gh-159739: Handle pyexpat configuration changes during callbacks (#160399)

A character data callback can disable buffering or remove the next event handler during a flush. Interning an attribute name can also change ordered_attributes while pyexpat is filling the attribute container. Using the freed buffer, cleared handler, or wrong container type can crash the interpreter. Clear the buffered length before dispatch, keep the active handler alive, recheck the buffer and handlers after callbacks, and use one attribute mode for the whole start event.

Author: jelle-openai

Lib/test/test_pyexpat.py

@@ -910,6 +910,34 @@ def test_disabling_buffer(self):
         parser.Parse(xml3, True)
         self.assertEqual(self.n, 12)
 
+    def test_disable_buffer_in_character_handler(self):
+        parser = expat.ParserCreate()
+        parser.buffer_size = 2
+        parser.buffer_text = True
+        seen = []
+
+        def consume(text):
+            seen.append(text)
+            if len(seen) == 1:
+                parser.buffer_text = False
+
+        parser.CharacterDataHandler = consume
+        parser.Parse(b'<r>a&#65;bc</r>', True)
+        self.assertEqual(''.join(seen), 'aAbc')
+
+    def test_clear_element_handler_during_flush(self):
+        parser = expat.ParserCreate()
+        parser.buffer_text = True
+        elements = []
+
+        def consume(text):
+            parser.StartElementHandler = None
+
+        parser.CharacterDataHandler = consume
+        parser.StartElementHandler = lambda name, attrs: elements.append(name)
+        parser.Parse(b'<r>abc<s/></r>', True)
+        self.assertEqual(elements, ['r'])
+
     def counting_handler(self, text):
         self.n += 1
 

Misc/NEWS.d/next/Library/2026-10-22-16-23-01.gh-issue-159739.XmlH3a.rst

@@ -0,0 +1,3 @@
+Fix crashes in :mod:`xml.parsers.expat` when a character data callback
+disables buffering or clears another handler during a flush, or when interning
+an attribute name changes :attr:`~xml.parsers.expat.xmlparser.ordered_attributes`.

Modules/pyexpat.c

@@ -317,7 +317,9 @@ call_with_frame(const char *funcname, int lineno, PyObject* func, PyObject* args
 {
     PyObject *res;
 
+    Py_INCREF(func);
     res = PyObject_Call(func, args, NULL);
+    Py_DECREF(func);
     if (res == NULL) {
         _PyTraceback_Add(funcname, __FILE__, lineno);
         XML_StopParser(self->itself, XML_FALSE);
@@ -357,19 +359,25 @@ call_character_handler(xmlparseobject *self, const XML_Char *buffer, int len)
     if (!have_handler(self, CharacterData))
         return -1;
 
-    args = PyTuple_New(1);
-    if (args == NULL)
-        return -1;
-    temp = (conv_string_len_to_unicode(buffer, len));
+    /* Copy the buffer before tuple allocation can trigger a GC callback. */
+    temp = conv_string_len_to_unicode(buffer, len);
     if (temp == NULL) {
-        Py_DECREF(args);
         flag_error(self);
         XML_SetCharacterDataHandler(self->itself,
                                     noop_character_data_handler);
         return -1;
     }
+    args = PyTuple_New(1);
+    if (args == NULL) {
+        Py_DECREF(temp);
+        return -1;
+    }
     PyTuple_SET_ITEM(args, 0, temp);
     /* temp is now a borrowed reference; consider it unused. */
+    if (!have_handler(self, CharacterData)) {
+        Py_DECREF(args);
+        return 0;
+    }
     self->in_callback = 1;
     temp = call_with_frame("CharacterData", __LINE__,
                            self->handlers[CharacterData], args, self);
@@ -389,12 +397,13 @@ call_character_handler(xmlparseobject *self, const XML_Char *buffer, int len)
 static int
 flush_character_buffer(xmlparseobject *self)
 {
-    int rc;
     if (self->buffer == NULL || self->buffer_used == 0)
         return 0;
-    rc = call_character_handler(self, self->buffer, self->buffer_used);
+    int len = self->buffer_used;
+    /* The callback may change buffering or install a new handler, which
+       also flushes. Do not expose the same pending data a second time. */
     self->buffer_used = 0;
-    return rc;
+    return call_character_handler(self, self->buffer, len);
 }
 
 static void
@@ -405,27 +414,22 @@ my_CharacterDataHandler(void *userData, const XML_Char *data, int len)
     if (PyErr_Occurred())
         return;
 
-    if (self->buffer == NULL)
+    if (self->buffer != NULL &&
+        len > (self->buffer_size - self->buffer_used)) {
+        if (flush_character_buffer(self) < 0)
+            return;
+        /* The callback may replace the handler or disable buffering. */
+        if (!have_handler(self, CharacterData))
+            return;
+    }
+    if (self->buffer == NULL ||
+        len > (self->buffer_size - self->buffer_used)) {
         call_character_handler(self, data, len);
+    }
     else {
-        if (len > (self->buffer_size - self->buffer_used)) {
-            if (flush_character_buffer(self) < 0)
-                return;
-            /* handler might have changed; drop the rest on the floor
-             * if there isn't a handler anymore
-             */
-            if (!have_handler(self, CharacterData))
-                return;
-        }
-        if (len > self->buffer_size) {
-            call_character_handler(self, data, len);
-            self->buffer_used = 0;
-        }
-        else {
-            memcpy(self->buffer + self->buffer_used,
-                   data, len * sizeof(XML_Char));
-            self->buffer_used += len;
-        }
+        memcpy(self->buffer + self->buffer_used,
+               data, len * sizeof(XML_Char));
+        self->buffer_used += len;
     }
 }
 
@@ -456,7 +460,9 @@ my_StartElementHandler(void *userData,
                 max += 2;
         }
         /* Build the container. */
-        if (self->ordered_attributes)
+        /* Intern-dictionary comparisons can change the mode mid-loop. */
+        int ordered_attributes = self->ordered_attributes;
+        if (ordered_attributes)
             container = PyList_New(max);
         else
             container = PyDict_New();
@@ -479,7 +485,7 @@ my_StartElementHandler(void *userData,
                 Py_DECREF(n);
                 return;
             }
-            if (self->ordered_attributes) {
+            if (ordered_attributes) {
                 PyList_SET_ITEM(container, i, n);
                 PyList_SET_ITEM(container, i+1, v);
             }
@@ -505,6 +511,10 @@ my_StartElementHandler(void *userData,
             return;
         }
         /* Container is now a borrowed reference; ignore it. */
+        if (!have_handler(self, StartElement)) {
+            Py_DECREF(args);
+            return;
+        }
         self->in_callback = 1;
         rv = call_with_frame("StartElement", __LINE__,
                              self->handlers[StartElement], args, self);
@@ -561,6 +571,10 @@ my_ ## NAME ## Handler PARAMS {                     \
         flag_error(self);                           \
         return RETURN_VARIABLE;                     \
     }                                               \
+    if (!have_handler(self, NAME)) {                \
+        Py_DECREF(args);                            \
+        return RETURN_VARIABLE;                     \
+    }                                               \
     self->in_callback = 1;                          \
     PyObject *rv = call_with_frame(                 \
         #NAME, __LINE__,                            \
@@ -709,6 +723,9 @@ my_ElementDeclHandler(void *userData,
             flag_error(self);
             goto finally;
         }
+        if (!have_handler(self, ElementDecl)) {
+            goto finally;
+        }
         self->in_callback = 1;
         rv = call_with_frame("ElementDecl", __LINE__,
                              self->handlers[ElementDecl], args, self);