gh-115398: Expose Expat >=2.6.0 reparse deferral API (CVE-2023-52425) (GH-115623)

Allow controlling Expat >=2.6.0 reparse deferral (CVE-2023-52425) by adding five new methods:

- `xml.etree.ElementTree.XMLParser.flush`
- `xml.etree.ElementTree.XMLPullParser.flush`
- `xml.parsers.expat.xmlparser.GetReparseDeferralEnabled`
- `xml.parsers.expat.xmlparser.SetReparseDeferralEnabled`
- `xml.sax.expatreader.ExpatParser.flush`

Based on the "flush" idea from https://github.com/python/cpython/pull/115138#issuecomment-1932444270 .

### Notes

- Please treat as a security fix related to CVE-2023-52425.

Includes code suggested-by: Snild Dolkow <snild@sony.com>
and by core dev Serhiy Storchaka.

5 files changed, 155 insertions, 2 deletions

diff --git a/Modules/_elementtree.c b/Modules/_elementtree.c
index 5445108..edd2f88 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -3895,6 +3895,40 @@ _elementtree_XMLParser_close_impl(XMLParserObject *self)
 }
 
 /*[clinic input]
+_elementtree.XMLParser.flush
+
+[clinic start generated code]*/
+
+static PyObject *
+_elementtree_XMLParser_flush_impl(XMLParserObject *self)
+/*[clinic end generated code: output=42fdb8795ca24509 input=effbecdb28715949]*/
+{
+    if (!_check_xmlparser(self)) {
+        return NULL;
+    }
+
+    elementtreestate *st = self->state;
+
+    if (EXPAT(st, SetReparseDeferralEnabled) == NULL) {
+        Py_RETURN_NONE;
+    }
+
+    // NOTE: The Expat parser in the C implementation of ElementTree is not
+    //       exposed to the outside; as a result we known that reparse deferral
+    //       is currently enabled, or we would not even have access to function
+    //       XML_SetReparseDeferralEnabled in the first place (which we checked
+    //       for, a few lines up).
+
+    EXPAT(st, SetReparseDeferralEnabled)(self->parser, XML_FALSE);
+
+    PyObject *res = expat_parse(st, self, "", 0, XML_FALSE);
+
+    EXPAT(st, SetReparseDeferralEnabled)(self->parser, XML_TRUE);
+
+    return res;
+}
+
+/*[clinic input]
 _elementtree.XMLParser.feed
 
     data: object
@@ -4288,6 +4322,7 @@ static PyType_Spec treebuilder_spec = {
 static PyMethodDef xmlparser_methods[] = {
     _ELEMENTTREE_XMLPARSER_FEED_METHODDEF
     _ELEMENTTREE_XMLPARSER_CLOSE_METHODDEF
+    _ELEMENTTREE_XMLPARSER_FLUSH_METHODDEF
     _ELEMENTTREE_XMLPARSER__PARSE_WHOLE_METHODDEF
     _ELEMENTTREE_XMLPARSER__SETEVENTS_METHODDEF
     {NULL, NULL}
diff --git a/Modules/clinic/_elementtree.c.h b/Modules/clinic/_elementtree.c.h
index 9622591..10b2dd1 100644
--- a/Modules/clinic/_elementtree.c.h
+++ b/Modules/clinic/_elementtree.c.h
@@ -1169,6 +1169,23 @@ _elementtree_XMLParser_close(XMLParserObject *self, PyObject *Py_UNUSED(ignored)
     return _elementtree_XMLParser_close_impl(self);
 }
 
+PyDoc_STRVAR(_elementtree_XMLParser_flush__doc__,
+"flush($self, /)\n"
+"--\n"
+"\n");
+
+#define _ELEMENTTREE_XMLPARSER_FLUSH_METHODDEF    \
+    {"flush", (PyCFunction)_elementtree_XMLParser_flush, METH_NOARGS, _elementtree_XMLParser_flush__doc__},
+
+static PyObject *
+_elementtree_XMLParser_flush_impl(XMLParserObject *self);
+
+static PyObject *
+_elementtree_XMLParser_flush(XMLParserObject *self, PyObject *Py_UNUSED(ignored))
+{
+    return _elementtree_XMLParser_flush_impl(self);
+}
+
 PyDoc_STRVAR(_elementtree_XMLParser_feed__doc__,
 "feed($self, data, /)\n"
 "--\n"
@@ -1219,4 +1236,4 @@ skip_optional:
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=218ec9e6a889f796 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=aed9f53eeb0404e0 input=a9049054013a1b77]*/
diff --git a/Modules/clinic/pyexpat.c.h b/Modules/clinic/pyexpat.c.h
index a5b93e6..343cb91 100644
--- a/Modules/clinic/pyexpat.c.h
+++ b/Modules/clinic/pyexpat.c.h
@@ -8,6 +8,53 @@ preserve
 #endif
 #include "pycore_modsupport.h"    // _PyArg_UnpackKeywords()
 
