cmd/sharefs: containerise filesystem daemon

This replaces the forking daemonise libfuse function which prevents Go callbacks from calling into the runtime. This also enforces least privilege on the daemon process.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/sharefs/fuse.go

@@ -12,22 +12,31 @@ extern void sharefs_destroy(void *private_data);
 
 typedef void (*closure)();
 static inline struct fuse_opt _FUSE_OPT_END() { return (struct fuse_opt)FUSE_OPT_END; };
-static inline int _fuse_main(int argc, char *argv[], const struct fuse_operations *op, void *user_data) { return fuse_main(argc, argv, op, user_data); }
 */
 import "C"
 import (
+	"context"
 	"encoding/gob"
+	"errors"
 	"fmt"
+	"io"
 	"log"
 	"os"
-	"path"
+	"os/exec"
+	"os/signal"
 	"runtime"
 	"runtime/cgo"
 	"strconv"
 	"syscall"
 	"unsafe"
 
+	"hakurei.app/container"
+	"hakurei.app/container/check"
+	"hakurei.app/container/std"
+	"hakurei.app/hst"
+	"hakurei.app/internal/helper/proc"
 	"hakurei.app/internal/info"
+	"hakurei.app/message"
 )
 
 type (
@@ -40,8 +49,11 @@ type (
 		// Whether sharefs_init failed.
 		initFailed bool
 
-		// Open file descriptor to backing directory.
-		Source int
+		// Open file descriptor to fuse.
+		Fuse int
+
+		// Pathname to open for dirfd.
+		Source *check.Absolute
 		// New uid and gid to set by sharefs_init when starting as root.
 		Setuid, Setgid int
 	}
@@ -71,22 +83,8 @@ func sharefs_init(_ *C.struct_fuse_conn_info, cfg *C.struct_fuse_config) unsafe.
 	setup := cgo.Handle(priv.setup).Value().(*setupState)
 
 	if os.Geteuid() == 0 {
-		if setup.Setuid <= 0 || setup.Setgid <= 0 {
-			log.Println("setuid and setgid must not be 0")
-			goto fail
-		}
-		if err := syscall.Setresgid(setup.Setgid, setup.Setgid, setup.Setgid); err != nil {
-			log.Printf("cannot set gid: %v", err)
-			goto fail
-		}
-		if err := syscall.Setgroups(nil); err != nil {
-			log.Printf("cannot set supplementary groups: %v", err)
-			goto fail
-		}
-		if err := syscall.Setresuid(setup.Setuid, setup.Setuid, setup.Setuid); err != nil {
-			log.Printf("cannot set uid: %v", err)
-			goto fail
-		}
+		log.Println("filesystem daemon must not run as root")
+		goto fail
 	}
 
 	cfg.use_ino = C.true
@@ -97,7 +95,16 @@ func sharefs_init(_ *C.struct_fuse_conn_info, cfg *C.struct_fuse_config) unsafe.
 	cfg.negative_timeout = 0
 
 	// all future filesystem operations happen through this dirfd
-	priv.dirfd = C.int(setup.Source)
+	if fd, err := syscall.Open(setup.Source.String(), syscall.O_DIRECTORY|syscall.O_RDONLY|syscall.O_CLOEXEC, 0); err != nil {
+		log.Printf("cannot open %q: %v", setup.Source, err)
+		goto fail
+	} else if err = syscall.Fchdir(fd); err != nil {
+		_ = syscall.Close(fd)
+		log.Printf("cannot enter %q: %s", setup.Source, err)
+		goto fail
+	} else {
+		priv.dirfd = C.int(fd)
+	}
 
