cmd: shim and init into separate binaries

This change also fixes a deadlock when shim fails to connect and complete the setup.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

cmd/fshim/ipc/payload.go

@@ -0,0 +1,42 @@
+package shim0
+
+import (
+	"encoding/gob"
+	"errors"
+	"net"
+
+	"git.ophivana.moe/security/fortify/helper/bwrap"
+	"git.ophivana.moe/security/fortify/internal/fmsg"
+)
+
+const Env = "FORTIFY_SHIM"
+
+type Payload struct {
+	// child full argv
+	Argv []string
+	// bwrap, target full exec path
+	Exec [2]string
+	// bwrap config
+	Bwrap *bwrap.Config
+	// whether to pass wayland fd
+	WL bool
+
+	// verbosity pass through
+	Verbose bool
+}
+
+func (p *Payload) Serve(conn *net.UnixConn, wl *Wayland) error {
+	if err := gob.NewEncoder(conn).Encode(*p); err != nil {
+		return fmsg.WrapErrorSuffix(err,
+			"cannot stream shim payload:")
+	}
+
+	if wl != nil {
+		if err := wl.WriteUnix(conn); err != nil {
+			return errors.Join(err, conn.Close())
+		}
+	}
+
+	return fmsg.WrapErrorSuffix(conn.Close(),
+		"cannot close setup connection:")
+}