+PyDoc_STRVAR(pyexpat_xmlparser_SetReparseDeferralEnabled__doc__,
+"SetReparseDeferralEnabled($self, enabled, /)\n"
+"--\n"
+"\n"
+"Enable/Disable reparse deferral; enabled by default with Expat >=2.6.0.");
+
+#define PYEXPAT_XMLPARSER_SETREPARSEDEFERRALENABLED_METHODDEF    \
+    {"SetReparseDeferralEnabled", (PyCFunction)pyexpat_xmlparser_SetReparseDeferralEnabled, METH_O, pyexpat_xmlparser_SetReparseDeferralEnabled__doc__},
+
+static PyObject *
+pyexpat_xmlparser_SetReparseDeferralEnabled_impl(xmlparseobject *self,
+                                                 int enabled);
+
+static PyObject *
+pyexpat_xmlparser_SetReparseDeferralEnabled(xmlparseobject *self, PyObject *arg)
+{
+    PyObject *return_value = NULL;
+    int enabled;
+
+    enabled = PyObject_IsTrue(arg);
+    if (enabled < 0) {
+        goto exit;
+    }
+    return_value = pyexpat_xmlparser_SetReparseDeferralEnabled_impl(self, enabled);
+
+exit:
+    return return_value;
+}
+
+PyDoc_STRVAR(pyexpat_xmlparser_GetReparseDeferralEnabled__doc__,
+"GetReparseDeferralEnabled($self, /)\n"
+"--\n"
+"\n"
+"Retrieve reparse deferral enabled status; always returns false with Expat <2.6.0.");
+
+#define PYEXPAT_XMLPARSER_GETREPARSEDEFERRALENABLED_METHODDEF    \
+    {"GetReparseDeferralEnabled", (PyCFunction)pyexpat_xmlparser_GetReparseDeferralEnabled, METH_NOARGS, pyexpat_xmlparser_GetReparseDeferralEnabled__doc__},
+
+static PyObject *
+pyexpat_xmlparser_GetReparseDeferralEnabled_impl(xmlparseobject *self);
+
+static PyObject *
+pyexpat_xmlparser_GetReparseDeferralEnabled(xmlparseobject *self, PyObject *Py_UNUSED(ignored))
+{
+    return pyexpat_xmlparser_GetReparseDeferralEnabled_impl(self);
+}
+
 PyDoc_STRVAR(pyexpat_xmlparser_Parse__doc__,
 "Parse($self, data, isfinal=False, /)\n"
 "--\n"
@@ -498,4 +545,4 @@ exit:
 #ifndef PYEXPAT_XMLPARSER_USEFOREIGNDTD_METHODDEF
     #define PYEXPAT_XMLPARSER_USEFOREIGNDTD_METHODDEF
 #endif /* !defined(PYEXPAT_XMLPARSER_USEFOREIGNDTD_METHODDEF) */
-/*[clinic end generated code: output=48c4296e43777df4 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=892e48e41f9b6e4b input=a9049054013a1b77]*/
diff --git a/Modules/expat/pyexpatns.h b/Modules/expat/pyexpatns.h
index d45d9b6..8ee03ef 100644
--- a/Modules/expat/pyexpatns.h
+++ b/Modules/expat/pyexpatns.h
@@ -108,6 +108,7 @@
 #define XML_SetNotStandaloneHandler     PyExpat_XML_SetNotStandaloneHandler
 #define XML_SetParamEntityParsing       PyExpat_XML_SetParamEntityParsing
 #define XML_SetProcessingInstructionHandler PyExpat_XML_SetProcessingInstructionHandler
+#define XML_SetReparseDeferralEnabled   PyExpat_XML_SetReparseDeferralEnabled
 #define XML_SetReturnNSTriplet          PyExpat_XML_SetReturnNSTriplet
 #define XML_SetSkippedEntityHandler     PyExpat_XML_SetSkippedEntityHandler
 #define XML_SetStartCdataSectionHandler PyExpat_XML_SetStartCdataSectionHandler
diff --git a/Modules/pyexpat.c b/Modules/pyexpat.c
index 62cd262..f04f96b 100644
--- a/Modules/pyexpat.c
+++ b/Modules/pyexpat.c
@@ -7,6 +7,7 @@
 #include "pycore_pyhash.h"        // _Py_HashSecret
 #include "pycore_traceback.h"     // _PyTraceback_Add()
 
