puid int) (fid int, ok bool) {
+	// refuse to run if fsurc is not protected correctly
+	if s, err := os.Stat(p); err != nil {
+		log.Fatal(err)
+	} else if s.Mode().Perm() != 0400 {
+		log.Fatal("bad fsurc perm")
+	} else if st := s.Sys().(*syscall.Stat_t); st.Uid != 0 || st.Gid != 0 {
+		log.Fatal("fsurc must be owned by uid 0")
+	}
+
+	if r, err := os.Open(p); err != nil {
+		log.Fatal(err)
+		return -1, false
+	} else {
+		s := bufio.NewScanner(r)
+		var line int
+		for s.Scan() {
+			line++
+
+			// <puid> <fid>
+			lf := strings.SplitN(s.Text(), " ", 2)
+			if len(lf) != 2 {
+				log.Fatalf("invalid entry on line %d", line)
+			}
+
+			var puid0 int
+			if puid0, err = parseUint32Fast(lf[0]); err != nil || puid0 < 1 {
+				log.Fatalf("invalid parent uid on line %d", line)
+			}
+
+			ok = puid0 == puid
+			if ok {
+				// allowed fid range 0 to 99
+				if fid, err = parseUint32Fast(lf[1]); err != nil || fid < 0 || fid > 99 {
+					log.Fatalf("invalid fortify uid on line %d", line)
+				}
+				return
+			}
+		}
+		if err = s.Err(); err != nil {
+			log.Fatalf("cannot read fsurc: %v", err)
+		}
+		return -1, false
+	}
+}
--- a/internal/app/app.go
+++ b/internal/app/app.go
@ -53,7 +53,7 @@ func (a *app) String() string {
 	}

 	if a.seal != nil {
-		return "(sealed fortified app as uid " + a.seal.sys.user.Uid + ")"
+		return "(sealed fortified app as uid " + a.seal.sys.user.us + ")"
 	}

