Add chroot for tar packing operations

Previously only unpack operations were supported with chroot.
This adds chroot support for packing operations.
This prevents potential breakouts when copying data from a container.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

daemon/archive.go

@@ -39,11 +39,11 @@ func extractArchive(i interface{}, src io.Reader, dst string, opts *archive.TarO
 	return chrootarchive.UntarWithRoot(src, dst, opts, root)
 }
 
-func archivePath(i interface{}, src string, opts *archive.TarOptions) (io.ReadCloser, error) {
+func archivePath(i interface{}, src string, opts *archive.TarOptions, root string) (io.ReadCloser, error) {
 	if ap, ok := i.(archiver); ok {
 		return ap.ArchivePath(src, opts)
 	}
-	return archive.TarWithOptions(src, opts)
+	return chrootarchive.Tar(src, opts, root)
 }
 
 // ContainerCopy performs a deprecated operation of archiving the resource at
@@ -239,7 +239,7 @@ func (daemon *Daemon) containerArchivePath(container *container.Container, path
 	sourceDir, sourceBase := driver.Dir(resolvedPath), driver.Base(resolvedPath)
 	opts := archive.TarResourceRebaseOpts(sourceBase, driver.Base(absPath))
 
-	data, err := archivePath(driver, sourceDir, opts)
+	data, err := archivePath(driver, sourceDir, opts, container.BaseFS.Path())
 	if err != nil {
 		return nil, nil, err
 	}
@@ -433,7 +433,7 @@ func (daemon *Daemon) containerCopy(container *container.Container, resource str
 	archive, err := archivePath(driver, basePath, &archive.TarOptions{
 		Compression:  archive.Uncompressed,
 		IncludeFiles: filter,
-	})
+	}, container.BaseFS.Path())
 	if err != nil {
 		return nil, err
 	}

daemon/export.go

@@ -70,7 +70,7 @@ func (daemon *Daemon) containerExport(container *container.Container) (arch io.R
 		Compression: archive.Uncompressed,
 		UIDMaps:     daemon.idMapping.UIDs(),
 		GIDMaps:     daemon.idMapping.GIDs(),
-	})
+	}, basefs.Path())
 	if err != nil {
 		rwlayer.Unmount()
 		return nil, err

pkg/chrootarchive/archive.go

@@ -87,3 +87,11 @@ func untarHandler(tarArchive io.Reader, dest string, options *archive.TarOptions
 
 	return invokeUnpack(r, dest, options, root)
 }
+
+// Tar tars the requested path while chrooted to the specified root.
+func Tar(srcPath string, options *archive.TarOptions, root string) (io.ReadCloser, error) {
+	if options == nil {
+		options = &archive.TarOptions{}
+	}
+	return invokePack(srcPath, options, root)
+}

pkg/chrootarchive/archive_unix.go

@@ -12,9 +12,11 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"strings"
 
 	"github.com/docker/docker/pkg/archive"
 	"github.com/docker/docker/pkg/reexec"
+	"github.com/pkg/errors"
 )
 
 // untar is the entry-point for docker-untar on re-exec. This is not used on
@@ -24,7 +26,7 @@ func untar() {
 	runtime.LockOSThread()
 	flag.Parse()
 
-	var options *archive.TarOptions
+	var options archive.TarOptions
 
 	//read the options from the pipe "ExtraFiles"
 	if err := json.NewDecoder(os.NewFile(3, "options")).Decode(&options); err != nil {
@@ -45,7 +47,7 @@ func untar() {
 		fatal(err)
 	}
 
-	if err := archive.Unpack(os.Stdin, dst, options); err != nil {
+	if err := archive.Unpack(os.Stdin, dst, &options); err != nil {
 		fatal(err)
 	}
 	// fully consume stdin in case it is zero padded
@@ -57,6 +59,9 @@ func untar() {
 }
 
 func invokeUnpack(decompressedArchive io.Reader, dest string, options *archive.TarOptions, root string) error {
+	if root == "" {
+		return errors.New("must specify a root to chroot to")
+	}
 
 	// We can't pass a potentially large exclude list directly via cmd line
 	// because we easily overrun the kernel's max argument/environment size
@@ -112,3 +117,92 @@ func invokeUnpack(decompressedArchive io.Reader, dest string, options *archive.T
 	}
 	return nil
 }
+
+func tar() {
+	runtime.LockOSThread()
+	flag.Parse()
+
+	src := flag.Arg(0)
+	var root string
+	if len(flag.Args()) > 1 {
+		root = flag.Arg(1)
+	}
+
+	if root == "" {
+		root = src
+	}
+
+	if err := realChroot(root); err != nil {
+		fatal(err)
+	}
+
+	var options archive.TarOptions
+	if err := json.NewDecoder(os.Stdin).Decode(&options); err != nil {
+		fatal(err)
+	}
+
+	rdr, err := archive.TarWithOptions(src, &options)
+	if err != nil {
+		fatal(err)
+	}
+	defer rdr.Close()
+
+	if _, err := io.Copy(os.Stdout, rdr); err != nil {
+		fatal(err)
+	}
+
+	os.Exit(0)
+}
+
+func invokePack(srcPath string, options *archive.TarOptions, root string) (io.ReadCloser, error) {
+	if root == "" {
+		return nil, errors.New("root path must not be empty")
+	}
+
+	relSrc, err := filepath.Rel(root, srcPath)
+	if err != nil {
+		return nil, err
+	}
+	if relSrc == "." {
+		relSrc = "/"
+	}
+	if relSrc[0] != '/' {
+		relSrc = "/" + relSrc
+	}
+
+	// make sure we didn't trim a trailing slash with the call to `Rel`
+	if strings.HasSuffix(srcPath, "/") && !strings.HasSuffix(relSrc, "/") {
+		relSrc += "/"
+	}
+
+	cmd := reexec.Command("docker-tar", relSrc, root)
+
+	errBuff := bytes.NewBuffer(nil)
+	cmd.Stderr = errBuff
+
+	tarR, tarW := io.Pipe()
+	cmd.Stdout = tarW
+
+	stdin, err := cmd.StdinPipe()
+	if err != nil {
+		return nil, errors.Wrap(err, "error getting options pipe for tar process")
+	}
+
+	if err := cmd.Start(); err != nil {
+		return nil, errors.Wrap(err, "tar error on re-exec cmd")
+	}
+
+	go func() {
+		err := cmd.Wait()
+		err = errors.Wrapf(err, "error processing tar file: %s", errBuff)
+		tarW.CloseWithError(err)
+	}()
+
+	if err := json.NewEncoder(stdin).Encode(options); err != nil {
+		stdin.Close()
+		return nil, errors.Wrap(err, "tar json encode to pipe failed")
+	}
+	stdin.Close()
+
+	return tarR, nil
+}