+#include <stdbool.h>
 #include <stddef.h>               // offsetof()
 #include "expat.h"
 #include "pyexpat.h"
@@ -81,6 +82,12 @@ typedef struct {
                                 /* NULL if not enabled */
     int buffer_size;            /* Size of buffer, in XML_Char units */
     int buffer_used;            /* Buffer units in use */
+    bool reparse_deferral_enabled; /* Whether to defer reparsing of
+                                   unfinished XML tokens; a de-facto cache of
+                                   what Expat has the authority on, for lack
+                                   of a getter API function
+                                   "XML_GetReparseDeferralEnabled" in Expat
+                                   2.6.0 */
     PyObject *intern;           /* Dictionary to intern strings */
     PyObject **handlers;
 } xmlparseobject;
@@ -704,6 +711,40 @@ get_parse_result(pyexpat_state *state, xmlparseobject *self, int rv)
 #define MAX_CHUNK_SIZE (1 << 20)
 
 /*[clinic input]
+pyexpat.xmlparser.SetReparseDeferralEnabled
+
+    enabled: bool
+    /
+
+Enable/Disable reparse deferral; enabled by default with Expat >=2.6.0.
+[clinic start generated code]*/
+
+static PyObject *
+pyexpat_xmlparser_SetReparseDeferralEnabled_impl(xmlparseobject *self,
+                                                 int enabled)
+/*[clinic end generated code: output=5ec539e3b63c8c49 input=021eb9e0bafc32c5]*/
+{
+#if XML_COMBINED_VERSION >= 20600
+    XML_SetReparseDeferralEnabled(self->itself, enabled ? XML_TRUE : XML_FALSE);
+    self->reparse_deferral_enabled = (bool)enabled;
+#endif
+    Py_RETURN_NONE;
+}
+
+/*[clinic input]
+pyexpat.xmlparser.GetReparseDeferralEnabled
+
+Retrieve reparse deferral enabled status; always returns false with Expat <2.6.0.
+[clinic start generated code]*/
+
+static PyObject *
+pyexpat_xmlparser_GetReparseDeferralEnabled_impl(xmlparseobject *self)
+/*[clinic end generated code: output=4e91312e88a595a8 input=54b5f11d32b20f3e]*/
+{
+    return PyBool_FromLong(self->reparse_deferral_enabled);
+}
+
+/*[clinic input]
 pyexpat.xmlparser.Parse
 
     cls: defining_class
@@ -1063,6 +1104,8 @@ static struct PyMethodDef xmlparse_methods[] = {
 #if XML_COMBINED_VERSION >= 19505
     PYEXPAT_XMLPARSER_USEFOREIGNDTD_METHODDEF
 #endif
+    PYEXPAT_XMLPARSER_SETREPARSEDEFERRALENABLED_METHODDEF
+    PYEXPAT_XMLPARSER_GETREPARSEDEFERRALENABLED_METHODDEF
     {NULL, NULL}  /* sentinel */
 };
 
@@ -1158,6 +1201,11 @@ newxmlparseobject(pyexpat_state *state, const char *encoding,
     self->ns_prefixes = 0;
     self->handlers = NULL;
     self->intern = Py_XNewRef(intern);
+#if XML_COMBINED_VERSION >= 20600
+    self->reparse_deferral_enabled = true;
+#else
+    self->reparse_deferral_enabled = false;
+#endif
 
     /* namespace_separator is either NULL or contains one char + \0 */
     self->itself = XML_ParserCreate_MM(encoding, &ExpatMemoryHandler,
@@ -2019,6 +2067,11 @@ pyexpat_exec(PyObject *mod)
 #else
     capi->SetHashSalt = NULL;
 #endif
+#if XML_COMBINED_VERSION >= 20600
+    capi->SetReparseDeferralEnabled = XML_SetReparseDeferralEnabled;
+#else
+    capi->SetReparseDeferralEnabled = NULL;
+#endif
 
     /* export using capsule */
     PyObject *capi_object = PyCapsule_New(capi, PyExpat_CAPSULE_NAME,