 	return "(unsealed fortified app)"
--- a/internal/app/app_nixos_test.go
+++ b/internal/app/app_nixos_test.go
@ -19,9 +19,12 @@ var testCasesNixos = []sealTestCase{
 	{
 		"nixos permissive defaults no enablements", new(stubNixOS),
 		&app.Config{
-			User:    "chronos",
 			Command: make([]string, 0),
-			Method:  "sudo",
+			Confinement: app.ConfinementConfig{
+				AppID:    0,
+				Username: "chronos",
+				Home:     "/home/chronos",
+			},
 		},
 		app.ID{
 			0x4a, 0x45, 0x0b, 0x65,
@ -29,11 +32,11 @@ var testCasesNixos = []sealTestCase{
 			0xbd, 0x01, 0x78, 0x0e,
 			0xb9, 0xa6, 0x07, 0xac,
 		},
-		system.New(150).
-			Ensure("/tmp/fortify.1971", 0701).
-			Ephemeral(system.Process, "/tmp/fortify.1971/4a450b6596d7bc15bd01780eb9a607ac", 0701).
+		system.New(1000000).
+			Ensure("/tmp/fortify.1971", 0711).
+			Ephemeral(system.Process, "/tmp/fortify.1971/4a450b6596d7bc15bd01780eb9a607ac", 0711).
 			Ensure("/tmp/fortify.1971/tmpdir", 0700).UpdatePermType(system.User, "/tmp/fortify.1971/tmpdir", acl.Execute).
-			Ensure("/tmp/fortify.1971/tmpdir/150", 01700).UpdatePermType(system.User, "/tmp/fortify.1971/tmpdir/150", acl.Read, acl.Write, acl.Execute).
+			Ensure("/tmp/fortify.1971/tmpdir/0", 01700).UpdatePermType(system.User, "/tmp/fortify.1971/tmpdir/0", acl.Read, acl.Write, acl.Execute).
 			Ensure("/run/user/1971/fortify", 0700).UpdatePermType(system.User, "/run/user/1971/fortify", acl.Execute).
 			Ensure("/run/user/1971", 0700).UpdatePermType(system.User, "/run/user/1971", acl.Execute). // this is ordered as is because the previous Ensure only calls mkdir if XDG_RUNTIME_DIR is unset
 			Ephemeral(system.Process, "/run/user/1971/fortify/4a450b6596d7bc15bd01780eb9a607ac", 0700).UpdatePermType(system.Process, "/run/user/1971/fortify/4a450b6596d7bc15bd01780eb9a607ac", acl.Execute).
@ -43,6 +46,7 @@ var testCasesNixos = []sealTestCase{
 			Net:      true,
 			UserNS:   true,
 			Clearenv: true,
+			Chdir:    "/home/chronos",
 			SetEnv: map[string]string{
 				"HOME":              "/home/chronos",
 				"SHELL":             "/run/current-system/sw/bin/zsh",
@ -182,10 +186,11 @@ var testCasesNixos = []sealTestCase{
 			Symlink("/fortify/etc/zprofile", "/etc/zprofile").
 			Symlink("/fortify/etc/zshenv", "/etc/zshenv").
 			Symlink("/fortify/etc/zshrc", "/etc/zshrc").
-			Bind("/tmp/fortify.1971/tmpdir/150", "/tmp", false, true).
+			Bind("/tmp/fortify.1971/tmpdir/0", "/tmp", false, true).
 			Tmpfs("/tmp/fortify.1971", 1048576).
 			Tmpfs("/run/user", 1048576).
 			Tmpfs("/run/user/65534", 8388608).
+			Bind("/home/chronos", "/home/chronos", false, true).
 			Bind("/tmp/fortify.1971/4a450b6596d7bc15bd01780eb9a607ac/passwd", "/etc/passwd").
 			Bind("/tmp/fortify.1971/4a450b6596d7bc15bd01780eb9a607ac/group", "/etc/group").
 			Tmpfs("/var/run/nscd", 8192),
@ -194,9 +199,12 @@ var testCasesNixos = []sealTestCase{
 		"nixos permissive defaults chromium", new(stubNixOS),
 		&app.Config{
 			ID:      "org.chromium.Chromium",
-			User:    "chronos",
 			Command: []string{"/run/current-system/sw/bin/zsh", "-c", "exec chromium "},
 			Confinement: app.ConfinementConfig{
+				AppID:    9,
+				Groups:   []string{"video"},
+				Username: "chronos",
+				Home:     "/home/chronos",
 				SessionBus: &dbus.Config{
 					Talk: []string{
 						"org.freedesktop.Notifications",
@ -230,7 +238,6 @@ var testCasesNixos = []sealTestCase{
 				},
 				Enablements: system.EWayland.Mask() | system.EDBus.Mask() | system.EPulse.Mask(),
 			},
-			Method: "systemd",
 		},
 		app.ID{
 			0xeb, 0xf0, 0x83, 0xd1,
@ -238,11 +245,11 @@ var testCasesNixos = []sealTestCase{
 			0x82, 0xd4, 0x13, 0x36,
 			0x9b, 0x64, 0xce, 0x7c,
 		},
-		system.New(150).
-			Ensure("/tmp/fortify.1971", 0701).
-			Ephemeral(system.Process, "/tmp/fortify.1971/ebf083d1b175911782d413369b64ce7c", 0701).
+		system.New(1000009).
+			Ensure("/tmp/fortify.1971", 0711).
+			Ephemeral(system.Process, "/tmp/fortify.1971/ebf083d1b175911782d413369b64ce7c", 0711).
 			Ensure("/tmp/fortify.1971/tmpdir", 0700).UpdatePermType(system.User, "/tmp/fortify.1971/tmpdir", acl.Execute).
-			Ensure("/tmp/fortify.1971/tmpdir/150", 01700).UpdatePermType(system.User, "/tmp/fortify.1971/tmpdir/150", acl.Read, acl.Write, acl.Execute).
+			Ensure("/tmp/fortify.1971/tmpdir/9", 01700).UpdatePermType(system.User, "/tmp/fortify.1971/tmpdir/9", acl.Read, acl.Write, acl.Execute).
 			Ensure("/run/user/1971/fortify", 0700).UpdatePermType(system.User, "/run/user/1971/fortify", acl.Execute).
 			Ensure("/run/user/1971", 0700).UpdatePermType(system.User, "/run/user/1971", acl.Execute). // this is ordered as is because the previous Ensure only calls mkdir if XDG_RUNTIME_DIR is unset
 			Ephemeral(system.Process, "/run/user/1971/fortify/ebf083d1b175911782d413369b64ce7c", 0700).UpdatePermType(system.Process, "/run/user/1971/fortify/ebf083d1b175911782d413369b64ce7c", acl.Execute).
@ -287,6 +294,7 @@ var testCasesNixos = []sealTestCase{
 		(&bwrap.Config{
 			Net:      true,
 			UserNS:   true,
+			Chdir:    "/home/chronos",
 			Clearenv: true,
 			SetEnv: map[string]string{
 				"DBUS_SESSION_BUS_ADDRESS": "unix:path=/run/user/65534/bus",
@ -434,10 +442,11 @@ var testCasesNixos = []sealTestCase{
 			Symlink("/fortify/etc/zprofile", "/etc/zprofile").
 			Symlink("/fortify/etc/zshenv", "/etc/zshenv").
 			Symlink("/fortify/etc/zshrc", "/etc/zshrc").
-			Bind("/tmp/fortify.1971/tmpdir/150", "/tmp", false, true).
+			Bind("/tmp/fortify.1971/tmpdir/9", "/tmp", false, true).
 			Tmpfs("/tmp/fortify.1971", 1048576).
 			Tmpfs("/run/user", 1048576).
 			Tmpfs("/run/user/65534", 8388608).
+			Bind("/home/chronos", "/home/chronos", false, true).
 			Bind("/tmp/fortify.1971/ebf083d1b175911782d413369b64ce7c/passwd", "/etc/passwd").
 			Bind("/tmp/fortify.1971/ebf083d1b175911782d413369b64ce7c/group", "/etc/group").
 			Bind("/run/user/1971/fortify/ebf083d1b175911782d413369b64ce7c/wayland", "/run/user/65534/wayland-0").
@ -504,23 +513,12 @@ func (s *stubNixOS) Executable() (string, error) {
 	return "/home/ophestra/.nix-profile/bin/fortify", nil
 }

-func (s *stubNixOS) Lookup(username string) (*user.User, error) {
-	if s.usernameErr != nil {
-		if err, ok := s.usernameErr[username]; ok {
-			return nil, err
-		}
-	}
-
-	switch username {
-	case "chronos":
-		return &user.User{
-			Uid:      "150",
-			Gid:      "101",
-			Username: "chronos",
-			HomeDir:  "/home/chronos",
-		}, nil
+func (s *stubNixOS) LookupGroup(name string) (*user.Group, error) {
+	switch name {
+	case "video":
+		return &user.Group{Gid: "26", Name: "video"}, nil
 	default:
-		return nil, user.UnknownUserError(username)
+		return nil, user.UnknownGroupError(name)
 	}
 }

