New submission from paul:
# xmlparser_setevents(XMLParserObject *self, PyObject* args)
# {
# ...
# /* clear out existing events */
# Py_CLEAR(target->start_event_obj);
# 1 Py_CLEAR(target->end_event_obj);
# Py_CLEAR(target->start_ns_event_obj);
# Py_CLEAR(target->end_ns_event_obj);
#
# ...
#
# seqlen = PySequence_Size(events_seq);
# for (i = 0; i < seqlen; ++i) {
# 3 PyObject *event_name_obj = PySequence_Fast_GET_ITEM(events_seq, i);
# ...
#
# if (event_name == NULL) {
# ...
# return NULL;
# } else if (strcmp(event_name, "start") == 0) {
# ...
# } else if (strcmp(event_name, "end") == 0) {
# Py_INCREF(event_name_obj);
# 2 Py_XDECREF(target->end_event_obj);
# target->end_event_obj = event_name_obj;
# }
# ...
# }
# ...
# }
#
# This one leverages nested _setevents invocations. First invocation sets
# target->end_event_obj to S1 instance. On seconds invocation,
# target->end_event_obj has refcnt==1, so DECREF at line 1 triggers
S1.__del__().
# Destructor invokes _setevents again and sets target->end_event_obj to a S3
# instance (with refcnt==1). After we return from nested call at line 1,
# execution continues until it hits an "end" element. At line 2 S3.__del__() is
# called and it deallocates "events_seq". This triggers a controlled OOB (we can
# call it a use after free too) read at line 3. We can control a PyObject
pointer.
#
# Program received signal SIGSEGV, Segmentation fault.
# 0x4068563b in xmlparser_setevents (self=0x40669e4c, args=([], [])) at
/home/p/Python-3.4.1/Modules/_elementtree.c:3560
# 3560 PyObject *event_name_obj =
PySequence_Fast_GET_ITEM(events_seq, i);
# (gdb) print i
# $1 = 1337
# (gdb) print *(PyListObject*)events_seq
# $2 = {ob_base = {ob_base = {_ob_next = 0x40669df4, _ob_prev = 0x4055f814,
ob_refcnt = 3, ob_type = 0x830e1c0 <PyList_Type>},
# ob_size = 0}, ob_item = 0x0, allocated = 0}
#
----------
files: poc_xml_setevents1.py
messages: 242320
nosy: pkt
priority: normal
severity: normal
status: open
title: Use after free in xmlparser_setevents (1)
type: crash
versions: Python 3.4
Added file: http://bugs.python.org/file39254/poc_xml_setevents1.py
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue24103>
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