runc can be confused to create empty files/directories on the host

@@ -313,7 +313,7 @@ func mountCgroupV1(m *configs.Mount, c *mountConfig) error {
 			// inside the tmpfs, so we don't want to resolve symlinks).
 			subsystemPath := filepath.Join(c.root, b.Destination)
 			subsystemName := filepath.Base(b.Destination)
-			if err := os.MkdirAll(subsystemPath, 0o755); err != nil {
+			if err := utils.MkdirAllInRoot(c.root, subsystemPath, 0o755); err != nil {
 				return err
 			}
 			if err := utils.WithProcfd(c.root, b.Destination, func(dstFd string) error {
@@ -505,15 +505,26 @@ func createMountpoint(rootfs string, m mountEntry) (string, error) {
 				return "", fmt.Errorf("%w: file bind mount over rootfs", errRootfsToFile)
 			}
 			// Make the parent directory.
-			if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
+			destDir, destBase := filepath.Split(dest)
+			destDirFd, err := utils.MkdirAllInRootOpen(rootfs, destDir, 0o755)
+			if err != nil {
 				return "", fmt.Errorf("make parent dir of file bind-mount: %w", err)
 			}
-			// Make the target file.
-			f, err := os.OpenFile(dest, os.O_CREATE, 0o755)
-			if err != nil {
-				return "", fmt.Errorf("create target of file bind-mount: %w", err)
+			defer destDirFd.Close()
+			// Make the target file. We want to avoid opening any file that is
+			// already there because it could be a "bad" file like an invalid
+			// device or hung tty that might cause a DoS, so we use mknodat.
+			// destBase does not contain any "/" components, and mknodat does
+			// not follow trailing symlinks, so we can safely just call mknodat
+			// here.
+			if err := unix.Mknodat(int(destDirFd.Fd()), destBase, unix.S_IFREG|0o644, 0); err != nil {
+				// If we get EEXIST, there was already an inode there and
+				// we can consider that a success.
+				if !errors.Is(err, unix.EEXIST) {
+					err = &os.PathError{Op: "mknod regular file", Path: dest, Err: err}
+					return "", fmt.Errorf("create target of file bind-mount: %w", err)
+				}
 			}
-			_ = f.Close()
 			// Nothing left to do.
 			return dest, nil
 		}
@@ -532,7 +543,7 @@ func createMountpoint(rootfs string, m mountEntry) (string, error) {
 		}
 	}
 
-	if err := os.MkdirAll(dest, 0o755); err != nil {
+	if err := utils.MkdirAllInRoot(rootfs, dest, 0o755); err != nil {
 		return "", err
 	}
 	return dest, nil
@@ -565,7 +576,7 @@ func mountToRootfs(c *mountConfig, m mountEntry) error {
 		} else if !fi.IsDir() {
 			return fmt.Errorf("filesystem %q must be mounted on ordinary directory", m.Device)
 		}
-		if err := os.MkdirAll(dest, 0o755); err != nil {
+		if err := utils.MkdirAllInRoot(rootfs, dest, 0o755); err != nil {
 			return err
 		}
 		// Selinux kernels do not support labeling of /proc or /sys.
@@ -928,7 +939,7 @@ func createDeviceNode(rootfs string, node *devices.Device, bind bool) error {
 	if dest == rootfs {
 		return fmt.Errorf("%w: mknod over rootfs", errRootfsToFile)
 	}
-	if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
+	if err := utils.MkdirAllInRoot(rootfs, filepath.Dir(dest), 0o755); err != nil {
 		return err
 	}
 	if bind {

@@ -6,7 +6,9 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"runtime"
 	"strconv"
+	"strings"
 	"syscall"
 	"unsafe"
 
