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/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
+	}
+}