cmd/fshim/ipc/shim/shim.go

@@ -0,0 +1,222 @@
+package shim
+
+import (
+	"errors"
+	"net"
+	"os"
+	"os/exec"
+	"sync"
+	"sync/atomic"
+	"syscall"
+	"time"
+
+	"git.ophivana.moe/security/fortify/acl"
+	shim0 "git.ophivana.moe/security/fortify/cmd/fshim/ipc"
+	"git.ophivana.moe/security/fortify/internal/fmsg"
+)
+
+const shimSetupTimeout = 5 * time.Second
+
+// used by the parent process
+
+type Shim struct {
+	// user switcher process
+	cmd *exec.Cmd
+	// uid of shim target user
+	uid uint32
+	// whether to check shim pid
+	checkPid bool
+	// user switcher executable path
+	executable string
+	// path to setup socket
+	socket string
+	// shim setup abort reason and completion
+	abort     chan error
+	abortErr  atomic.Pointer[error]
+	abortOnce sync.Once
+	// 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 (s *Shim) String() string {
+	if s.cmd == nil {
+		return "(unused shim manager)"
+	}
+	return s.cmd.String()
+}
+
+func (s *Shim) Unwrap() *exec.Cmd {
+	return s.cmd
+}
+
+func (s *Shim) Abort(err error) {
+	s.abortOnce.Do(func() {
+		s.abortErr.Store(&err)
+		// s.abort is buffered so this will never block
+		s.abort <- err
+	})
+}
+
+func (s *Shim) AbortWait(err error) {
+	s.Abort(err)
+	<-s.abort
+}
+
+type CommandBuilder func(shimEnv string) (args []string)
+
+func (s *Shim) Start(f CommandBuilder) (*time.Time, error) {
+	var (
+		cf     chan *net.UnixConn
+		accept func()
+	)
+
+	// listen on setup socket
+	if c, a, err := s.serve(); err != nil {
+		return nil, fmsg.WrapErrorSuffix(err,
+			"cannot listen on shim setup socket:")
+	} else {
+		// accepts a connection after each call to accept
+		// connections are sent to the channel cf
+		cf, accept = c, a
+	}
+
+	// start user switcher process and save time
+	s.cmd = exec.Command(s.executable, f(shim0.Env+"="+s.socket)...)
+	s.cmd.Env = []string{}
+	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
+	if err := s.cmd.Start(); err != nil {
+		return nil, fmsg.WrapErrorSuffix(err,
+			"cannot start user switcher:")
+	}
+	startTime := time.Now().UTC()
+
+	// kill shim if something goes wrong and an error is returned
+	killShim := func() {
+		if err := s.cmd.Process.Signal(os.Interrupt); err != nil {
+			fmsg.Println("cannot terminate shim on faulted setup:", err)
+		}
+	}
+	defer func() { killShim() }()
+
+	accept()
+	var conn *net.UnixConn
+	select {
+	case c := <-cf:
+		if c == nil {
+			return &startTime, fmsg.WrapErrorSuffix(*s.abortErr.Load(), "cannot accept call on setup socket:")
+		} else {
+			conn = c
+		}
+	case <-time.After(shimSetupTimeout):
+		err := errors.New("timed out waiting for shim")
+		s.AbortWait(err)
+		return &startTime, err
+	}
+
+	// authenticate against called provided uid and shim pid
+	if cred, err := peerCred(conn); err != nil {
+		return &startTime, fmsg.WrapErrorSuffix(*s.abortErr.Load(), "cannot retrieve shim credentials:")
+	} else if cred.Uid != s.uid {
+		fmsg.Printf("process %d owned by user %d tried to connect, expecting %d",
+			cred.Pid, cred.Uid, s.uid)
+		err = errors.New("compromised fortify build")
+		s.Abort(err)
+		return &startTime, err
+	} else if s.checkPid && 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")
+		s.Abort(err)
+		return &startTime, err
+	}
+
+	// serve payload and wayland fd if enabled
+	// this also closes the connection
+	err := s.payload.Serve(conn, s.wl)
+	if err == nil {
+		killShim = func() {}
+	}
+	s.Abort(err) // aborting with nil indicates success
+	return &startTime, err
+}
+
+func (s *Shim) serve() (chan *net.UnixConn, func(), error) {
+	if s.abort != nil {
+		panic("attempted to serve shim setup twice")
+	}
+	s.abort = make(chan error, 1)
+
+	cf := make(chan *net.UnixConn)
+	accept := make(chan struct{}, 1)
+
+	if l, err := net.ListenUnix("unix", &net.UnixAddr{Name: s.socket, Net: "unix"}); err != nil {
+		return nil, nil, err
+	} else {
+		l.SetUnlinkOnClose(true)
+
+		fmsg.VPrintf("listening on shim setup socket %q", s.socket)
+		if err = acl.UpdatePerm(s.socket, int(s.uid), acl.Read, acl.Write, acl.Execute); err != nil {
+			fmsg.Println("cannot append ACL entry to shim setup socket:", err)
+			s.Abort(err) // ensures setup socket cleanup
+		}
+
+		go func() {
+			cfWg := new(sync.WaitGroup)
+			for {
+				select {
+				case err = <-s.abort:
+					if err != nil {
+						fmsg.VPrintln("aborting shim setup, reason:", err)
+					}
+					if err = l.Close(); err != nil {
+						fmsg.Println("cannot close setup socket:", err)
+					}
+					close(s.abort)
+					go func() {
+						cfWg.Wait()
+						close(cf)
+					}()
+					return
+				case <-accept:
+					cfWg.Add(1)
+					go func() {
+						defer cfWg.Done()
+						if conn, err0 := l.AcceptUnix(); err0 != nil {
+							// breaks loop
+							s.Abort(err0)
+							// receiver sees nil value and loads err0 stored during abort
+							cf <- nil
+						} else {
+							cf <- conn
+						}
+					}()
+				}
+			}
+		}()
+	}
+
+	return cf, func() { accept <- struct{}{} }, nil
+}
+
+// peerCred fetches peer credentials of conn
+func peerCred(conn *net.UnixConn) (ucred *syscall.Ucred, err error) {
+	var raw syscall.RawConn
+	if raw, err = conn.SyscallConn(); err != nil {
+		return
+	}
+
+	err0 := raw.Control(func(fd uintptr) {
+		ucred, err = syscall.GetsockoptUcred(int(fd), syscall.SOL_SOCKET, syscall.SO_PEERCRED)
+	})
+	err = errors.Join(err, err0)
+	return
+}