@@ -214,3 +216,42 @@ func SetLinuxPersonality(personality int) error {
 	}
 	return nil
 }
+
+func prepareAt(dir *os.File, path string) (int, string) {
+	if dir == nil {
+		return unix.AT_FDCWD, path
+	}
+
+	// Rather than just filepath.Join-ing path here, do it manually so the
+	// error and handle correctly indicate cases like path=".." as being
+	// relative to the correct directory. The handle.Name() might end up being
+	// wrong but because this is (currently) only used in MkdirAllInRoot, that
+	// isn't a problem.
+	dirName := dir.Name()
+	if !strings.HasSuffix(dirName, "/") {
+		dirName += "/"
+	}
+	fullPath := dirName + path
+
+	return int(dir.Fd()), fullPath
+}
+
+func Openat(dir *os.File, path string, flags int, mode uint32) (*os.File, error) {
+	dirFd, fullPath := prepareAt(dir, path)
+	fd, err := unix.Openat(dirFd, path, flags, mode)
+	if err != nil {
+		return nil, &os.PathError{Op: "openat", Path: fullPath, Err: err}
+	}
+	runtime.KeepAlive(dir)
+	return os.NewFile(uintptr(fd), fullPath), nil
+}
+
+func Mkdirat(dir *os.File, path string, mode uint32) error {
+	dirFd, fullPath := prepareAt(dir, path)
+	err := unix.Mkdirat(dirFd, path, mode)
+	if err != nil {
+		err = &os.PathError{Op: "mkdirat", Path: fullPath, Err: err}
+	}
+	runtime.KeepAlive(dir)
+	return err
+}

@@ -3,6 +3,7 @@
 package utils
 
 import (
+	"errors"
 	"fmt"
 	"math"
 	"os"
@@ -13,6 +14,8 @@ import (
 	"sync"
 	_ "unsafe" // for go:linkname
 
+	"github.com/opencontainers/runc/libcontainer/system"
+
 	securejoin "github.com/cyphar/filepath-securejoin"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
@@ -275,3 +278,112 @@ func IsLexicallyInRoot(root, path string) bool {
 	}
 	return strings.HasPrefix(path, root)
 }
+
+// MkdirAllInRootOpen attempts to make
+//
+//	path, _ := securejoin.SecureJoin(root, unsafePath)
+//	os.MkdirAll(path, mode)
+//	os.Open(path)
+//
+// safer against attacks where components in the path are changed between
+// SecureJoin returning and MkdirAll (or Open) being called. In particular, we
+// try to detect any symlink components in the path while we are doing the
+// MkdirAll.
+//
+// NOTE: Unlike os.MkdirAll, mode is not Go's os.FileMode, it is the unix mode
+// (the suid/sgid/sticky bits are not the same as for os.FileMode).
+//
+// NOTE: If unsafePath is a subpath of root, we assume that you have already
+// called SecureJoin and so we use the provided path verbatim without resolving
+// any symlinks (this is done in a way that avoids symlink-exchange races).
+// This means that the path also must not contain ".." elements, otherwise an
+// error will occur.
+//
+// This is a somewhat less safe alternative to
+// <https://github.com/cyphar/filepath-securejoin/pull/13>, but it should
+// detect attempts to trick us into creating directories outside of the root.
+// We should migrate to securejoin.MkdirAll once it is merged.
+func MkdirAllInRootOpen(root, unsafePath string, mode uint32) (_ *os.File, Err error) {
+	// If the path is already "within" the root, use it verbatim.
+	fullPath := unsafePath
+	if !IsLexicallyInRoot(root, unsafePath) {
+		var err error
+		fullPath, err = securejoin.SecureJoin(root, unsafePath)
+		if err != nil {
+			return nil, err
+		}
+	}
+	subPath, err := filepath.Rel(root, fullPath)
+	if err != nil {
+		return nil, err
+	}
+
+	// Check for any silly mode bits.
+	if mode&^0o7777 != 0 {
+		return nil, fmt.Errorf("tried to include non-mode bits in MkdirAll mode: 0o%.3o", mode)
+	}
+
+	currentDir, err := os.OpenFile(root, unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
+	if err != nil {
+		return nil, fmt.Errorf("open root handle: %w", err)
+	}
+	defer func() {
+		if Err != nil {
+			currentDir.Close()
+		}
+	}()
+
+	for _, part := range strings.Split(subPath, string(filepath.Separator)) {
+		switch part {
+		case "", ".":
+			// Skip over no-op components.
+			continue
+		case "..":
+			return nil, fmt.Errorf("possible breakout detected: found %q component in SecureJoin subpath %s", part, subPath)
+		}
+
+		nextDir, err := system.Openat(currentDir, part, unix.O_DIRECTORY|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0)
+		switch {
+		case err == nil:
+			// Update the currentDir.
+			_ = currentDir.Close()
+			currentDir = nextDir
+
+		case errors.Is(err, unix.ENOTDIR):
+			// This might be a symlink or some other random file. Either way,
+			// error out.
+			return nil, fmt.Errorf("cannot mkdir in %s/%s: %w", currentDir.Name(), part, unix.ENOTDIR)
+
+		case errors.Is(err, os.ErrNotExist):
+			// Luckily, mkdirat will not follow trailing symlinks, so this is
+			// safe to do as-is.
+			if err := system.Mkdirat(currentDir, part, mode); err != nil {
+				return nil, err
+			}
+			// Open the new directory. There is a race here where an attacker
+			// could swap the directory with a different directory, but
+			// MkdirAll's fuzzy semantics mean we don't care about that.
+			nextDir, err := system.Openat(currentDir, part, unix.O_DIRECTORY|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0)
+			if err != nil {
+				return nil, fmt.Errorf("open newly created directory: %w", err)
+			}
+			// Update the currentDir.
+			_ = currentDir.Close()
+			currentDir = nextDir
+
+		default:
+			return nil, err
+		}
+	}
+	return currentDir, nil
+}
+
+// MkdirAllInRoot is a wrapper around MkdirAllInRootOpen which closes the
+// returned handle, for callers that don't need to use it.
+func MkdirAllInRoot(root, unsafePath string, mode uint32) error {
+	f, err := MkdirAllInRootOpen(root, unsafePath, mode)
+	if err == nil {
+		_ = f.Close()
+	}
+	return err
+}