@ -586,10 +584,6 @@ func (s *stubNixOS) Stdout() io.Writer {
 	panic("requested stdout")
 }

-func (s *stubNixOS) FshimPath() string {
-	return "/nix/store/00000000000000000000000000000000-fortify-0.0.10/bin/.fshim"
-}
-
 func (s *stubNixOS) Paths() linux.Paths {
 	return linux.Paths{
 		SharePath:   "/tmp/fortify.1971",
@ -598,6 +592,10 @@ func (s *stubNixOS) Paths() linux.Paths {
 	}
 }

+func (s *stubNixOS) Uid(aid int) (int, error) {
+	return 1000000 + 0*10000 + aid, nil
+}
+
 func (s *stubNixOS) SdBooted() bool {
 	return true
 }
--- a/internal/app/config.go
+++ b/internal/app/config.go
@ -15,12 +15,8 @@ const fTmp = "/fortify"
 type Config struct {
 	// D-Bus application ID
 	ID string `json:"id"`
-	// username of the target user to switch to
-	User string `json:"user"`
 	// value passed through to the child process as its argv
 	Command []string `json:"command"`
-	// string representation of the child's launch method
-	Method string `json:"method"`

 	// child confinement configuration
 	Confinement ConfinementConfig `json:"confinement"`
@ -28,6 +24,14 @@ type Config struct {

 // ConfinementConfig defines fortified child's confinement
 type ConfinementConfig struct {
+	// numerical application id, determines uid in the init namespace
+	AppID int `json:"app_id"`
+	// list of supplementary groups to inherit
+	Groups []string `json:"groups"`
+	// passwd username in the sandbox, defaults to chronos
+	Username string `json:"username,omitempty"`
+	// home directory in sandbox
+	Home string `json:"home"`
 	// bwrap sandbox confinement configuration
 	Sandbox *SandboxConfig `json:"sandbox"`

@ -169,8 +173,7 @@ func (s *SandboxConfig) Bwrap(os linux.System) (*bwrap.Config, error) {
 // Template returns a fully populated instance of Config.
 func Template() *Config {
 	return &Config{
-		ID:   "org.chromium.Chromium",
-		User: "chronos",
+		ID: "org.chromium.Chromium",
 		Command: []string{
 			"chromium",
 			"--ignore-gpu-blocklist",
@ -178,8 +181,11 @@ func Template() *Config {
 			"--enable-features=UseOzonePlatform",
 			"--ozone-platform=wayland",
 		},
-		Method: "sudo",
 		Confinement: ConfinementConfig{
+			AppID:    9,
+			Groups:   []string{"video"},
+			Username: "chronos",
+			Home:     "/var/lib/persist/home/org.chromium.Chromium",
 			Sandbox: &SandboxConfig{
 				Hostname:     "localhost",
 				UserNS:       true,
--- a/internal/app/launch.machinectl.go
+++ b/internal/app/launch.machinectl.go
@ -1,57 +0,0 @@
-package app
-
-import (
-	"strings"
-
-	"git.ophivana.moe/security/fortify/internal/fmsg"
-)
-
-func (a *app) commandBuilderMachineCtl(shimEnv string) (args []string) {
-	args = make([]string, 0, 9+len(a.seal.sys.bwrap.SetEnv))
-
-	// shell --uid=$USER
-	args = append(args, "shell", "--uid="+a.seal.sys.user.Username)
-
-	// --quiet
-	if !fmsg.Verbose() {
-		args = append(args, "--quiet")
-	}
-
-	// environ
-	envQ := make([]string, 0, len(a.seal.sys.bwrap.SetEnv)+1)
-	for k, v := range a.seal.sys.bwrap.SetEnv {
-		envQ = append(envQ, "-E"+k+"="+v)
-	}
-	// add shim payload to environment for shim path
-	envQ = append(envQ, "-E"+shimEnv)
-	args = append(args, envQ...)
-
-	// -- .host
-	args = append(args, "--", ".host")
-
-	// /bin/sh -c
-	if sh, err := a.os.LookPath("sh"); err != nil {
-		// hardcode /bin/sh path since it exists more often than not
-		args = append(args, "/bin/sh", "-c")
-	} else {
-		args = append(args, sh, "-c")
-	}
-
-	// build inner command expression ran as target user
-	innerCommand := strings.Builder{}
-
-	// apply custom environment variables to activation environment
-	innerCommand.WriteString("dbus-update-activation-environment --systemd")
-	for k := range a.seal.sys.bwrap.SetEnv {
-		innerCommand.WriteString(" " + k)
-	}
-	innerCommand.WriteString("; ")
-
-	// launch fortify shim
-	innerCommand.WriteString("exec " + a.os.FshimPath())
-
-	// append inner command
-	args = append(args, innerCommand.String())
-
-	return
-}
--- a/internal/app/launch.sudo.go
+++ b/internal/app/launch.sudo.go
@ -1,30 +0,0 @@
-package app
-
-import (
-	"git.ophivana.moe/security/fortify/internal/fmsg"
-)
-
-const (
-	sudoAskPass = "SUDO_ASKPASS"
-)
-
-func (a *app) commandBuilderSudo(shimEnv string) (args []string) {
-	args = make([]string, 0, 8)
-
-	// -Hiu $USER
-	args = append(args, "-Hiu", a.seal.sys.user.Username)
-
-	// -A?
-	if _, ok := a.os.LookupEnv(sudoAskPass); ok {
-		fmsg.VPrintln(sudoAskPass, "set, adding askpass flag")
-		args = append(args, "-A")
-	}
-
-	// shim payload
-	args = append(args, shimEnv)
-
-	// -- $@
-	args = append(args, "--", a.os.FshimPath())
-
-	return
-}
--- a/internal/app/seal.go
+++ b/internal/app/seal.go
@ -2,8 +2,8 @@ package app

 import (
 	"errors"
+	"fmt"
 	"io/fs"
-	"os/user"
 	"path"
 	"strconv"

