bpo-31728: Prevent crashes in _elementtree due to unsafe cleanup of Element.text and Element.tail (#3924)

Lib/test/test_xml_etree_c.py

@@ -85,6 +85,38 @@ class MiscTests(unittest.TestCase):
         # and so destroy the parser
         support.gc_collect()
 
+    def test_bpo_31728(self):
+        # A crash or an assertion failure shouldn't happen, in case garbage
+        # collection triggers a call to clear() or a reading of text or tail,
+        # while a setter or clear() or __setstate__() is already running.
+        elem = cET.Element('elem')
+        class X:
+            def __del__(self):
+                elem.text
+                elem.tail
+                elem.clear()
+
+        elem.text = X()
+        elem.clear()  # shouldn't crash
+
+        elem.tail = X()
+        elem.clear()  # shouldn't crash
+
+        elem.text = X()
+        elem.text = X()  # shouldn't crash
+        elem.clear()
+
+        elem.tail = X()
+        elem.tail = X()  # shouldn't crash
+        elem.clear()
+
+        elem.text = X()
+        elem.__setstate__({'tag': 42})  # shouldn't cause an assertion failure
+        elem.clear()
+
+        elem.tail = X()
+        elem.__setstate__({'tag': 42})  # shouldn't cause an assertion failure
+
 
 @unittest.skipUnless(cET, 'requires _elementtree')
 class TestAliasWorking(unittest.TestCase):

Misc/NEWS.d/next/Library/2017-10-08-23-28-30.bpo-31728.XrVMME.rst

@@ -0,0 +1,2 @@
+Prevent crashes in `_elementtree` due to unsafe cleanup of `Element.text`
+and `Element.tail`. Patch by Oren Milman.

Modules/_elementtree.c

@@ -61,15 +61,22 @@ do { memory -= size; printf("%8d - %s\n", memory, comment); } while (0)
 #define JOIN_SET(p, flag) ((void*) ((uintptr_t) (JOIN_OBJ(p)) | (flag)))
 #define JOIN_OBJ(p) ((PyObject*) ((uintptr_t) (p) & ~(uintptr_t)1))
 
+/* Py_SETREF for a PyObject* that uses a join flag. */
+Py_LOCAL_INLINE(void)
+_set_joined_ptr(PyObject **p, PyObject *new_joined_ptr)
+{
+    PyObject *tmp = JOIN_OBJ(*p);
+    *p = new_joined_ptr;
+    Py_DECREF(tmp);
+}
+
 /* Py_CLEAR for a PyObject* that uses a join flag. Pass the pointer by
  * reference since this function sets it to NULL.
 */
 static void _clear_joined_ptr(PyObject **p)
 {
     if (*p) {
-        PyObject *tmp = JOIN_OBJ(*p);
-        *p = NULL;
-        Py_DECREF(tmp);
+        _set_joined_ptr(p, NULL);
     }
 }
 
@@ -356,7 +363,6 @@ static int
 element_init(PyObject *self, PyObject *args, PyObject *kwds)
 {
     PyObject *tag;
-    PyObject *tmp;
     PyObject *attrib = NULL;
     ElementObject *self_elem;
 
@@ -397,15 +403,11 @@ element_init(PyObject *self, PyObject *args, PyObject *kwds)
     Py_INCREF(tag);
     Py_XSETREF(self_elem->tag, tag);
 
-    tmp = self_elem->text;
     Py_INCREF(Py_None);
-    self_elem->text = Py_None;
-    Py_DECREF(JOIN_OBJ(tmp));
+    _set_joined_ptr(&self_elem->text, Py_None);
 
-    tmp = self_elem->tail;
     Py_INCREF(Py_None);
-    self_elem->tail = Py_None;
-    Py_DECREF(JOIN_OBJ(tmp));
+    _set_joined_ptr(&self_elem->tail, Py_None);
 
     return 0;
 }
@@ -675,12 +677,10 @@ _elementtree_Element_clear_impl(ElementObject *self)
     dealloc_extra(self);
 