@@ -253,7 +253,7 @@ func mountCgroupV1(m *configs.Mount, c *mountConfig) error {
 		if c.cgroupns {
 			subsystemPath := filepath.Join(c.root, b.Destination)
 			subsystemName := filepath.Base(b.Destination)
-			if err := os.MkdirAll(subsystemPath, 0o755); err != nil {
+			if err := utils.MkdirAllInRoot(c.root, subsystemPath, 0o755); err != nil {
 				return err
 			}
 			if err := utils.WithProcfd(c.root, b.Destination, func(procfd string) error {
@@ -406,15 +406,26 @@ func createMountpoint(rootfs string, m *configs.Mount, mountFd *int, source stri
 				return "", fmt.Errorf("%w: file bind mount over rootfs", errRootfsToFile)
 			}
 			// Make the parent directory.
-			if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
+			destDir, destBase := filepath.Split(dest)
+			destDirFd, err := utils.MkdirAllInRootOpen(rootfs, destDir, 0o755)
+			if err != nil {
 				return "", fmt.Errorf("make parent dir of file bind-mount: %w", err)
 			}
-			// Make the target file.
-			f, err := os.OpenFile(dest, os.O_CREATE, 0o755)
-			if err != nil {
-				return "", fmt.Errorf("create target of file bind-mount: %w", err)
+			defer destDirFd.Close()
+			// Make the target file. We want to avoid opening any file that is
+			// already there because it could be a "bad" file like an invalid
+			// device or hung tty that might cause a DoS, so we use mknodat.
+			// destBase does not contain any "/" components, and mknodat does
+			// not follow trailing symlinks, so we can safely just call mknodat
+			// here.
+			if err := unix.Mknodat(int(destDirFd.Fd()), destBase, unix.S_IFREG|0o644, 0); err != nil {
+				// If we get EEXIST, there was already an inode there and
+				// we can consider that a success.
+				if !errors.Is(err, unix.EEXIST) {
+					err = &os.PathError{Op: "mknod regular file", Path: dest, Err: err}
+					return "", fmt.Errorf("create target of file bind-mount: %w", err)
+				}
 			}
-			_ = f.Close()
 			// Nothing left to do.
 			return dest, nil
 		}
@@ -433,7 +444,7 @@ func createMountpoint(rootfs string, m *configs.Mount, mountFd *int, source stri
 		}
 	}
 
