gh-143200: fix UAFs in Element.__{set,get}item__ when the element is concurrently mutated (GH-143226)

(cherry picked from commit b6b0e14)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: picnixz

Lib/test/test_xml_etree.py

@@ -2964,32 +2964,72 @@ def __del__(self):
         elem = b.close()
         self.assertEqual(elem[0].tail, 'ABCDEFGHIJKL')
 
-    def test_subscr(self):
-        # Issue #27863
+    def test_subscr_with_clear(self):
+        # See https://github.com/python/cpython/issues/143200.
+        self.do_test_subscr_with_mutating_slice(use_clear_method=True)
+
+    def test_subscr_with_delete(self):
+        # See https://github.com/python/cpython/issues/72050.
+        self.do_test_subscr_with_mutating_slice(use_clear_method=False)
+
+    def do_test_subscr_with_mutating_slice(self, *, use_clear_method):
         class X:
+            def __init__(self, i=0):
+                self.i = i
             def __index__(self):
-                del e[:]
-                return 1
+                if use_clear_method:
+                    e.clear()
+                else:
+                    del e[:]
+                return self.i
 
-        e = ET.Element('elem')
-        e.append(ET.Element('child'))
-        e[:X()]  # shouldn't crash
+        for s in self.get_mutating_slices(X, 10):
+            with self.subTest(s):
+                e = ET.Element('elem')
+                e.extend([ET.Element(f'c{i}') for i in range(10)])
+                e[s]  # shouldn't crash
 
-        e.append(ET.Element('child'))
-        e[0:10:X()]  # shouldn't crash
+    def test_ass_subscr_with_mutating_slice(self):
+        # See https://github.com/python/cpython/issues/72050
+        # and https://github.com/python/cpython/issues/143200.
 
-    def test_ass_subscr(self):
-        # Issue #27863
         class X:
+            def __init__(self, i=0):
+                self.i = i
             def __index__(self):
                 e[:] = []
-                return 1
+                return self.i
+
+        for s in self.get_mutating_slices(X, 10):
+            with self.subTest(s):
+                e = ET.Element('elem')
+                e.extend([ET.Element(f'c{i}') for i in range(10)])
+                e[s] = []  # shouldn't crash
+
+    def get_mutating_slices(self, index_class, n_children):
+        self.assertGreaterEqual(n_children, 10)
+        return [
+            slice(index_class(), None, None),
+            slice(index_class(2), None, None),
+            slice(None, index_class(), None),
+            slice(None, index_class(2), None),
+            slice(0, 2, index_class(1)),
+            slice(0, 2, index_class(2)),
+            slice(0, n_children, index_class(1)),
+            slice(0, n_children, index_class(2)),
+            slice(0, 2 * n_children, index_class(1)),
+            slice(0, 2 * n_children, index_class(2)),
+        ]
 
-        e = ET.Element('elem')
-        for _ in range(10):
-            e.insert(0, ET.Element('child'))
+    def test_ass_subscr_with_mutating_iterable_value(self):
+        class V:
+            def __iter__(self):
+                e.clear()
+                return iter([ET.Element('a'), ET.Element('b')])
 
-        e[0:10:X()] = []  # shouldn't crash
+        e = ET.Element('elem')
+        e.extend([ET.Element(f'c{i}') for i in range(10)])
+        e[:] = V()
 
     def test_treebuilder_start(self):
         # Issue #27863

Misc/NEWS.d/next/Library/2025-12-27-15-41-27.gh-issue-143200.2hEUAl.rst

@@ -0,0 +1,4 @@
+:mod:`xml.etree.ElementTree`: fix use-after-free crashes in
+:meth:`~object.__getitem__` and :meth:`~object.__setitem__` methods of
+:class:`~xml.etree.ElementTree.Element` when the element is concurrently
+mutated. Patch by Bénédikt Tran.

Modules/_elementtree.c

@@ -1803,16 +1803,20 @@ element_subscr(PyObject* self_, PyObject* item)
         return element_getitem(self_, i);
     }
     else if (PySlice_Check(item)) {
+        // Note: 'slicelen' is computed once we are sure that 'self->extra'
+        // cannot be mutated by user-defined code.
+        // See https://github.com/python/cpython/issues/143200.
         Py_ssize_t start, stop, step, slicelen, i;
         size_t cur;
         PyObject* list;
 
-        if (!self->extra)
-            return PyList_New(0);
-
         if (PySlice_Unpack(item, &start, &stop, &step) < 0) {
             return NULL;
         }
+
+        if (self->extra == NULL) {
+            return PyList_New(0);
+        }
         slicelen = PySlice_AdjustIndices(self->extra->length, &start, &stop,
                                          step);
 
@@ -1855,28 +1859,26 @@ element_ass_subscr(PyObject* self_, PyObject* item, PyObject* value)
         return element_setitem(self_, i, value);
     }
     else if (PySlice_Check(item)) {
+        // Note: 'slicelen' is computed once we are sure that 'self->extra'
+        // cannot be mutated by user-defined code.
+        // See https://github.com/python/cpython/issues/143200.
         Py_ssize_t start, stop, step, slicelen, newlen, i;
         size_t cur;
 
         PyObject* recycle = NULL;
         PyObject* seq;
 
-        if (!self->extra) {
-            if (create_extra(self, NULL) < 0)
-                return -1;
-        }
-
         if (PySlice_Unpack(item, &start, &stop, &step) < 0) {
             return -1;
         }
-        slicelen = PySlice_AdjustIndices(self->extra->length, &start, &stop,
-                                         step);
 
         if (value == NULL) {
             /* Delete slice */
-            size_t cur;
-            Py_ssize_t i;
-
+            if (self->extra == NULL) {
+                return 0;
+            }
+            slicelen = PySlice_AdjustIndices(self->extra->length, &start, &stop,
+                                             step);
             if (slicelen <= 0)
                 return 0;
 
@@ -1945,8 +1947,16 @@ element_ass_subscr(PyObject* self_, PyObject* item, PyObject* value)
         }
         newlen = PySequence_Fast_GET_SIZE(seq);
 
-        if (step !=  1 && newlen != slicelen)
-        {
+        if (self->extra == NULL) {
+            if (create_extra(self, NULL) < 0) {
+                Py_DECREF(seq);
+                return -1;
+            }
+        }
+        slicelen = PySlice_AdjustIndices(self->extra->length, &start, &stop,
+                                         step);
+
+        if (step != 1 && newlen != slicelen) {
             Py_DECREF(seq);
             PyErr_Format(PyExc_ValueError,
                 "attempt to assign sequence of size %zd "