@ -15,24 +15,10 @@ import (
 	"git.ophivana.moe/security/fortify/internal/system"
 )

-const (
-	LaunchMethodSudo uint8 = iota
-	LaunchMethodMachineCtl
-)
-
-var method = [...]string{
-	LaunchMethodSudo:       "sudo",
-	LaunchMethodMachineCtl: "systemd",
-}
-
 var (
 	ErrConfig = errors.New("no configuration to seal")
-	ErrUser   = errors.New("unknown user")
-	ErrLaunch = errors.New("invalid launch method")
-
-	ErrSudo       = errors.New("sudo not available")
-	ErrSystemd    = errors.New("systemd not available")
-	ErrMachineCtl = errors.New("machinectl not available")
+	ErrUser   = errors.New("invalid aid")
+	ErrHome   = errors.New("invalid home directory")
 )

 // appSeal seals the application with child-related information
@ -51,15 +37,11 @@ type appSeal struct {
 	// persistent process state store
 	store state.Store

-	// uint8 representation of launch method sealed from config
-	launchOption uint8
 	// process-specific share directory path
 	share string
 	// process-specific share directory path local to XDG_RUNTIME_DIR
 	shareLocal string

-	// path to launcher program
-	toolPath string
 	// pass-through enablement tracking from config
 	et system.Enablements

@ -98,34 +80,6 @@ func (a *app) Seal(config *Config) error {
 	seal.fid = config.ID
 	seal.command = config.Command

-	// parses launch method text and looks up tool path
-	switch config.Method {
-	case method[LaunchMethodSudo]:
-		seal.launchOption = LaunchMethodSudo
-		if sudoPath, err := a.os.LookPath("sudo"); err != nil {
-			return fmsg.WrapError(ErrSudo,
-				"sudo not found")
-		} else {
-			seal.toolPath = sudoPath
-		}
-	case method[LaunchMethodMachineCtl]:
-		seal.launchOption = LaunchMethodMachineCtl
-		if !a.os.SdBooted() {
-			return fmsg.WrapError(ErrSystemd,
-				"system has not been booted with systemd as init system")
-		}
-
-		if machineCtlPath, err := a.os.LookPath("machinectl"); err != nil {
-			return fmsg.WrapError(ErrMachineCtl,
-				"machinectl not found")
-		} else {
-			seal.toolPath = machineCtlPath
-		}
-	default:
-		return fmsg.WrapError(ErrLaunch,
-			"invalid launch method")
-	}
-
 	// create seal system component
 	seal.sys = new(appSealSys)

@ -138,16 +92,44 @@ func (a *app) Seal(config *Config) error {
 	seal.sys.mappedIDString = strconv.Itoa(seal.sys.mappedID)
 	seal.sys.runtime = path.Join("/run/user", seal.sys.mappedIDString)

-	// look up user from system
-	if u, err := a.os.Lookup(config.User); err != nil {
-		if errors.As(err, new(user.UnknownUserError)) {
-			return fmsg.WrapError(ErrUser, "unknown user", config.User)
-		} else {
-			// unreachable
-			panic(err)
-		}
+	// validate uid and set user info
+	if config.Confinement.AppID < 0 || config.Confinement.AppID > 9999 {
+		return fmsg.WrapError(ErrUser,
+			fmt.Sprintf("aid %d out of range", config.Confinement.AppID))
 	} else {
-		seal.sys.user = u
+		seal.sys.user = appUser{
+			aid:      config.Confinement.AppID,
+			as:       strconv.Itoa(config.Confinement.AppID),
+			home:     config.Confinement.Home,
+			username: config.Confinement.Username,
+		}
+		if seal.sys.user.username == "" {
+			seal.sys.user.username = "chronos"
+		}
+		if seal.sys.user.home == "" || !path.IsAbs(seal.sys.user.home) {
+			return fmsg.WrapError(ErrHome,
+				fmt.Sprintf("invalid home directory %q", seal.sys.user.home))
+		}
+
+		// invoke fsu for full uid
+		if u, err := a.os.Uid(seal.sys.user.aid); err != nil {
+			return fmsg.WrapErrorSuffix(err,
+				"cannot obtain uid from fsu:")
+		} else {
+			seal.sys.user.uid = u
+			seal.sys.user.us = strconv.Itoa(u)
+		}
+
+		// resolve supplementary group ids from names
+		seal.sys.user.supp = make([]string, len(config.Confinement.Groups))
+		for i, name := range config.Confinement.Groups {
+			if g, err := a.os.LookupGroup(name); err != nil {
+				return fmsg.WrapError(err,
+					fmt.Sprintf("unknown group %q", name))
+			} else {
+				seal.sys.user.supp[i] = g.Gid
+			}
+		}
 	}

