# Program received signal SIGSEGV, Segmentation fault.
# 0x40036740 in encoder_listencode_dict (s=0x405b43fc, acc=0xbf86438c, dct=<D at remote 0x405b2fb4>, indent_level=0)
#     at /home/p/Python-3.4.1/Modules/_json.c:1557
# 1557                PyList_SET_ITEM(items, i, item);
# (gdb)  print *(PyListObject*)items
# $1 = {ob_base = {ob_base = {_ob_next = 0x405bcab4, _ob_prev = 0x40591184, ob_refcnt = 2, ob_type = 0x830e1c0 <PyList_Type>}, 
#     ob_size = 0}, ob_item = 0x0, allocated = 0}
# (gdb) print i
# $2 = 112233
# 
# "items" was cleared in __hash__, so we get a wild write at a controlled address.

# Program received signal SIGSEGV, Segmentation fault.
# 0x080f2c17 in PyObject_GetAttr (v=<unknown at remote 0x40573d8c>, name='match') at Objects/object.c:872
# 872         if (tp->tp_getattro != NULL)
# (gdb) bt
# #0  0x080f2c17 in PyObject_GetAttr (v=<unknown at remote 0x40573d8c>, name='match') at Objects/object.c:872
# #1  0x080f2b42 in _PyObject_GetAttrId (v=<unknown at remote 0x40573d8c>, name=0x8328354 <PyId_match.9432>) at Objects/object.c:835
# #2  0x0809c3a6 in _PyObject_CallMethodId (o=<unknown at remote 0x40573d8c>, name=0x8328354 <PyId_match.9432>, format=0x829552c "O")
#     at Objects/abstract.c:2215
# #3  0x0817e48b in check_matched (obj=<unknown at remote 0x40573d8c>, arg='c') at Python/_warnings.c:28
# #4  0x0817e88b in get_filter (category=<type at remote 0x830bf80>, text='', lineno=4, module='c', item=0xbfa87c88)
# (gdb) frame 4
# #4  0x0817e88b in get_filter (category=<type at remote 0x830bf80>, text='', lineno=4, module='c', item=0xbfa87c88)
#     at Python/_warnings.c:152
# 152             good_mod = check_matched(mod, module);
# (gdb) print *mod
# $1 = {_ob_next = 0xdbdbdbdb, _ob_prev = 0xdbdbdbdb, ob_refcnt = -606348325, ob_type = 0xdbdbdbdb}
# 
# "mod" object is deleted in "match" method. Use after free.
#

In 3.5 the segfault occurs at

#0  0x00007ffff073e55e in encoder_listencode_dict (s=s@entry=0x7ffff09aa988, acc=acc@entry=0x7fffffffcf20, dct=dct@entry=<D at remote 0x7ffff094b958>, indent_level=indent_level@entry=0)
    at /home/heimes/dev/python/cpython/Modules/_json.c:1686
#1  0x00007ffff073ee85 in encoder_listencode_obj (s=s@entry=0x7ffff09aa988, acc=acc@entry=0x7fffffffcf20, obj=<D at remote 0x7ffff094b958>, indent_level=0) at /home/heimes/dev/python/cpython/Modules/_json.c:1561
#2  0x00007ffff073f392 in encoder_call (self=<_json.Encoder at remote 0x7ffff09aa988>, args=(<D at remote 0x7ffff094b958>, 0), kwds=0x0) at /home/heimes/dev/python/cpython/Modules/_json.c:1386
#3  0x000000000044edf8 in PyObject_Call (func=func@entry=<_json.Encoder at remote 0x7ffff09aa988>, arg=arg@entry=(<D at remote 0x7ffff094b958>, 0), kw=kw@entry=0x0) at Objects/abstract.c:2147
#4  0x000000000052be56 in do_call (func=func@entry=<_json.Encoder at remote 0x7ffff09aa988>, pp_stack=pp_stack@entry=0x7fffffffd098, na=na@entry=2, nk=nk@entry=0) at Python/ceval.c:4515
#5  0x000000000053663c in call_function (pp_stack=pp_stack@entry=0x7fffffffd098, oparg=oparg@entry=2) at Python/ceval.c:4311

Issue for poc_enc_dict3.py is here: https://bugs.python.org/issue24105

Please ignore first and third message.

Thanks Paul!

May I ask how you found that many use-after-free bugs? Are you using some sort of tool for static code analysis or fuzzying?

New changeset ffc1f9d1c8b3 by Benjamin Peterson in branch '3.3':
be more robust against the filters list changing under us (closes #24096)
https://hg.python.org/cpython/rev/ffc1f9d1c8b3

New changeset 47f4c3a5d86a by Benjamin Peterson in branch '3.4':
merge 3.3 (#24096)
https://hg.python.org/cpython/rev/47f4c3a5d86a

New changeset bfea101f9402 by Benjamin Peterson in branch 'default':
merge 3.4 (#24096)
https://hg.python.org/cpython/rev/bfea101f9402