gh-126033: fix UAF in xml.etree.ElementTree.Element.remove when concurrent mutations happen (#126124)

Lib/test/test_xml_etree.py

@@ -18,9 +18,11 @@
 import textwrap
 import types
 import unittest
+import unittest.mock as mock
 import warnings
 import weakref
 
+from contextlib import nullcontext
 from functools import partial
 from itertools import product, islice
 from test import support
@@ -121,6 +123,21 @@
 </foo>
 """
 
+def is_python_implementation():
+    assert ET is not None, "ET must be initialized"
+    assert pyET is not None, "pyET must be initialized"
+    return ET is pyET
+
+
+def equal_wrapper(cls):
+    """Mock cls.__eq__ to check whether it has been called or not.
+
+    The behaviour of cls.__eq__ (side-effects included) is left as is.
+    """
+    eq = cls.__eq__
+    return mock.patch.object(cls, "__eq__", autospec=True, wraps=eq)
+
+
 def checkwarnings(*filters, quiet=False):
     def decorator(test):
         def newtest(*args, **kwargs):
@@ -2642,6 +2659,7 @@ def test_pickle_issue18997(self):
 
 
 class BadElementTest(ElementTestCase, unittest.TestCase):
+
     def test_extend_mutable_list(self):
         class X:
             @property
@@ -2680,18 +2698,168 @@ class Y(X, ET.Element):
         e = ET.Element('foo')
         e.extend(L)
 
-    def test_remove_with_mutating(self):
+    def test_remove_with_clear_assume_missing(self):
-        class X(ET.Element):
+        # gh-126033: Check that a concurrent clear() for an assumed-to-be
-            def __eq__(self, o):
+        # missing element does not make the interpreter crash.
-                del e[:]
+        self.do_test_remove_with_clear(raises=True)
-                return False
-        e = ET.Element('foo')
-        e.extend([X('bar')])
-        self.assertRaises(ValueError, e.remove, ET.Element('baz'))
 