cmd/fshim/ipc/wayland.go

@@ -0,0 +1,75 @@
+package shim0
+
+import (
+	"fmt"
+	"net"
+	"sync"
+	"syscall"
+
+	"git.ophivana.moe/security/fortify/internal/fmsg"
+)
+
+// Wayland implements wayland mediation.
+type Wayland struct {
+	// wayland socket path
+	Path string
+
+	// wayland connection
+	conn *net.UnixConn
+
+	connErr error
+	sync.Once
+	// wait for wayland client to exit
+	done chan struct{}
+}
+
+func (wl *Wayland) WriteUnix(conn *net.UnixConn) error {
+	// connect to host wayland socket
+	if f, err := net.DialUnix("unix", nil, &net.UnixAddr{Name: wl.Path, Net: "unix"}); err != nil {
+		return fmsg.WrapErrorSuffix(err,
+			fmt.Sprintf("cannot connect to wayland at %q:", wl.Path))
+	} else {
+		fmsg.VPrintf("connected to wayland at %q", wl.Path)
+		wl.conn = f
+	}
+
+	// set up for passing wayland socket
+	if rc, err := wl.conn.SyscallConn(); err != nil {
+		return fmsg.WrapErrorSuffix(err, "cannot obtain raw wayland connection:")
+	} else {
+		ec := make(chan error)
+		go func() {
+			// pass wayland connection fd
+			if err = rc.Control(func(fd uintptr) {
+				if _, _, err = conn.WriteMsgUnix(nil, syscall.UnixRights(int(fd)), nil); err != nil {
+					ec <- fmsg.WrapErrorSuffix(err, "cannot pass wayland connection to shim:")
+					return
+				}
+				ec <- nil
+
+				// block until shim exits
+				<-wl.done
+				fmsg.VPrintln("releasing wayland connection")
+			}); err != nil {
+				ec <- fmsg.WrapErrorSuffix(err, "cannot obtain wayland connection fd:")
+				return
+			}
+		}()
+		return <-ec
+	}
+}
+
+func (wl *Wayland) Close() error {
+	wl.Do(func() {
+		close(wl.done)
+		wl.connErr = wl.conn.Close()
+	})
+
+	return wl.connErr
+}
+
+func NewWayland() *Wayland {
+	wl := new(Wayland)
+	wl.done = make(chan struct{})
+	return wl
+}

