libcontainer: create Cwd when it does not exist

libcontainer/rootfs_linux.go

@@ -40,7 +40,8 @@ func needsSetupDev(config *configs.Config) bool {
 // prepareRootfs sets up the devices, mount points, and filesystems for use
 // inside a new mount namespace. It doesn't set anything as ro. You must call
 // finalizeRootfs after this function to finish setting up the rootfs.
-func prepareRootfs(pipe io.ReadWriter, config *configs.Config) (err error) {
+func prepareRootfs(pipe io.ReadWriter, iConfig *initConfig) (err error) {
+	config := iConfig.Config
 	if err := prepareRoot(config); err != nil {
 		return newSystemErrorWithCause(err, "preparing rootfs")
 	}
@@ -80,6 +81,7 @@ func prepareRootfs(pipe io.ReadWriter, config *configs.Config) (err error) {
 	// The hooks are run after the mounts are setup, but before we switch to the new
 	// root, so that the old root is still available in the hooks for any mount
 	// manipulations.
+	// Note that iConfig.Cwd is not guaranteed to exist here.
 	if err := syncParentHooks(pipe); err != nil {
 		return err
 	}
@@ -111,6 +113,14 @@ func prepareRootfs(pipe io.ReadWriter, config *configs.Config) (err error) {
 		}
 	}
 
+	if cwd := iConfig.Cwd; cwd != "" {
+		// Note that spec.Process.Cwd can contain unclean value like  "../../../../foo/bar...".
+		// However, we are safe to call MkDirAll directly because we are in the jail here.
+		if err := os.MkdirAll(cwd, 0755); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }
 

libcontainer/standard_init_linux.go

@@ -68,7 +68,7 @@ func (l *linuxStandardInit) Init() error {
 
 	// prepareRootfs() can be executed only for a new mount namespace.
 	if l.config.Config.Namespaces.Contains(configs.NEWNS) {
-		if err := prepareRootfs(l.pipe, l.config.Config); err != nil {
+		if err := prepareRootfs(l.pipe, l.config); err != nil {
 			return err
 		}
 	}