 	// map sandbox config to bwrap
@ -230,15 +212,10 @@ func (a *app) Seal(config *Config) error {
 	// open process state store
 	// the simple store only starts holding an open file after first action
 	// store activity begins after Start is called and must end before Wait
-	seal.store = state.NewSimple(seal.RunDirPath, seal.sys.user.Uid)
+	seal.store = state.NewSimple(seal.RunDirPath, seal.sys.user.as)

-	// parse string UID
-	if u, err := strconv.Atoi(seal.sys.user.Uid); err != nil {
-		// unreachable unless kernel bug
-		panic("uid parse")
-	} else {
-		seal.sys.I = system.New(u)
-	}
+	// initialise system interface with full uid
+	seal.sys.I = system.New(seal.sys.user.uid)

 	// pass through enablements
 	seal.et = config.Confinement.Enablements
@ -249,11 +226,8 @@ func (a *app) Seal(config *Config) error {
 	}

 	// verbose log seal information
-	fmsg.VPrintln("created application seal as user",
-		seal.sys.user.Username, "("+seal.sys.user.Uid+"),",
-		"method:", config.Method+",",
-		"launcher:", seal.toolPath+",",
-		"command:", config.Command)
+	fmsg.VPrintf("created application seal for uid %s (%s) groups: %v, command: %s",
+		seal.sys.user.us, seal.sys.user.username, config.Confinement.Groups, config.Command)

 	// seal app and release lock
 	a.seal = seal
--- a/internal/app/share.display.go
+++ b/internal/app/share.display.go
@ -58,7 +58,7 @@ func (seal *appSeal) shareDisplay(os linux.System) error {
 			return fmsg.WrapError(ErrXDisplay,
 				"DISPLAY is not set")
 		} else {
-			seal.sys.ChangeHosts(seal.sys.user.Username)
+			seal.sys.ChangeHosts(seal.sys.user.us)
 			seal.sys.bwrap.SetEnv[display] = d
 			seal.sys.bwrap.Bind("/tmp/.X11-unix", "/tmp/.X11-unix")
 		}
--- a/internal/app/share.system.go
+++ b/internal/app/share.system.go
@ -16,12 +16,12 @@ const (
 func (seal *appSeal) shareSystem() {
 	// ensure Share (e.g. `/tmp/fortify.%d`)
 	// acl is unnecessary as this directory is world executable
-	seal.sys.Ensure(seal.SharePath, 0701)
+	seal.sys.Ensure(seal.SharePath, 0711)

 	// ensure process-specific share (e.g. `/tmp/fortify.%d/%s`)
 	// acl is unnecessary as this directory is world executable
 	seal.share = path.Join(seal.SharePath, seal.id)
-	seal.sys.Ephemeral(system.Process, seal.share, 0701)
+	seal.sys.Ephemeral(system.Process, seal.share, 0711)

 	// ensure child tmpdir parent directory (e.g. `/tmp/fortify.%d/tmpdir`)
 	targetTmpdirParent := path.Join(seal.SharePath, "tmpdir")
@ -29,7 +29,7 @@ func (seal *appSeal) shareSystem() {
 	seal.sys.UpdatePermType(system.User, targetTmpdirParent, acl.Execute)

 	// ensure child tmpdir (e.g. `/tmp/fortify.%d/tmpdir/%d`)
-	targetTmpdir := path.Join(targetTmpdirParent, seal.sys.user.Uid)
+	targetTmpdir := path.Join(targetTmpdirParent, seal.sys.user.as)
 	seal.sys.Ensure(targetTmpdir, 01700)
 	seal.sys.UpdatePermType(system.User, targetTmpdir, acl.Read, acl.Write, acl.Execute)
 	seal.sys.bwrap.Bind(targetTmpdir, "/tmp", false, true)
@ -49,15 +49,21 @@ func (seal *appSeal) sharePasswd(os linux.System) {
 	// generate /etc/passwd
 	passwdPath := path.Join(seal.share, "passwd")
 	username := "chronos"
-	if seal.sys.user.Username != "" {
-		username = seal.sys.user.Username
-		seal.sys.bwrap.SetEnv["USER"] = seal.sys.user.Username
+	if seal.sys.user.username != "" {
+		username = seal.sys.user.username
 	}
 	homeDir := "/var/empty"
-	if seal.sys.user.HomeDir != "" {
-		homeDir = seal.sys.user.HomeDir
-		seal.sys.bwrap.SetEnv["HOME"] = seal.sys.user.HomeDir
+	if seal.sys.user.home != "" {
+		homeDir = seal.sys.user.home
 	}
+
+	// bind home directory
+	seal.sys.bwrap.Bind(homeDir, homeDir, false, true)
+	seal.sys.bwrap.Chdir = homeDir
+
+	seal.sys.bwrap.SetEnv["USER"] = username
+	seal.sys.bwrap.SetEnv["HOME"] = homeDir
+
 	passwd := username + ":x:" + seal.sys.mappedIDString + ":" + seal.sys.mappedIDString + ":Fortify:" + homeDir + ":" + sh + "\n"
 	seal.sys.Write(passwdPath, passwd)

--- a/internal/app/start.go
+++ b/internal/app/start.go
@ -41,19 +41,13 @@ func (a *app) Start() error {
 		}
 	}

