1 files changed, 34 insertions, 7 deletions

diff --git a/Modules/_elementtree.c b/Modules/_elementtree.c
index 8c3efa36353..fe24629f9f6 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -811,6 +811,8 @@ _elementtree_Element___deepcopy___impl(ElementObject *self, PyObject *memo)
 
     PyTypeObject *tp = Py_TYPE(self);
     elementtreestate *st = get_elementtree_state_by_type(tp);
+    // The deepcopy() helper takes care of incrementing the refcount
+    // of the object to copy so to avoid use-after-frees.
     tag = deepcopy(st, self->tag, memo);
     if (!tag)
         return NULL;
@@ -845,11 +847,13 @@ _elementtree_Element___deepcopy___impl(ElementObject *self, PyObject *memo)
 
     assert(!element->extra || !element->extra->length);
     if (self->extra) {
-        if (element_resize(element, self->extra->length) < 0)
+        Py_ssize_t expected_count = self->extra->length;
+        if (element_resize(element, expected_count) < 0) {
+            assert(!element->extra->length);
             goto error;
+        }
 
-        // TODO(picnixz): check for an evil child's __deepcopy__ on 'self'
-        for (i = 0; i < self->extra->length; i++) {
+        for (i = 0; self->extra && i < self->extra->length; i++) {
             PyObject* child = deepcopy(st, self->extra->children[i], memo);
             if (!child || !Element_Check(st, child)) {
                 if (child) {
@@ -859,11 +863,24 @@ _elementtree_Element___deepcopy___impl(ElementObject *self, PyObject *memo)
                 element->extra->length = i;
                 goto error;
             }
+            if (self->extra && expected_count != self->extra->length) {
+                // 'self->extra' got mutated and 'element' may not have
+                // sufficient space to hold the next iteration's item.
+                expected_count = self->extra->length;
+                if (element_resize(element, expected_count) < 0) {
+                    Py_DECREF(child);
+                    element->extra->length = i;
+                    goto error;
+                }
+            }
             element->extra->children[i] = child;
         }
 
         assert(!element->extra->length);
-        element->extra->length = self->extra->length;
+        // The original 'self->extra' may be gone at this point if deepcopy()
+        // has side-effects. However, 'i' is the number of copied items that
+        // we were able to successfully copy.
+        element->extra->length = i;
     }
 
     /* add object to memo dictionary (so deepcopy won't visit it again) */
@@ -906,13 +923,20 @@ deepcopy(elementtreestate *st, PyObject *object, PyObject *memo)
                     break;
                 }
             }
-            if (simple)
+            if (simple) {
                 return PyDict_Copy(object);
+            }
             /* Fall through to general case */
         }
         else if (Element_CheckExact(st, object)) {
-            return _elementtree_Element___deepcopy___impl(
+            // The __deepcopy__() call may call arbitrary code even if the
+            // object to copy is a built-in XML element (one of its children
+            // any of its parents in its own __deepcopy__() implementation).
+            Py_INCREF(object);
+            PyObject *res = _elementtree_Element___deepcopy___impl(
                 (ElementObject *)object, memo);
+            Py_DECREF(object);
+            return res;
         }
     }
 
@@ -923,8 +947,11 @@ deepcopy(elementtreestate *st, PyObject *object, PyObject *memo)
         return NULL;
     }
 
+    Py_INCREF(object);
     PyObject *args[2] = {object, memo};
-    return PyObject_Vectorcall(st->deepcopy_obj, args, 2, NULL);
+    PyObject *res = PyObject_Vectorcall(st->deepcopy_obj, args, 2, NULL);
+    Py_DECREF(object);
+    return res;
 }