     Py_INCREF(Py_None);
-    Py_DECREF(JOIN_OBJ(self->text));
-    self->text = Py_None;
+    _set_joined_ptr(&self->text, Py_None);
 
     Py_INCREF(Py_None);
-    Py_DECREF(JOIN_OBJ(self->tail));
-    self->tail = Py_None;
+    _set_joined_ptr(&self->tail, Py_None);
 
     Py_RETURN_NONE;
 }
@@ -702,13 +702,11 @@ _elementtree_Element___copy___impl(ElementObject *self)
     if (!element)
         return NULL;
 
-    Py_DECREF(JOIN_OBJ(element->text));
-    element->text = self->text;
-    Py_INCREF(JOIN_OBJ(element->text));
+    Py_INCREF(JOIN_OBJ(self->text));
+    _set_joined_ptr(&element->text, self->text);
 
-    Py_DECREF(JOIN_OBJ(element->tail));
-    element->tail = self->tail;
-    Py_INCREF(JOIN_OBJ(element->tail));
+    Py_INCREF(JOIN_OBJ(self->tail));
+    _set_joined_ptr(&element->tail, self->tail);
 
     if (self->extra) {
         if (element_resize(element, self->extra->length) < 0) {
@@ -776,14 +774,12 @@ _elementtree_Element___deepcopy___impl(ElementObject *self, PyObject *memo)
     text = deepcopy(JOIN_OBJ(self->text), memo);
     if (!text)
         goto error;
-    Py_DECREF(element->text);
-    element->text = JOIN_SET(text, JOIN_GET(self->text));
+    _set_joined_ptr(&element->text, JOIN_SET(text, JOIN_GET(self->text)));
 
     tail = deepcopy(JOIN_OBJ(self->tail), memo);
     if (!tail)
         goto error;
-    Py_DECREF(element->tail);
-    element->tail = JOIN_SET(tail, JOIN_GET(self->tail));
+    _set_joined_ptr(&element->tail, JOIN_SET(tail, JOIN_GET(self->tail)));
 
     if (self->extra) {
         if (element_resize(element, self->extra->length) < 0)
@@ -968,13 +964,13 @@ element_setstate_from_attributes(ElementObject *self,
     Py_INCREF(tag);
     Py_XSETREF(self->tag, tag);
 
-    _clear_joined_ptr(&self->text);
-    self->text = text ? JOIN_SET(text, PyList_CheckExact(text)) : Py_None;
-    Py_INCREF(JOIN_OBJ(self->text));
+    text = text ? JOIN_SET(text, PyList_CheckExact(text)) : Py_None;
+    Py_INCREF(JOIN_OBJ(text));
+    _set_joined_ptr(&self->text, text);
 
-    _clear_joined_ptr(&self->tail);
-    self->tail = tail ? JOIN_SET(tail, PyList_CheckExact(tail)) : Py_None;
-    Py_INCREF(JOIN_OBJ(self->tail));
+    tail = tail ? JOIN_SET(tail, PyList_CheckExact(tail)) : Py_None;
+    Py_INCREF(JOIN_OBJ(tail));
+    _set_joined_ptr(&self->tail, tail);
 
     /* Handle ATTRIB and CHILDREN. */
     if (!children && !attrib)
@@ -2009,8 +2005,7 @@ element_text_setter(ElementObject *self, PyObject *value, void *closure)
 {
     _VALIDATE_ATTR_VALUE(value);
     Py_INCREF(value);
-    Py_DECREF(JOIN_OBJ(self->text));
-    self->text = value;
+    _set_joined_ptr(&self->text, value);
     return 0;
 }
 
@@ -2019,8 +2014,7 @@ element_tail_setter(ElementObject *self, PyObject *value, void *closure)
 {
     _VALIDATE_ATTR_VALUE(value);
     Py_INCREF(value);
-    Py_DECREF(JOIN_OBJ(self->tail));
-    self->tail = value;
+    _set_joined_ptr(&self->tail, value);
     return 0;
 }