gh-145986: Avoid unbound C recursion in conv_content_model in pyexpat.c (CVE 2026-4224) (#145987)

Fix C stack overflow (CVE-2026-4224) when an Expat parser
with a registered `ElementDeclHandler` parses inline DTD
containing deeply nested content model.

---------

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

Author: StanFromIreland

Lib/test/test_pyexpat.py

@@ -701,6 +701,25 @@ def test_trigger_leak(self):
         parser.ElementDeclHandler = lambda _1, _2: None
         self.assertRaises(TypeError, parser.Parse, data, True)
 
+    @support.skip_if_unlimited_stack_size
+    @support.skip_emscripten_stack_overflow()
+    @support.skip_wasi_stack_overflow()
+    def test_deeply_nested_content_model(self):
+        # This should raise a RecursionError and not crash.
+        # See https://github.com/python/cpython/issues/145986.
+        N = 500_000
+        data = (
+            b'<!DOCTYPE root [\n<!ELEMENT root '
+            + b'(a, ' * N + b'a' + b')' * N
+            + b'>\n]>\n<root/>\n'
+        )
+
+        parser = expat.ParserCreate()
+        parser.ElementDeclHandler = lambda _1, _2: None
+        with support.infinite_recursion():
+            with self.assertRaises(RecursionError):
+                parser.Parse(data)
+
 class MalformedInputTest(unittest.TestCase):
     def test1(self):
         xml = b"\0\r\n"

Misc/NEWS.d/next/Security/2026-03-14-17-31-39.gh-issue-145986.ifSSr8.rst

@@ -0,0 +1,4 @@
+:mod:`xml.parsers.expat`: Fixed a crash caused by unbounded C recursion when
+converting deeply nested XML content models with
+:meth:`~xml.parsers.expat.xmlparser.ElementDeclHandler`.
+This addresses :cve:`2026-4224`.

Modules/pyexpat.c

@@ -3,6 +3,7 @@
 #endif
 
 #include "Python.h"
+#include "pycore_ceval.h"         // _Py_EnterRecursiveCall()
 #include "pycore_import.h"        // _PyImport_SetModule()
 #include "pycore_pyhash.h"        // _Py_HashSecret
 #include "pycore_traceback.h"     // _PyTraceback_Add()
@@ -607,6 +608,10 @@ static PyObject *
 conv_content_model(XML_Content * const model,
                    PyObject *(*conv_string)(void *))
 {
+    if (_Py_EnterRecursiveCall(" in conv_content_model")) {
+        return NULL;
+    }
+
     PyObject *result = NULL;
     PyObject *children = PyTuple_New(model->numchildren);
     int i;
@@ -618,14 +623,16 @@ conv_content_model(XML_Content * const model,
                                                  conv_string);
             if (child == NULL) {
                 Py_XDECREF(children);
-                return NULL;
+                goto done;
             }
             PyTuple_SET_ITEM(children, i, child);
         }
         result = Py_BuildValue("(iiO&N)",
                                model->type, model->quant,
                                conv_string, model->name, children);
     }
+done:
+    _Py_LeaveRecursiveCall();
     return result;
 }