-	if err := os.MkdirAll(dest, 0o755); err != nil {
+	if err := utils.MkdirAllInRoot(rootfs, dest, 0o755); err != nil {
 		return "", err
 	}
 	return dest, nil
@@ -463,7 +474,7 @@ func mountToRootfs(m *configs.Mount, c *mountConfig) error {
 		} else if !fi.IsDir() {
 			return fmt.Errorf("filesystem %q must be mounted on ordinary directory", m.Device)
 		}
-		if err := os.MkdirAll(dest, 0o755); err != nil {
+		if err := utils.MkdirAllInRoot(rootfs, dest, 0o755); err != nil {
 			return err
 		}
 		// Selinux kernels do not support labeling of /proc or /sys.
@@ -751,7 +762,7 @@ func createDeviceNode(rootfs string, node *devices.Device, bind bool) error {
 	if dest == rootfs {
 		return fmt.Errorf("%w: mknod over rootfs", errRootfsToFile)
 	}
-	if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
+	if err := utils.MkdirAllInRoot(rootfs, filepath.Dir(dest), 0o755); err != nil {
 		return err
 	}
 	if bind {

@@ -6,6 +6,8 @@ package system
 import (
 	"os"
 	"os/exec"
+	"runtime"
+	"strings"
 	"unsafe"
 
 	"golang.org/x/sys/unix"
@@ -102,3 +104,42 @@ func GetSubreaper() (int, error) {
 
 	return int(i), nil
 }
+
+func prepareAt(dir *os.File, path string) (int, string) {
+	if dir == nil {
+		return unix.AT_FDCWD, path
+	}
+
+	// Rather than just filepath.Join-ing path here, do it manually so the
+	// error and handle correctly indicate cases like path=".." as being
+	// relative to the correct directory. The handle.Name() might end up being
+	// wrong but because this is (currently) only used in MkdirAllInRoot, that
+	// isn't a problem.
+	dirName := dir.Name()
+	if !strings.HasSuffix(dirName, "/") {
+		dirName += "/"
+	}
+	fullPath := dirName + path
+
+	return int(dir.Fd()), fullPath
+}
+
+func Openat(dir *os.File, path string, flags int, mode uint32) (*os.File, error) {
+	dirFd, fullPath := prepareAt(dir, path)
+	fd, err := unix.Openat(dirFd, path, flags, mode)
+	if err != nil {
+		return nil, &os.PathError{Op: "openat", Path: fullPath, Err: err}
+	}
+	runtime.KeepAlive(dir)
+	return os.NewFile(uintptr(fd), fullPath), nil
+}
+
+func Mkdirat(dir *os.File, path string, mode uint32) error {
+	dirFd, fullPath := prepareAt(dir, path)
+	err := unix.Mkdirat(dirFd, path, mode)
+	if err != nil {
+		err = &os.PathError{Op: "mkdirat", Path: fullPath, Err: err}
+	}
+	runtime.KeepAlive(dir)
+	return err
+}

@@ -4,12 +4,17 @@
 package utils
 
 import (
+	"errors"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
 	_ "unsafe" // for go:linkname
 
+	"github.com/opencontainers/runc/libcontainer/system"
+
+	securejoin "github.com/cyphar/filepath-securejoin"
 	"golang.org/x/sys/unix"
 )
 
