internal/safepath: Add linux implementation

All subpath components are opened with openat, relative to the base
volume directory and checked against the volume escape.
The final file descriptor is mounted from the /proc/self/fd/<fd> to a
temporary mount point owned by the daemon and then passed to the
underlying container runtime.
Temporary mountpoint is removed after the container is started.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

internal/safepath/join_linux.go

@@ -0,0 +1,154 @@
+package safepath
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strconv"
+
+	"github.com/containerd/log"
+	"github.com/moby/sys/mount"
+	"github.com/pkg/errors"
+	"golang.org/x/sys/unix"
+)
+
+// Join makes sure that the concatenation of path and subpath doesn't
+// resolve to a path outside of path and returns a path to a temporary file that is
+// a bind mount to the the exact same file/directory that was validated.
+//
+// After use, it is the caller's responsibility to call Close on the returned
+// SafePath object, which will unmount the temporary file/directory
+// and remove it.
+func Join(path, subpath string) (*SafePath, error) {
+	base, subpart, err := evaluatePath(path, subpath)
+	if err != nil {
+		return nil, err
+	}
+
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+	fd, err := safeOpenFd(base, subpart)
+	if err != nil {
+		return nil, err
+	}
+
+	defer unix.Close(fd)
+
+	tmpMount, err := tempMountPoint(fd)
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to create temporary file for safe mount")
+	}
+
+	pid := strconv.Itoa(unix.Gettid())
+	// Using explicit pid path, because /proc/self/fd/<fd> fails with EACCES
+	// when running under "Enhanced Container Isolation" in Docker Desktop
+	// which uses sysbox runtime under the hood.
+	// TODO(vvoland): Investigate.
+	mountSource := "/proc/" + pid + "/fd/" + strconv.Itoa(fd)
+
+	if err := unix.Mount(mountSource, tmpMount, "none", unix.MS_BIND, ""); err != nil {
+		os.Remove(tmpMount)
+		return nil, errors.Wrap(err, "failed to mount resolved path")
+	}
+
+	return &SafePath{
+		path:          tmpMount,
+		sourceBase:    base,
+		sourceSubpath: subpart,
+		cleanup:       cleanupSafePath(tmpMount),
+	}, nil
+}
+
+// safeOpenFd opens the file at filepath.Join(path, subpath) in O_PATH
+// mode and returns the file descriptor if subpath is within the subtree
+// rooted at path. It is an error if any of components of path or subpath
+// are symbolic links.
+//
+// It is a caller's responsibility to close the returned file descriptor, if no
+// error was returned.
+func safeOpenFd(path, subpath string) (int, error) {
+	// Open base volume path (_data directory).
+	prevFd, err := unix.Open(path, unix.O_PATH|unix.O_DIRECTORY|unix.O_CLOEXEC|unix.O_NOFOLLOW, 0)
+	if err != nil {
+		return -1, &ErrNotAccessible{Path: path, Cause: err}
+	}
+	defer unix.Close(prevFd)
+
+	// Try to use the Openat2 syscall first (available on Linux 5.6+).
+	fd, err := unix.Openat2(prevFd, subpath, &unix.OpenHow{
+		Flags:   unix.O_PATH | unix.O_CLOEXEC,
+		Mode:    0,
+		Resolve: unix.RESOLVE_BENEATH | unix.RESOLVE_NO_MAGICLINKS | unix.RESOLVE_NO_SYMLINKS,
+	})
+
+	switch {
+	case errors.Is(err, unix.ENOSYS):
+		// Openat2 is not available, fallback to Openat loop.
+		return softOpenat2(prevFd, subpath)
+	case errors.Is(err, unix.EXDEV):
+		return -1, &ErrEscapesBase{Base: path, Subpath: subpath}
+	case errors.Is(err, unix.ENOENT), errors.Is(err, unix.ELOOP):
+		return -1, &ErrNotAccessible{Path: filepath.Join(path, subpath), Cause: err}
+	case err != nil:
+		return -1, &os.PathError{Op: "openat2", Path: subpath, Err: err}
+	}
+
+	// Openat2 is available and succeeded.
+	return fd, nil
+}
+
+func softOpenat2(baseFd int, subpath string) (int, error) {
+	return -1, errors.New("temporary stub, will be removed in later commit")
+}
+
+// tempMountPoint creates a temporary file/directory to act as mount
+// point for the file descriptor.
+func tempMountPoint(sourceFd int) (string, error) {
+	var stat unix.Stat_t
+	err := unix.Fstat(sourceFd, &stat)
+	if err != nil {
+		return "", errors.Wrap(err, "failed to Fstat mount source fd")
+	}
+
+	isDir := (stat.Mode & unix.S_IFMT) == unix.S_IFDIR
+	if isDir {
+		return os.MkdirTemp("", "safe-mount")
+	}
+
+	f, err := os.CreateTemp("", "safe-mount")
+	if err != nil {
+		return "", err
+	}
+
+	p := f.Name()
+	if err := f.Close(); err != nil {
+		return "", err
+	}
+	return p, nil
+}
+
+// cleanupSafePaths returns a function that unmounts the path and removes the
+// mountpoint.
+func cleanupSafePath(path string) func() error {
+	return func() error {
+		log.G(context.TODO()).WithField("path", path).Debug("removing safe temp mount")
+
+		if err := unix.Unmount(path, unix.MNT_DETACH); err != nil {
+			if errors.Is(err, unix.EINVAL) {
+				log.G(context.TODO()).WithField("path", path).Warn("safe temp mount no longer exists?")
+				return nil
+			}
+			return errors.Wrapf(err, "error unmounting safe mount %s", path)
+		}
+		if err := os.Remove(path); err != nil {
+			if errors.Is(err, os.ErrNotExist) {
+				log.G(context.TODO()).WithField("path", path).Warn("safe temp mount no longer exists?")
+				return nil
+			}
+			return errors.Wrapf(err, "failed to delete temporary safe mount")
+		}
+
+		return nil
+	}
+}