-	// select command builder
-	var commandBuilder shim.CommandBuilder
-	switch a.seal.launchOption {
-	case LaunchMethodSudo:
-		commandBuilder = a.commandBuilderSudo
-	case LaunchMethodMachineCtl:
-		commandBuilder = a.commandBuilderMachineCtl
-	default:
-		panic("unreachable")
-	}
-
 	// construct shim manager
-	a.shim = shim.New(a.seal.toolPath, uint32(a.seal.sys.UID()), path.Join(a.seal.share, "shim"), a.seal.wl,
+	a.shim = shim.New(
+		uint32(a.seal.sys.UID()),
+		a.seal.sys.user.as,
+		a.seal.sys.user.supp,
+		path.Join(a.seal.share, "shim"),
+		a.seal.wl,
 		&shim0.Payload{
 			Argv:  a.seal.command,
 			Exec:  shimExec,
@ -62,9 +56,6 @@ func (a *app) Start() error {

 			Verbose: fmsg.Verbose(),
 		},
-		// checkPid is impossible at the moment since there is no reliable way to obtain shim's pid
-		// this feature is disabled here until sudo is replaced by fortify suid wrapper
-		false,
 	)

 	// startup will go ahead, commit system setup
@ -73,7 +64,7 @@ func (a *app) Start() error {
 	}
 	a.seal.sys.needRevert = true

-	if startTime, err := a.shim.Start(commandBuilder); err != nil {
+	if startTime, err := a.shim.Start(); err != nil {
 		return err
 	} else {
 		// shim start and setup success, create process state
@ -81,7 +72,6 @@ func (a *app) Start() error {
 			PID:        a.shim.Unwrap().Process.Pid,
 			Command:    a.seal.command,
 			Capability: a.seal.et,
-			Method:     method[a.seal.launchOption],
 			Argv:       a.shim.Unwrap().Args,
 			Time:       *startTime,
 		}
@ -166,20 +156,31 @@ func (a *app) Wait() (int, error) {
 		// failure prior to process start
 		r = 255
 	} else {
-		// wait for process and resolve exit code
-		if err := cmd.Wait(); err != nil {
-			var exitError *exec.ExitError
-			if !errors.As(err, &exitError) {
-				// should be unreachable
-				a.waitErr = err
-			}
+		wait := make(chan error, 1)
+		go func() { wait <- cmd.Wait() }()

-			// store non-zero return code
-			r = exitError.ExitCode()
-		} else {
-			r = cmd.ProcessState.ExitCode()
+		select {
+		// wait for process and resolve exit code
+		case err := <-wait:
+			if err != nil {
+				var exitError *exec.ExitError
+				if !errors.As(err, &exitError) {
+					// should be unreachable
+					a.waitErr = err
+				}
+
+				// store non-zero return code
+				r = exitError.ExitCode()
+			} else {
+				r = cmd.ProcessState.ExitCode()
+			}
+			fmsg.VPrintf("process %d exited with exit code %d", cmd.Process.Pid, r)
+
+		// alternative exit path when kill was unsuccessful
+		case err := <-a.shim.WaitFallback():
+			r = 255
+			fmsg.Printf("cannot terminate shim on faulted setup: %v", err)
 		}
-		fmsg.VPrintf("process %d exited with exit code %d", cmd.Process.Pid, r)
 	}

 	// child process exited, resume output
--- a/internal/app/system.go
+++ b/internal/app/system.go
@ -1,8 +1,6 @@
 package app

 import (
-	"os/user"
-
 	"git.ophivana.moe/security/fortify/dbus"
 	"git.ophivana.moe/security/fortify/helper/bwrap"
 	"git.ophivana.moe/security/fortify/internal/linux"
@ -18,7 +16,7 @@ type appSealSys struct {
 	// default formatted XDG_RUNTIME_DIR of User
 	runtime string
 	// target user sealed from config
-	user *user.User
+	user appUser

 	// mapped uid and gid in user namespace
 	mappedID int
@ -32,6 +30,26 @@ type appSealSys struct {
 	// protected by upstream mutex
 }

+type appUser struct {
+	// full uid resolved by fsu
+	uid int
+	// string representation of uid
+	us string
+
+	// supplementary group ids
+	supp []string
+
+	// application id
+	aid int
+	// string representation of aid
+	as string
+
+	// app user home directory
+	home string
+	// passwd database username
+	username string
+}
+
 // shareAll calls all share methods in sequence
 func (seal *appSeal) shareAll(bus [2]*dbus.Config, os linux.System) error {
 	if seal.shared {
--- a/internal/fmsg/defer.go
+++ b/internal/fmsg/defer.go
@ -56,9 +56,10 @@ func Exit(code int) {
 	os.Exit(code)
 }

-func Withhold() {
+func Suspend() {
 	dequeueOnce.Do(dequeue)
 	if wstate.CompareAndSwap(false, true) {
+		queueSync.Wait()
 		withhold <- struct{}{}
 	}
 }
--- a/internal/linux/interface.go
+++ b/internal/linux/interface.go
@ -22,8 +22,8 @@ type System interface {
 	LookPath(file string) (string, error)
 	// Executable provides [os.Executable].
 	Executable() (string, error)
-	// Lookup provides [user.Lookup].
-	Lookup(username string) (*user.User, error)
+	// LookupGroup provides [user.LookupGroup].
+	LookupGroup(name string) (*user.Group, error)
 	// ReadDir provides [os.ReadDir].
 	ReadDir(name string) ([]fs.DirEntry, error)
 	// Stat provides [os.Stat].
@ -35,10 +35,10 @@ type System interface {
 	// Stdout provides [os.Stdout].
 	Stdout() io.Writer