@@ -130,3 +135,112 @@ func IsLexicallyInRoot(root, path string) bool {
 	}
 	return strings.HasPrefix(path, root)
 }
+
+// MkdirAllInRootOpen attempts to make
+//
+//	path, _ := securejoin.SecureJoin(root, unsafePath)
+//	os.MkdirAll(path, mode)
+//	os.Open(path)
+//
+// safer against attacks where components in the path are changed between
+// SecureJoin returning and MkdirAll (or Open) being called. In particular, we
+// try to detect any symlink components in the path while we are doing the
+// MkdirAll.
+//
+// NOTE: Unlike os.MkdirAll, mode is not Go's os.FileMode, it is the unix mode
+// (the suid/sgid/sticky bits are not the same as for os.FileMode).
+//
+// NOTE: If unsafePath is a subpath of root, we assume that you have already
+// called SecureJoin and so we use the provided path verbatim without resolving
+// any symlinks (this is done in a way that avoids symlink-exchange races).
+// This means that the path also must not contain ".." elements, otherwise an
+// error will occur.
+//
+// This is a somewhat less safe alternative to
+// <https://github.com/cyphar/filepath-securejoin/pull/13>, but it should
+// detect attempts to trick us into creating directories outside of the root.
+// We should migrate to securejoin.MkdirAll once it is merged.
+func MkdirAllInRootOpen(root, unsafePath string, mode uint32) (_ *os.File, Err error) {
+	// If the path is already "within" the root, use it verbatim.
+	fullPath := unsafePath
+	if !IsLexicallyInRoot(root, unsafePath) {
+		var err error
+		fullPath, err = securejoin.SecureJoin(root, unsafePath)
+		if err != nil {
+			return nil, err
+		}
+	}
+	subPath, err := filepath.Rel(root, fullPath)
+	if err != nil {
+		return nil, err
+	}
+
+	// Check for any silly mode bits.
+	if mode&^0o7777 != 0 {
+		return nil, fmt.Errorf("tried to include non-mode bits in MkdirAll mode: 0o%.3o", mode)
+	}
+
+	currentDir, err := os.OpenFile(root, unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
+	if err != nil {
+		return nil, fmt.Errorf("open root handle: %w", err)
+	}
+	defer func() {
+		if Err != nil {
+			currentDir.Close()
+		}
+	}()
+
+	for _, part := range strings.Split(subPath, string(filepath.Separator)) {
+		switch part {
+		case "", ".":
+			// Skip over no-op components.
+			continue
+		case "..":
+			return nil, fmt.Errorf("possible breakout detected: found %q component in SecureJoin subpath %s", part, subPath)
+		}
+
+		nextDir, err := system.Openat(currentDir, part, unix.O_DIRECTORY|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0)
+		switch {
+		case err == nil:
+			// Update the currentDir.
+			_ = currentDir.Close()
+			currentDir = nextDir
+
+		case errors.Is(err, unix.ENOTDIR):
+			// This might be a symlink or some other random file. Either way,
+			// error out.
+			return nil, fmt.Errorf("cannot mkdir in %s/%s: %w", currentDir.Name(), part, unix.ENOTDIR)
+
+		case errors.Is(err, os.ErrNotExist):
+			// Luckily, mkdirat will not follow trailing symlinks, so this is
+			// safe to do as-is.
+			if err := system.Mkdirat(currentDir, part, mode); err != nil {
+				return nil, err
+			}
+			// Open the new directory. There is a race here where an attacker
+			// could swap the directory with a different directory, but
+			// MkdirAll's fuzzy semantics mean we don't care about that.
+			nextDir, err := system.Openat(currentDir, part, unix.O_DIRECTORY|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0)
+			if err != nil {
+				return nil, fmt.Errorf("open newly created directory: %w", err)
+			}
+			// Update the currentDir.
+			_ = currentDir.Close()
+			currentDir = nextDir
+
+		default:
+			return nil, err
+		}
+	}
+	return currentDir, nil
+}
+
+// MkdirAllInRoot is a wrapper around MkdirAllInRootOpen which closes the
+// returned handle, for callers that don't need to use it.
+func MkdirAllInRoot(root, unsafePath string, mode uint32) error {
+	f, err := MkdirAllInRootOpen(root, unsafePath, mode)
+	if err == nil {
+		_ = f.Close()
+	}
+	return err
+}

