Prohibit def_method on marshal-loaded ERB instances

Extends the @_init guard to def_method so that an ERB object created
via Marshal.load (which bypasses initialize) raises ArgumentError
instead of evaluating arbitrary source. def_module and def_class both
delegate to def_method and are covered by the same check.

Co-authored-by: Tristan Madani <TristanInSec@gmail.com>

Author: k0kubun

lib/erb.rb

@@ -1087,6 +1087,9 @@ def new_toplevel(vars = nil)
   # ```
   #
   def def_method(mod, methodname, fname='(ERB)')
+    unless @_init.equal?(self.class.singleton_class)
+      raise ArgumentError, "not initialized"
+    end
     src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
     mod.module_eval do
       eval(src, binding, fname, -1)

test/erb/test_erb.rb

@@ -664,6 +664,33 @@ def test_prohibited_marshal_load
     assert_raise(ArgumentError) {erb.result}
   end
 
+  def test_prohibited_marshal_load_def_method
+    erb = ERB.allocate
+    erb.instance_variable_set(:@src, "")
+    erb.instance_variable_set(:@lineno, 1)
+    erb.instance_variable_set(:@_init, true)
+    erb = Marshal.load(Marshal.dump(erb))
+    assert_raise(ArgumentError) {erb.def_method(Class.new, 'render')}
+  end
+
+  def test_prohibited_marshal_load_def_module
+    erb = ERB.allocate
+    erb.instance_variable_set(:@src, "")
+    erb.instance_variable_set(:@lineno, 1)
+    erb.instance_variable_set(:@_init, true)
+    erb = Marshal.load(Marshal.dump(erb))
+    assert_raise(ArgumentError) {erb.def_module}
+  end
+
+  def test_prohibited_marshal_load_def_class
+    erb = ERB.allocate
+    erb.instance_variable_set(:@src, "")
+    erb.instance_variable_set(:@lineno, 1)
+    erb.instance_variable_set(:@_init, true)
+    erb = Marshal.load(Marshal.dump(erb))
+    assert_raise(ArgumentError) {erb.def_class}
+  end
+
   def test_multi_line_comment_lineno
     erb = ERB.new(<<~EOS)
       <%= __LINE__ %>