-	// FshimPath returns an absolute path to the fshim binary.
-	FshimPath() string
 	// Paths returns a populated [Paths] struct.
 	Paths() Paths
+	// Uid invokes fsu and returns target uid.
+	Uid(aid int) (int, error)
 	// SdBooted implements https://www.freedesktop.org/software/systemd/man/sd_booted.html
 	SdBooted() bool
 }
--- a/internal/linux/std.go
+++ b/internal/linux/std.go
@ -7,6 +7,7 @@ import (
 	"os"
 	"os/exec"
 	"os/user"
+	"strconv"
 	"sync"

 	"git.ophivana.moe/security/fortify/internal"
@ -21,41 +22,75 @@ type Std struct {
 	sdBooted     bool
 	sdBootedOnce sync.Once

-	fshim     string
-	fshimOnce sync.Once
+	uidOnce sync.Once
+	uidCopy map[int]struct {
+		uid int
+		err error
+	}
+	uidMu sync.RWMutex
 }

-func (s *Std) Geteuid() int                               { return os.Geteuid() }
-func (s *Std) LookupEnv(key string) (string, bool)        { return os.LookupEnv(key) }
-func (s *Std) TempDir() string                            { return os.TempDir() }
-func (s *Std) LookPath(file string) (string, error)       { return exec.LookPath(file) }
-func (s *Std) Executable() (string, error)                { return os.Executable() }
-func (s *Std) Lookup(username string) (*user.User, error) { return user.Lookup(username) }
-func (s *Std) ReadDir(name string) ([]os.DirEntry, error) { return os.ReadDir(name) }
-func (s *Std) Stat(name string) (fs.FileInfo, error)      { return os.Stat(name) }
-func (s *Std) Open(name string) (fs.File, error)          { return os.Open(name) }
-func (s *Std) Exit(code int)                              { fmsg.Exit(code) }
-func (s *Std) Stdout() io.Writer                          { return os.Stdout }
+func (s *Std) Geteuid() int                                 { return os.Geteuid() }
+func (s *Std) LookupEnv(key string) (string, bool)          { return os.LookupEnv(key) }
+func (s *Std) TempDir() string                              { return os.TempDir() }
+func (s *Std) LookPath(file string) (string, error)         { return exec.LookPath(file) }
+func (s *Std) Executable() (string, error)                  { return os.Executable() }
+func (s *Std) LookupGroup(name string) (*user.Group, error) { return user.LookupGroup(name) }
+func (s *Std) ReadDir(name string) ([]os.DirEntry, error)   { return os.ReadDir(name) }
+func (s *Std) Stat(name string) (fs.FileInfo, error)        { return os.Stat(name) }
+func (s *Std) Open(name string) (fs.File, error)            { return os.Open(name) }
+func (s *Std) Exit(code int)                                { fmsg.Exit(code) }
+func (s *Std) Stdout() io.Writer                            { return os.Stdout }

 const xdgRuntimeDir = "XDG_RUNTIME_DIR"

-func (s *Std) FshimPath() string {
-	s.fshimOnce.Do(func() {
-		p, ok := internal.Path(internal.Fshim)
-		if !ok {
-			fmsg.Fatal("invalid fshim path, this copy of fortify is not compiled correctly")
-		}
-		s.fshim = p
-	})
-
-	return s.fshim
-}
-
 func (s *Std) Paths() Paths {
 	s.pathsOnce.Do(func() { CopyPaths(s, &s.paths) })
 	return s.paths
 }

+func (s *Std) Uid(aid int) (int, error) {
+	s.uidOnce.Do(func() {
+		s.uidCopy = make(map[int]struct {
+			uid int
+			err error
+		})
+	})
+
+	s.uidMu.RLock()
+	if u, ok := s.uidCopy[aid]; ok {
+		s.uidMu.RUnlock()
+		return u.uid, u.err
+	}
+
+	s.uidMu.RUnlock()
+	s.uidMu.Lock()
+	defer s.uidMu.Unlock()
+
+	u := struct {
+		uid int
+		err error
+	}{}
+	defer func() { s.uidCopy[aid] = u }()
+
+	u.uid = -1
+	if fsu, ok := internal.Check(internal.Fsu); !ok {
+		fmsg.Fatal("invalid fsu path, this copy of fshim is not compiled correctly")
+		panic("unreachable")
+	} else {
+		cmd := exec.Command(fsu)
+		cmd.Path = fsu
+		cmd.Stderr = os.Stderr // pass through fatal messages
+		cmd.Env = []string{"FORTIFY_APP_ID=" + strconv.Itoa(aid)}
+		cmd.Dir = "/"
+		var p []byte
+		if p, u.err = cmd.Output(); u.err == nil {
+			u.uid, u.err = strconv.Atoi(string(p))
+		}
+		return u.uid, u.err
+	}
+}
+
 func (s *Std) SdBooted() bool {
 	s.sdBootedOnce.Do(func() { s.sdBooted = copySdBooted() })
 	return s.sdBooted
--- a/internal/path.go
+++ b/internal/path.go
@ -4,7 +4,6 @@ import "path"

 var (
 	Fsu   = compPoison
-	Fshim = compPoison
 	Finit = compPoison
 )

--- a/internal/state/print.go
+++ b/internal/state/print.go
@ -67,10 +67,10 @@ func (s *simpleStore) mustPrintLauncherState(w **tabwriter.Writer, now time.Time

 				// write header when initialising
 				if !fmsg.Verbose() {
-					_, _ = fmt.Fprintln(*w, "\tUID\tPID\tUptime\tEnablements\tMethod\tCommand")
+					_, _ = fmt.Fprintln(*w, "\tPID\tApp\tUptime\tEnablements\tCommand")
 				} else {
 					// argv is emitted in body when verbose
-					_, _ = fmt.Fprintln(*w, "\tUID\tPID\tArgv")
+					_, _ = fmt.Fprintln(*w, "\tPID\tApp\tArgv")
 				}
 			}