@@ -1276,8 +1276,7 @@ func (c *linuxContainer) restoreNetwork(req *criurpc.CriuReq, criuOpts *CriuOpts
 // restore using CRIU. This function is inspired from the code in
 // rootfs_linux.go
 func (c *linuxContainer) makeCriuRestoreMountpoints(m *configs.Mount) error {
-	switch m.Device {
-	case "cgroup":
+	if m.Device == "cgroup" {
 		// No mount point(s) need to be created:
 		//
 		// * for v1, mount points are saved by CRIU because
@@ -1286,26 +1285,11 @@ func (c *linuxContainer) makeCriuRestoreMountpoints(m *configs.Mount) error {
 		// * for v2, /sys/fs/cgroup is a real mount, but
 		//   the mountpoint appears as soon as /sys is mounted
 		return nil
-	case "bind":
-		// The prepareBindMount() function checks if source
-		// exists. So it cannot be used for other filesystem types.
-		// TODO: pass something else than nil? Not sure if criu is
-		// impacted by issue #2484
-		if err := prepareBindMount(m, c.config.Rootfs, nil); err != nil {
-			return err
-		}
-	default:
-		// for all other filesystems just create the mountpoints
-		dest, err := securejoin.SecureJoin(c.config.Rootfs, m.Destination)
-		if err != nil {
-			return err
-		}
-		if err := checkProcMount(c.config.Rootfs, dest, m, ""); err != nil {
-			return err
-		}
-		if err := os.MkdirAll(dest, 0o755); err != nil {
-			return err
-		}
+	}
+	// TODO: pass something else than nil? Not sure if criu is
+	// impacted by issue #2484
+	if _, err := createMountpoint(c.config.Rootfs, m, nil, ""); err != nil {
+		return fmt.Errorf("create criu restore mount for %s mount: %w", m.Destination, err)
 	}
 	return nil
 }

@@ -224,36 +224,6 @@ func mountCmd(cmd configs.Command) error {
 	return nil
 }
 
-func prepareBindMount(m *configs.Mount, rootfs string, mountFd *int) error {
-	source := m.Source
-	if mountFd != nil {
-		source = "/proc/self/fd/" + strconv.Itoa(*mountFd)
-	}
-
-	stat, err := os.Stat(source)
-	if err != nil {
-		// error out if the source of a bind mount does not exist as we will be
-		// unable to bind anything to it.
-		return err
-	}
-	// ensure that the destination of the bind mount is resolved of symlinks at mount time because
-	// any previous mounts can invalidate the next mount's destination.
-	// this can happen when a user specifies mounts within other mounts to cause breakouts or other
-	// evil stuff to try to escape the container's rootfs.
-	var dest string
-	if dest, err = securejoin.SecureJoin(rootfs, m.Destination); err != nil {
-		return err
-	}
-	if err := checkProcMount(rootfs, dest, m, source); err != nil {
-		return err
-	}
-	if err := createIfNotExists(dest, stat.IsDir()); err != nil {
-		return err
-	}
-
-	return nil
-}
-
 func mountCgroupV1(m *configs.Mount, c *mountConfig) error {
 	binds, err := getCgroupMounts(m)
 	if err != nil {
@@ -282,6 +252,7 @@ func mountCgroupV1(m *configs.Mount, c *mountConfig) error {
 	for _, b := range binds {
 		if c.cgroupns {
 			subsystemPath := filepath.Join(c.root, b.Destination)
+			subsystemName := filepath.Base(b.Destination)
 			if err := os.MkdirAll(subsystemPath, 0o755); err != nil {
 				return err
 			}
@@ -292,7 +263,7 @@ func mountCgroupV1(m *configs.Mount, c *mountConfig) error {
 				}
 				var (
 					source = "cgroup"
-					data   = filepath.Base(subsystemPath)
+					data   = subsystemName
 				)
 				if data == "systemd" {
 					data = cgroups.CgroupNamePrefix + data
@@ -322,14 +293,7 @@ func mountCgroupV1(m *configs.Mount, c *mountConfig) error {
 }
 