 	return ctx.private_data
 
@@ -177,8 +184,20 @@ func parseOpts(args *C.struct_fuse_args, setup *setupState) (ok bool) {
 		if v, err := strconv.Atoi(C.GoString(unsafeOpts.setup)); err != nil || v < 3 {
 			log.Println("invalid value for option setup")
 			return false
-		} else if err = gob.NewDecoder(os.NewFile(uintptr(v), "setup")).Decode(setup); err != nil {
-			log.Println(err)
+		} else {
+			r := os.NewFile(uintptr(v), "setup")
+			defer func() {
+				if err = r.Close(); err != nil {
+					log.Println(err)
+				}
+			}()
+			if err = gob.NewDecoder(r).Decode(setup); err != nil {
+				log.Println(err)
+				return false
+			}
+		}
+		if setup.Fuse < 3 {
+			log.Println("invalid file descriptor", setup.Fuse)
 			return false
 		}
 		return true
@@ -187,23 +206,11 @@ func parseOpts(args *C.struct_fuse_args, setup *setupState) (ok bool) {
 	if unsafeOpts.source == nil {
 		showHelp(args)
 		return false
-	} else if source := C.GoString(unsafeOpts.source); !path.IsAbs(source) {
-		log.Println("source is not absolute")
-		return false
-	} else if fd, err := syscall.Open(source, syscall.O_DIRECTORY|syscall.O_RDONLY, 0); err != nil {
-		log.Printf("cannot open source: %v", err)
-		return false
-	} else if err = syscall.Fchdir(fd); err != nil {
-		_ = syscall.Close(fd)
-		log.Printf("cannot enter source: %s", err)
+	} else if a, err := check.NewAbs(C.GoString(unsafeOpts.source)); err != nil {
+		log.Println(err)
 		return false
 	} else {
-		setup.Source = fd
-		defer func() {
-			if !ok {
-				_ = syscall.Close(fd)
-			}
-		}()
+		setup.Source = a
 	}
 
 	if unsafeOpts.setuid == nil {
@@ -244,7 +251,9 @@ func unsafeAddArgument(args *C.struct_fuse_args, arg string) {
 	C.fuse_opt_add_arg(args, (*C.char)(unsafe.Pointer(unsafe.StringData(arg))))
 }
 
-func _main(argc int, argv **C.char) int {
+func _main(argc int, argv **C.char) (exitCode int) {
+	msg := message.New(log.Default())
+	container.TryArgv0(msg)
 	runtime.LockOSThread()
 
 	// don't mask creation mode, kernel already did that
@@ -286,11 +295,24 @@ func _main(argc int, argv **C.char) int {
 	if opts.show_help != 0 {
 		showHelp(&args)
 		return 0
-	}
-
-	if opts.show_help == 0 && opts.mountpoint == nil {
+	} else if opts.mountpoint == nil {
 		log.Println("no mountpoint specified")
 		return 2
+	} else {
+		// hack to keep fuse_parse_cmdline happy in the container
+		mountpoint := C.GoString(opts.mountpoint)
+		pathnameArg := -1
+		for i, arg := range os.Args {
+			if arg == mountpoint {
+				pathnameArg = i
+				break
+			}
+		}
+		if pathnameArg < 0 {
+			log.Println("mountpoint must be absolute")
+			return 2
+		}
+		os.Args[pathnameArg] = container.Nonexistent
 	}
 