cmd/fshim/main.go

@@ -0,0 +1,194 @@
+package main
+
+import (
+	"encoding/gob"
+	"errors"
+	"net"
+	"os"
+	"path"
+	"strconv"
+	"syscall"
+
+	init0 "git.ophivana.moe/security/fortify/cmd/finit/ipc"
+	shim "git.ophivana.moe/security/fortify/cmd/fshim/ipc"
+	"git.ophivana.moe/security/fortify/helper"
+	"git.ophivana.moe/security/fortify/internal"
+	"git.ophivana.moe/security/fortify/internal/fmsg"
+)
+
+// everything beyond this point runs as unconstrained target user
+// proceed with caution!
+
+func main() {
+	// sharing stdout with fortify
+	// USE WITH CAUTION
+	fmsg.SetPrefix("shim")
+
+	// setting this prevents ptrace
+	if err := internal.PR_SET_DUMPABLE__SUID_DUMP_DISABLE(); err != nil {
+		fmsg.Fatalf("cannot set SUID_DUMP_DISABLE: %s", err)
+		panic("unreachable")
+	}
+
+	// re-exec
+	if len(os.Args) > 0 && (os.Args[0] != "fshim" || len(os.Args) != 1) && path.IsAbs(os.Args[0]) {
+		if err := syscall.Exec(os.Args[0], []string{"fshim"}, os.Environ()); err != nil {
+			fmsg.Println("cannot re-exec self:", err)
+			// continue anyway
+		}
+	}
+
+	// lookup socket path from environment
+	var socketPath string
+	if s, ok := os.LookupEnv(shim.Env); !ok {
+		fmsg.Fatal("FORTIFY_SHIM not set")
+		panic("unreachable")
+	} else {
+		socketPath = s
+	}
+
+	// check path to finit
+	var finitPath string
+	if p, ok := internal.Path(internal.Finit); !ok {
+		fmsg.Fatal("invalid finit path, this copy of fshim is not compiled correctly")
+	} else {
+		finitPath = p
+	}
+
+	// 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)
+		panic("unreachable")
+	} else {
+		conn = c
+	}
+
+	// decode payload gob stream
+	var payload shim.Payload
+	if err := gob.NewDecoder(conn).Decode(&payload); err != nil {
+		fmsg.Fatal("cannot decode shim payload:", err)
+	} else {
+		fmsg.SetVerbose(payload.Verbose)
+	}
+
+	if payload.Bwrap == nil {
+		fmsg.Fatal("bwrap config not supplied")
+	}
+
+	// receive wayland fd over socket
+	wfd := -1
+	if payload.WL {
+		if fd, err := receiveWLfd(conn); err != nil {
+			fmsg.Fatal("cannot receive wayland fd:", err)
+		} else {
+			wfd = fd
+		}
+	}
+
+	// close setup socket
+	if err := conn.Close(); err != nil {
+		fmsg.Println("cannot close setup socket:", err)
+		// not fatal
+	}
+
+	var ic init0.Payload
+
+	// resolve argv0
+	ic.Argv = payload.Argv
+	if len(ic.Argv) > 0 {
+		// looked up from $PATH by parent
+		ic.Argv0 = payload.Exec[1]
+	} else {
+		// no argv, look up shell instead
+		var ok bool
+		if ic.Argv0, ok = os.LookupEnv("SHELL"); !ok {
+			fmsg.Fatal("no command was specified and $SHELL was unset")
+		}
+
+		ic.Argv = []string{ic.Argv0}
+	}
+
+	conf := payload.Bwrap
+
+	var extraFiles []*os.File
+
+	// pass wayland fd
+	if wfd != -1 {
+		if f := os.NewFile(uintptr(wfd), "wayland"); f != nil {
+			ic.WL = 3 + len(extraFiles)
+			extraFiles = append(extraFiles, f)
+		}
+	} else {
+		ic.WL = -1
+	}
+
+	// share config pipe
+	if r, w, err := os.Pipe(); err != nil {
+		fmsg.Fatal("cannot pipe:", err)
+	} else {
+		conf.SetEnv[init0.Env] = strconv.Itoa(3 + len(extraFiles))
+		extraFiles = append(extraFiles, r)
+
+		fmsg.VPrintln("transmitting config to init")
+		go func() {
+			// stream config to pipe
+			if err = gob.NewEncoder(w).Encode(&ic); err != nil {
+				fmsg.Fatal("cannot transmit init config:", err)
+			}
+		}()
+	}
+
+	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)
+	} else {
+		cmd := b.Unwrap()
+		cmd.Stdin, cmd.Stdout, cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
+		cmd.ExtraFiles = extraFiles
+
+		if fmsg.Verbose() {
+			fmsg.VPrintln("bwrap args:", conf.Args())
+		}
+
+		// run and pass through exit code
+		if err = b.Start(); err != nil {
+			fmsg.Fatal("cannot start target process:", err)
+		} else if err = b.Wait(); err != nil {
+			fmsg.VPrintln("wait:", err)
+		}
+		if b.Unwrap().ProcessState != nil {
+			fmsg.Exit(b.Unwrap().ProcessState.ExitCode())
+		} else {
+			fmsg.Exit(127)
+		}
+	}
+}
+
+func receiveWLfd(conn *net.UnixConn) (int, error) {
+	oob := make([]byte, syscall.CmsgSpace(4)) // single fd
+
+	if _, oobn, _, _, err := conn.ReadMsgUnix(nil, oob); err != nil {
+		return -1, err
+	} else if len(oob) != oobn {
+		return -1, errors.New("invalid message length")
+	}
+
+	var msg syscall.SocketControlMessage
+	if messages, err := syscall.ParseSocketControlMessage(oob); err != nil {
+		return -1, err
+	} else if len(messages) != 1 {
+		return -1, errors.New("unexpected message count")
+	} else {
+		msg = messages[0]
+	}
+
+	if fds, err := syscall.ParseUnixRights(&msg); err != nil {
+		return -1, err
+	} else if len(fds) != 1 {
+		return -1, errors.New("unexpected fd count")
+	} else {
+		return fds[0], nil
+	}
+}