pkg/chrootarchive/archive_unix_test.go

@@ -3,11 +3,14 @@
 package chrootarchive
 
 import (
+	gotar "archive/tar"
 	"bytes"
 	"io"
 	"io/ioutil"
 	"os"
+	"path"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/docker/docker/pkg/archive"
@@ -75,3 +78,94 @@ func TestUntarWithMaliciousSymlinks(t *testing.T) {
 	assert.NilError(t, err)
 	assert.Equal(t, string(hostData), "pwn3d")
 }
+
+// Test for CVE-2018-15664
+// Assures that in the case where an "attacker" controlled path is a symlink to
+// some path outside of a container's rootfs that we do not unwittingly leak
+// host data into the archive.
+func TestTarWithMaliciousSymlinks(t *testing.T) {
+	dir, err := ioutil.TempDir("", t.Name())
+	assert.NilError(t, err)
+	// defer os.RemoveAll(dir)
+	t.Log(dir)
+
+	root := filepath.Join(dir, "root")
+
+	err = os.MkdirAll(root, 0755)
+	assert.NilError(t, err)
+
+	hostFileData := []byte("I am a host file")
+
+	// Add a file into a directory above root
+	// Ensure that we can't access this file while tarring.
+	err = ioutil.WriteFile(filepath.Join(dir, "host-file"), hostFileData, 0644)
+	assert.NilError(t, err)
+
+	safe := filepath.Join(root, "safe")
+	err = unix.Symlink(dir, safe)
+	assert.NilError(t, err)
+
+	data := filepath.Join(dir, "data")
+	err = os.MkdirAll(data, 0755)
+	assert.NilError(t, err)
+
+	type testCase struct {
+		p        string
+		includes []string
+	}
+
+	cases := []testCase{
+		{p: safe, includes: []string{"host-file"}},
+		{p: safe + "/", includes: []string{"host-file"}},
+		{p: safe, includes: nil},
+		{p: safe + "/", includes: nil},
+		{p: root, includes: []string{"safe/host-file"}},
+		{p: root, includes: []string{"/safe/host-file"}},
+		{p: root, includes: nil},
+	}
+
+	maxBytes := len(hostFileData)
+
+	for _, tc := range cases {
+		t.Run(path.Join(tc.p+"_"+strings.Join(tc.includes, "_")), func(t *testing.T) {
+			// Here if we use archive.TarWithOptions directly or change the "root" parameter
+			// to be the same as "safe", data from the host will be leaked into the archive
+			var opts *archive.TarOptions
+			if tc.includes != nil {
+				opts = &archive.TarOptions{
+					IncludeFiles: tc.includes,
+				}
+			}
+			rdr, err := Tar(tc.p, opts, root)
+			assert.NilError(t, err)
+			defer rdr.Close()
+
+			tr := gotar.NewReader(rdr)
+			assert.Assert(t, !isDataInTar(t, tr, hostFileData, int64(maxBytes)), "host data leaked to archive")
+		})
+	}
+}
+
+func isDataInTar(t *testing.T, tr *gotar.Reader, compare []byte, maxBytes int64) bool {
+	for {
+		h, err := tr.Next()
+		if err == io.EOF {
+			break
+		}
+		assert.NilError(t, err)
+
+		if h.Size == 0 {
+			continue
+		}
+		assert.Assert(t, h.Size <= maxBytes, "%s: file size exceeds max expected size %d: %d", h.Name, maxBytes, h.Size)
+
+		data := make([]byte, int(h.Size))
+		_, err = io.ReadFull(tr, data)
+		assert.NilError(t, err)
+		if bytes.Contains(data, compare) {
+			return true
+		}
+	}
+
+	return false
+}

pkg/chrootarchive/archive_windows.go

@@ -20,3 +20,10 @@ func invokeUnpack(decompressedArchive io.ReadCloser,
 	// do the unpack. We call inline instead within the daemon process.
 	return archive.Unpack(decompressedArchive, longpath.AddPrefix(dest), options)
 }
+
+func invokePack(srcPath string, options *archive.TarOptions, root string) (io.ReadCloser, error) {
+	// Windows is different to Linux here because Windows does not support
+	// chroot. Hence there is no point sandboxing a chrooted process to
+	// do the pack. We call inline instead within the daemon process.
+	return archive.TarWithOptions(srcPath, options)
+}

pkg/chrootarchive/init_unix.go

@@ -14,6 +14,7 @@ import (
 func init() {
 	reexec.Register("docker-applyLayer", applyLayer)
 	reexec.Register("docker-untar", untar)
+	reexec.Register("docker-tar", tar)
 }
 
 func fatal(err error) {