 func mountCgroupV2(m *configs.Mount, c *mountConfig) error {
-	dest, err := securejoin.SecureJoin(c.root, m.Destination)
-	if err != nil {
-		return err
-	}
-	if err := os.MkdirAll(dest, 0o755); err != nil {
-		return err
-	}
-	err = utils.WithProcfd(c.root, m.Destination, func(procfd string) error {
+	err := utils.WithProcfd(c.root, m.Destination, func(procfd string) error {
 		return mount(m.Source, m.Destination, procfd, "cgroup2", uintptr(m.Flags), m.Data)
 	})
 	if err == nil || !(errors.Is(err, unix.EPERM) || errors.Is(err, unix.EBUSY)) {
@@ -411,6 +375,70 @@ func doTmpfsCopyUp(m *configs.Mount, rootfs, mountLabel string) (Err error) {
 	})
 }
 
+var errRootfsToFile = errors.New("config tries to change rootfs to file")
+
+func createMountpoint(rootfs string, m *configs.Mount, mountFd *int, source string) (string, error) {
+	dest, err := securejoin.SecureJoin(rootfs, m.Destination)
+	if err != nil {
+		return "", err
+	}
+	if err := checkProcMount(rootfs, dest, m, source); err != nil {
+		return "", fmt.Errorf("check proc-safety of %s mount: %w", m.Destination, err)
+	}
+
+	switch m.Device {
+	case "bind":
+		source := m.Source
+		if mountFd != nil {
+			source = "/proc/self/fd/" + strconv.Itoa(*mountFd)
+		}
+
+		fi, err := os.Stat(source)
+		if err != nil {
+			// Error out if the source of a bind mount does not exist as we
+			// will be unable to bind anything to it.
+			return "", fmt.Errorf("bind mount source stat: %w", err)
+		}
+		// If the original source is not a directory, make the target a file.
+		if !fi.IsDir() {
+			// Make sure we aren't tricked into trying to make the root a file.
+			if rootfs == dest {
+				return "", fmt.Errorf("%w: file bind mount over rootfs", errRootfsToFile)
+			}
+			// Make the parent directory.
+			if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
+				return "", fmt.Errorf("make parent dir of file bind-mount: %w", err)
+			}
+			// Make the target file.
+			f, err := os.OpenFile(dest, os.O_CREATE, 0o755)
+			if err != nil {
+				return "", fmt.Errorf("create target of file bind-mount: %w", err)
+			}
+			_ = f.Close()
+			// Nothing left to do.
+			return dest, nil
+		}
+
+	case "tmpfs":
+		// If the original target exists, copy the mode for the tmpfs mount.
+		if stat, err := os.Stat(dest); err == nil {
+			dt := fmt.Sprintf("mode=%04o", syscallMode(stat.Mode()))
+			if m.Data != "" {
+				dt = dt + "," + m.Data
+			}
+			m.Data = dt
+
+			// Nothing left to do.
+			return dest, nil
+		}
+	}
+
+	if err := os.MkdirAll(dest, 0o755); err != nil {
+		return "", err
+	}
+	return dest, nil
+}
+
 func mountToRootfs(m *configs.Mount, c *mountConfig) error {
 	rootfs := c.root
 
@@ -442,46 +470,27 @@ func mountToRootfs(m *configs.Mount, c *mountConfig) error {
 		return mountPropagate(m, rootfs, "", nil)
 	}
 