-        e = ET.Element('foo')
+    def test_remove_with_clear_assume_existing(self):
-        e.extend([ET.Element('bar')])
+        # gh-126033: Check that a concurrent clear() for an assumed-to-be
-        self.assertRaises(ValueError, e.remove, X('baz'))
+        # existing element does not make the interpreter crash.
+        self.do_test_remove_with_clear(raises=False)
+
+    def do_test_remove_with_clear(self, *, raises):
+
+        # Until the discrepency between "del root[:]" and "root.clear()" is
+        # resolved, we need to keep two tests. Previously, using "del root[:]"
+        # did not crash with the reproducer of gh-126033 while "root.clear()"
+        # did.
+
+        class E(ET.Element):
+            """Local class to be able to mock E.__eq__ for introspection."""
+
+        class X(E):
+            def __eq__(self, o):
+                del root[:]
+                return not raises
+
+        class Y(E):
+            def __eq__(self, o):
+                root.clear()
+                return not raises
+
+        if raises:
+            get_checker_context = lambda: self.assertRaises(ValueError)
+        else:
+            get_checker_context = nullcontext
+
+        self.assertIs(E.__eq__, object.__eq__)
+
+        for Z, side_effect in [(X, 'del root[:]'), (Y, 'root.clear()')]:
+            self.enterContext(self.subTest(side_effect=side_effect))
+
+            # test removing R() from [U()]
+            for R, U, description in [
+                (E, Z, "remove missing E() from [Z()]"),
+                (Z, E, "remove missing Z() from [E()]"),
+                (Z, Z, "remove missing Z() from [Z()]"),
+            ]:
+                with self.subTest(description):
+                    root = E('top')
+                    root.extend([U('one')])
+                    with get_checker_context():
+                        root.remove(R('missing'))
+
+            # test removing R() from [U(), V()]
+            cases = self.cases_for_remove_missing_with_mutations(E, Z)
+            for R, U, V, description in cases:
+                with self.subTest(description):
+                    root = E('top')
+                    root.extend([U('one'), V('two')])
+                    with get_checker_context():
+                        root.remove(R('missing'))
+
+            # Test removing root[0] from [Z()].
+            #
+            # Since we call root.remove() with root[0], Z.__eq__()
+            # will not be called (we branch on the fast Py_EQ path).
+            with self.subTest("remove root[0] from [Z()]"):
+                root = E('top')
+                root.append(Z('rem'))
+                with equal_wrapper(E) as f, equal_wrapper(Z) as g:
+                    root.remove(root[0])
+                f.assert_not_called()
+                g.assert_not_called()
+
+            # Test removing root[1] (of type R) from [U(), R()].
+            is_special = is_python_implementation() and raises and Z is Y
+            if is_python_implementation() and raises and Z is Y:
+                # In pure Python, using root.clear() sets the children
+                # list to [] without calling list.clear().
+                #
+                # For this reason, the call to root.remove() first
+                # checks root[0] and sets the children list to []
+                # since either root[0] or root[1] is an evil element.
+                #
+                # Since checking root[1] still uses the old reference
+                # to the children list, PyObject_RichCompareBool() branches
+                # to the fast Py_EQ path and Y.__eq__() is called exactly
+                # once (when checking root[0]).
+                continue
+            else:
+                cases = self.cases_for_remove_existing_with_mutations(E, Z)
+                for R, U, description in cases:
+                    with self.subTest(description):
+                        root = E('top')
+                        root.extend([U('one'), R('rem')])
+                        with get_checker_context():
+                            root.remove(root[1])
+
+    def test_remove_with_mutate_root_assume_missing(self):
+        # gh-126033: Check that a concurrent mutation for an assumed-to-be
+        # missing element does not make the interpreter crash.
+        self.do_test_remove_with_mutate_root(raises=True)
+
+    def test_remove_with_mutate_root_assume_existing(self):
+        # gh-126033: Check that a concurrent mutation for an assumed-to-be
+        # existing element does not make the interpreter crash.
+        self.do_test_remove_with_mutate_root(raises=False)
+
+    def do_test_remove_with_mutate_root(self, *, raises):
+        E = ET.Element
+
+        class Z(E):
+            def __eq__(self, o):
+                del root[0]
+                return not raises
+
+        if raises:
+            get_checker_context = lambda: self.assertRaises(ValueError)
+        else:
+            get_checker_context = nullcontext
+
+        # test removing R() from [U(), V()]
+        cases = self.cases_for_remove_missing_with_mutations(E, Z)
+        for R, U, V, description in cases:
+            with self.subTest(description):
+                root = E('top')
+                root.extend([U('one'), V('two')])
+                with get_checker_context():
+                    root.remove(R('missing'))
+
+        # test removing root[1] (of type R) from [U(), R()]
+        cases = self.cases_for_remove_existing_with_mutations(E, Z)
+        for R, U, description in cases:
+            with self.subTest(description):
+                root = E('top')
+                root.extend([U('one'), R('rem')])
+                with get_checker_context():
+                    root.remove(root[1])
+
+    def cases_for_remove_missing_with_mutations(self, E, Z):
+        # Cases for removing R() from [U(), V()].
+        # The case U = V = R = E is not interesting as there is no mutation.
+        for U, V in [(E, Z), (Z, E), (Z, Z)]:
+            description = (f"remove missing {E.__name__}() from "
+                           f"[{U.__name__}(), {V.__name__}()]")
+            yield E, U, V, description
+
+        for U, V in [(E, E), (E, Z), (Z, E), (Z, Z)]:
+            description = (f"remove missing {Z.__name__}() from "
+                           f"[{U.__name__}(), {V.__name__}()]")
+            yield Z, U, V, description
+
+    def cases_for_remove_existing_with_mutations(self, E, Z):
+        # Cases for removing root[1] (of type R) from [U(), R()].
+        # The case U = R = E is not interesting as there is no mutation.
+        for U, R, description in [
+            (E, Z, "remove root[1] from [E(), Z()]"),
+            (Z, E, "remove root[1] from [Z(), E()]"),
+            (Z, Z, "remove root[1] from [Z(), Z()]"),
+        ]:
+            description = (f"remove root[1] (of type {R.__name__}) "
+                           f"from [{U.__name__}(), {R.__name__}()]")
+            yield R, U, description
 