 	if !parseOpts(&args, &setup) {
@@ -302,7 +324,7 @@ func _main(argc int, argv **C.char) int {
 			log.Println("setuid and setgid must not be 0")
 			return 1
 		}
-	} else if setup.Setuid > 0 || setup.Setgid > 0 {
+	} else if setup.Fuse < 3 && (setup.Setuid > 0 || setup.Setgid > 0) {
 		log.Println("setuid and setgid has no effect when not starting as root")
 		return 1
 	}
@@ -334,18 +356,136 @@ func _main(argc int, argv **C.char) int {
 		return 3
 	}
 	defer C.fuse_destroy(fuse)
+	se := C.fuse_get_session(fuse)
+
+	if setup.Fuse < 3 {
+		// unconfined, set up mount point and container
+		if C.fuse_mount(fuse, opts.mountpoint) != 0 {
+			return 4
+		}
+		// unmounted by initial process
+		defer func() {
+			if exitCode == 5 {
+				C.fuse_unmount(fuse)
+			}
+		}()
+
+		if os.Geteuid() == 0 {
+			if setup.Setuid <= 0 || setup.Setgid <= 0 {
+				log.Println("setuid and setgid must not be 0")
+				return 5
+			}
+			if err := syscall.Setresgid(setup.Setgid, setup.Setgid, setup.Setgid); err != nil {
+				log.Printf("cannot set gid: %v", err)
+				return 5
+			}
+			if err := syscall.Setgroups(nil); err != nil {
+				log.Printf("cannot set supplementary groups: %v", err)
+				return 5
+			}
+			if err := syscall.Setresuid(setup.Setuid, setup.Setuid, setup.Setuid); err != nil {
+				log.Printf("cannot set uid: %v", err)
+				return 5
+			}
+		}
+
+		msg.SwapVerbose(opts.debug != 0)
+		ctx := context.Background()
+		if opts.foreground != 0 {
+			c, cancel := signal.NotifyContext(ctx, syscall.SIGINT, syscall.SIGTERM)
+			defer cancel()
+			ctx = c
+		}
+		z := container.New(ctx, msg)
+		z.AllowOrphan = opts.foreground == 0
+		z.Env = os.Environ()
+
+		// keep fuse_parse_cmdline happy in the container
+		z.Tmpfs(check.MustAbs(container.Nonexistent), 1<<10, 0755)
+
+		if a, err := check.NewAbs(container.MustExecutable(msg)); err != nil {
+			log.Println(err)
+			return 5
+		} else {
+			z.Path = a
+		}
+		z.Args = os.Args
+		z.ForwardCancel = true
+		z.SeccompPresets |= std.PresetStrict
+		z.ParentPerm = 0700
+		z.Bind(setup.Source, setup.Source, std.BindWritable)
+		if !z.AllowOrphan {
+			z.WaitDelay = hst.WaitDelayMax
+			z.Stdin, z.Stdout, z.Stderr = os.Stdin, os.Stdout, os.Stderr
+		}
+		z.Bind(z.Path, z.Path, 0)
+		setup.Fuse = int(proc.ExtraFileSlice(&z.ExtraFiles, os.NewFile(uintptr(C.fuse_session_fd(se)), "fuse")))
+
+		var setupWriter io.WriteCloser
+		if fd, w, err := container.Setup(&z.ExtraFiles); err != nil {
+			log.Println(err)
+			return 5
+		} else {
+			z.Args = append(z.Args, "-osetup="+strconv.Itoa(fd))
+			setupWriter = w
+		}
+
+		if err := z.Start(); err != nil {
+			if m, ok := message.GetMessage(err); ok {
+				log.Println(m)
+			} else {
+				log.Println(err)
+			}
+			return 5
+		}
+		if err := z.Serve(); err != nil {
+			if m, ok := message.GetMessage(err); ok {
+				log.Println(m)
+			} else {
+				log.Println(err)
+			}
+			return 5
+		}
+
+		if err := gob.NewEncoder(setupWriter).Encode(&setup); err != nil {
+			log.Println(err)
+			return 5
+		} else if err = setupWriter.Close(); err != nil {
+			log.Println(err)
+		}
+
+		if !z.AllowOrphan {
+			if err := z.Wait(); err != nil {
+				var exitError *exec.ExitError
+				if !errors.As(err, &exitError) || exitError == nil {
+					log.Println(err)
+					return 5
+				}
+				switch code := exitError.ExitCode(); syscall.Signal(code & 0x7f) {
+				case syscall.SIGINT:
+				case syscall.SIGTERM:
+
+				default:
+					return code
+				}
+			}
+		}
+		return 0
+	} else { // confined
+		C.free(unsafe.Pointer(opts.mountpoint))
+		// must be heap allocated
+		opts.mountpoint = C.CString("/dev/fd/" + strconv.Itoa(setup.Fuse))
+
+		if err := os.Chdir("/"); err != nil {
+			log.Println(err)
+		}
+	}
 
 	if C.fuse_mount(fuse, opts.mountpoint) != 0 {
 		return 4
 	}
 	defer C.fuse_unmount(fuse)
 
-	// TODO(ophestra): spawn container here, set PR_SET_NO_NEW_PRIVS and enforce landlock
-	if C.fuse_daemonize(opts.foreground) != 0 {
-		return 5
-	}
-
-	se := C.fuse_get_session(fuse)
 	if C.fuse_set_signal_handlers(se) != 0 {
 		return 6
 	}

cmd/sharefs/test/test.py

@@ -33,11 +33,6 @@ check_bad_opts_output("allow_other", "sharefs: setuid and setgid must not be 0\n
 check_bad_opts_output("setuid=1023", "sharefs: setuid and setgid must not be 0\n", privileged=True)
 check_bad_opts_output("setgid=1023", "sharefs: setuid and setgid must not be 0\n", privileged=True)
 
-# Bad backing directory:
-check_bad_opts_output("clone_fd", "sharefs: cannot open source: no such file or directory\n", source="/proc/nonexistent")
-check_bad_opts_output("clone_fd", "sharefs: cannot open source: not a directory\n", source="/proc/self/exe")
-check_bad_opts_output("clone_fd", "sharefs: cannot open source: permission denied\n", source="/root")
-
 # Make sure nothing actually got mounted:
 machine.fail("umount /mnt")
 machine.succeed("rmdir /mnt")

nixos.nix

@@ -88,7 +88,6 @@ in
                 "noatime"
                 "auto_unmount"
                 "allow_other"
-                "clone_fd"
                 "setuid=$(id -u ${cfg.sharefs.user})"
                 "setgid=$(id -g ${cfg.sharefs.group})"
                 "source=${cfg.sharefs.source}"