@ -96,13 +96,13 @@ func (s *simpleStore) mustPrintLauncherState(w **tabwriter.Writer, now time.Time
 				}

 				if !fmsg.Verbose() {
-					_, _ = fmt.Fprintf(*w, "\t%s\t%d\t%s\t%s\t%s\t%s\n",
-						s.path[len(s.path)-1], state.PID, now.Sub(state.Time).Round(time.Second).String(), strings.TrimPrefix(ets.String(), ", "), state.Method,
+					_, _ = fmt.Fprintf(*w, "\t%d\t%s\t%s\t%s\t%s\n",
+						state.PID, s.path[len(s.path)-1], now.Sub(state.Time).Round(time.Second).String(), strings.TrimPrefix(ets.String(), ", "),
 						state.Command)
 				} else {
 					// emit argv instead when verbose
-					_, _ = fmt.Fprintf(*w, "\t%s\t%d\t%s\n",
-						s.path[len(s.path)-1], state.PID, state.Argv)
+					_, _ = fmt.Fprintf(*w, "\t%d\t%s\t%s\n",
+						state.PID, s.path[len(s.path)-1], state.Argv)
 				}
 			}

--- a/internal/state/state.go
+++ b/internal/state/state.go
@ -33,8 +33,6 @@ type State struct {
 	// capability enablements applied to child
 	Capability system.Enablements

-	// user switch method
-	Method string
 	// full argv whe launching
 	Argv []string
 	// process start time
--- a/main.go
+++ b/main.go
@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"flag"
 	"fmt"
+	"strings"
 	"text/tabwriter"

 	"git.ophivana.moe/security/fortify/dbus"
@ -29,6 +30,20 @@ func init() {

 var os = new(linux.Std)

+type gl []string
+
+func (g *gl) String() string {
+	if g == nil {
+		return "<nil>"
+	}
+	return strings.Join(*g, " ")
+}
+
+func (g *gl) Set(v string) error {
+	*g = append(*g, v)
+	return nil
+}
+
 func main() {
 	if err := internal.PR_SET_DUMPABLE__SUID_DUMP_DISABLE(); err != nil {
 		fmsg.Printf("cannot set SUID_DUMP_DISABLE: %s", err)
@ -135,10 +150,11 @@ func main() {
 			mpris             bool
 			dbusVerbose       bool

+			aid         int
+			groups      gl
+			homeDir     string
 			userName    string
 			enablements [system.ELen]bool
-
-			launchMethodText string
 		)

 		set.StringVar(&dbusConfigSession, "dbus-config", "builtin", "Path to D-Bus proxy config file, or \"builtin\" for defaults")
@ -147,29 +163,34 @@ func main() {
 		set.BoolVar(&mpris, "mpris", false, "Allow owning MPRIS D-Bus path, has no effect if custom config is available")
 		set.BoolVar(&dbusVerbose, "dbus-log", false, "Force logging in the D-Bus proxy")

-		set.StringVar(&userName, "u", "chronos", "Passwd name of user to run as")
+		set.IntVar(&aid, "a", 0, "Fortify application ID")
+		set.Var(&groups, "g", "Groups inherited by the app process")
+		set.StringVar(&homeDir, "d", "/var/empty", "Application home directory")
+		set.StringVar(&userName, "u", "chronos", "Passwd name within sandbox")
 		set.BoolVar(&enablements[system.EWayland], "wayland", false, "Share Wayland socket")
 		set.BoolVar(&enablements[system.EX11], "X", false, "Share X11 socket and allow connection")
 		set.BoolVar(&enablements[system.EDBus], "dbus", false, "Proxy D-Bus connection")
 		set.BoolVar(&enablements[system.EPulse], "pulse", false, "Share PulseAudio socket and cookie")

-		methodHelpString := "Method of launching the child process, can be one of \"sudo\""
-		if os.SdBooted() {
-			methodHelpString += ", \"systemd\""
-		}
-		set.StringVar(&launchMethodText, "method", "sudo", methodHelpString)
-
 		// Ignore errors; set is set for ExitOnError.
 		_ = set.Parse(args[1:])

 		// initialise config from flags
 		config := &app.Config{
 			ID:      dbusID,
-			User:    userName,
 			Command: set.Args(),
-			Method:  launchMethodText,
 		}

+		if aid < 0 || aid > 9999 {
+			fmsg.Fatalf("aid %d out of range", aid)
+			panic("unreachable")
+		}
+
+		config.Confinement.AppID = aid
+		config.Confinement.Groups = groups
+		config.Confinement.Home = homeDir
+		config.Confinement.Username = userName
+
 		// enablements from flags
 		for i := system.Enablement(0); i < system.Enablement(system.ELen); i++ {
 			if enablements[i] {
--- a/package.nix
+++ b/package.nix
@ -30,11 +30,12 @@ buildGoModule rec {
        "-w"
        "-X"
        "main.Fmain=${placeholder "out"}/bin/.fortify-wrapped"
+        "-X"
+        "main.Fshim=${placeholder "out"}/bin/fshim"
      ]
      {
        Version = "v${version}";
        Fsu = "/run/wrappers/bin/fsu";
-        Fshim = "${placeholder "out"}/bin/fshim";
        Finit = "${placeholder "out"}/bin/finit";
      };