     @support.infinite_recursion(25)
     def test_recursive_repr(self):

Misc/NEWS.d/next/Library/2024-10-29-12-59-45.gh-issue-126033.sM3uCn.rst

@@ -0,0 +1,3 @@
+:mod:`xml.etree.ElementTree`: Fix a crash in :meth:`Element.remove
+<xml.etree.ElementTree.Element.remove>` when the element is
+concurrently mutated. Patch by Bénédikt Tran.

Modules/_elementtree.c

@@ -850,6 +850,7 @@ _elementtree_Element___deepcopy___impl(ElementObject *self, PyObject *memo)
         if (element_resize(element, self->extra->length) < 0)
             goto error;
 
+        // TODO(picnixz): check for an evil child's __deepcopy__ on 'self'
         for (i = 0; i < self->extra->length; i++) {
             PyObject* child = deepcopy(st, self->extra->children[i], memo);
             if (!child || !Element_Check(st, child)) {
@@ -1629,42 +1630,47 @@ _elementtree_Element_remove_impl(ElementObject *self, PyObject *subelement)
 /*[clinic end generated code: output=38fe6c07d6d87d1f input=6133e1d05597d5ee]*/
 {
     Py_ssize_t i;
-    int rc;
+    // When iterating over the list of children, we need to check that the
-    PyObject *found;
+    // list is not cleared (self->extra != NULL) and that we are still within
-
+    // the correct bounds (i < self->extra->length).
-    if (!self->extra) {
+    //
-        /* element has no children, so raise exception */
+    // We deliberately avoid protecting against children lists that grow
-        PyErr_SetString(
+    // faster than the index since list objects do not protect against it.
-            PyExc_ValueError,
+    int rc = 0;
-            "list.remove(x): x not in list"
+    for (i = 0; self->extra && i < self->extra->length; i++) {
-            );
+        if (self->extra->children[i] == subelement) {
-        return NULL;
+            rc = 1;
-    }
-
-    for (i = 0; i < self->extra->length; i++) {
-        if (self->extra->children[i] == subelement)
             break;
-        rc = PyObject_RichCompareBool(self->extra->children[i], subelement, Py_EQ);
+        }
-        if (rc > 0)
+        PyObject *child = Py_NewRef(self->extra->children[i]);
-            break;
+        rc = PyObject_RichCompareBool(child, subelement, Py_EQ);
-        if (rc < 0)
+        Py_DECREF(child);
+        if (rc < 0) {
             return NULL;
+        }
+        else if (rc > 0) {
+            break;
+        }
     }
 
-    if (i >= self->extra->length) {
+    if (rc == 0) {
-        /* subelement is not in children, so raise exception */
+        PyErr_SetString(PyExc_ValueError, "list.remove(x): x not in list");
-        PyErr_SetString(
-            PyExc_ValueError,
-            "list.remove(x): x not in list"
-            );
         return NULL;
     }
 
-    found = self->extra->children[i];
+    // An extra check must be done if the mutation occurs at the very last
+    // step and removes or clears the 'extra' list (the condition on the
+    // length would not be satisfied any more).
+    if (self->extra == NULL || i >= self->extra->length) {
+        Py_RETURN_NONE;
+    }
+
+    PyObject *found = self->extra->children[i];
 
     self->extra->length--;
-    for (; i < self->extra->length; i++)
+    for (; i < self->extra->length; i++) {
         self->extra->children[i] = self->extra->children[i+1];
+    }
 
     Py_DECREF(found);
     Py_RETURN_NONE;