-	mountLabel := c.label
 	mountFd := c.fd
-	dest, err := securejoin.SecureJoin(rootfs, m.Destination)
+	dest, err := createMountpoint(rootfs, m, mountFd, m.Source)
 	if err != nil {
-		return err
+		return fmt.Errorf("create mount destination for %s mount: %w", m.Destination, err)
 	}
+	mountLabel := c.label
 
 	switch m.Device {
 	case "mqueue":
-		if err := os.MkdirAll(dest, 0o755); err != nil {
-			return err
-		}
 		if err := mountPropagate(m, rootfs, "", nil); err != nil {
 			return err
 		}
 		return label.SetFileLabel(dest, mountLabel)
 	case "tmpfs":
-		if stat, err := os.Stat(dest); err != nil {
-			if err := os.MkdirAll(dest, 0o755); err != nil {
-				return err
-			}
-		} else {
-			dt := fmt.Sprintf("mode=%04o", syscallMode(stat.Mode()))
-			if m.Data != "" {
-				dt = dt + "," + m.Data
-			}
-			m.Data = dt
-		}
-
 		if m.Extensions&configs.EXT_COPYUP == configs.EXT_COPYUP {
 			err = doTmpfsCopyUp(m, rootfs, mountLabel)
 		} else {
 			err = mountPropagate(m, rootfs, mountLabel, nil)
 		}
-
 		return err
 	case "bind":
-		if err := prepareBindMount(m, rootfs, mountFd); err != nil {
-			return err
-		}
 		if err := mountPropagate(m, rootfs, mountLabel, mountFd); err != nil {
 			return err
 		}
@@ -509,12 +518,6 @@ func mountToRootfs(m *configs.Mount, c *mountConfig) error {
 		}
 		return mountCgroupV1(m, c)
 	default:
-		if err := checkProcMount(rootfs, dest, m, m.Source); err != nil {
-			return err
-		}
-		if err := os.MkdirAll(dest, 0o755); err != nil {
-			return err
-		}
 		return mountPropagate(m, rootfs, mountLabel, mountFd)
 	}
 	if err := setRecAttr(m, rootfs); err != nil {
@@ -745,6 +748,9 @@ func createDeviceNode(rootfs string, node *devices.Device, bind bool) error {
 	if err != nil {
 		return err
 	}
+	if dest == rootfs {
+		return fmt.Errorf("%w: mknod over rootfs", errRootfsToFile)
+	}
 	if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
 		return err
 	}
@@ -1011,26 +1017,6 @@ func chroot() error {
 	return nil
 }
 
-// createIfNotExists creates a file or a directory only if it does not already exist.
-func createIfNotExists(path string, isDir bool) error {
-	if _, err := os.Stat(path); err != nil {
-		if os.IsNotExist(err) {
-			if isDir {
-				return os.MkdirAll(path, 0o755)
-			}
-			if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
-				return err
-			}
-			f, err := os.OpenFile(path, os.O_CREATE, 0o755)
-			if err != nil {
-				return err
-			}
-			_ = f.Close()
-		}
-	}
-	return nil
-}
-
 // readonlyPath will make a path read only.
 func readonlyPath(path string) error {
 	if err := mount(path, path, "", "", unix.MS_BIND|unix.MS_REC, ""); err != nil {

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"os"
 	"strconv"
+	"strings"
 	_ "unsafe" // for go:linkname
 
 	"golang.org/x/sys/unix"
@@ -115,3 +116,17 @@ func NewSockPair(name string) (parent *os.File, child *os.File, err error) {
 	}
 	return os.NewFile(uintptr(fds[1]), name+"-p"), os.NewFile(uintptr(fds[0]), name+"-c"), nil
 }
+
+// IsLexicallyInRoot is shorthand for strings.HasPrefix(path+"/", root+"/"),
+// but properly handling the case where path or root are "/".
+//
+// NOTE: The return value only make sense if the path doesn't contain "..".
+func IsLexicallyInRoot(root, path string) bool {
+	if root != "/" {
+		root += "/"
+	}
+	if path != "/" {
+		path += "/"
+	}
+	return strings.HasPrefix(path, root)
+}