Thomas E Hybel reports:

This vulnerability exists in /Modules/_operator.c in the function
methodcaller_new.

Here is the problematic code:

    mc = PyObject_GC_New(methodcallerobject, &methodcaller_type);
    if (mc == NULL)
        return NULL;
   
    newargs = PyTuple_GetSlice(args, 1, PyTuple_GET_SIZE(args));
    if (newargs == NULL) {
        Py_DECREF(mc);
        return NULL;
    }

We first allocate an "mc" object. Then we call PyTuple_GetSlice. If that fails,
e.g. because we're out of memory, then we call Py_DECREF(mc). But mc's variables
have not been initialized yet. methodcaller_dealloc will therefore free several
arbitrary pointers.

This could be fixed by setting mc's member variables to NULL right after
allocating it.

Proof-of-concept script:

--- begin script ---

import operator

args = ("AAAA",)*0x10000000
ag = operator.methodcaller(*args)

--- end script ---

(Note that this PoC only works if the machine runs out of memory at the right
time; you may have to experiment with the size of "args." This was tested on a
32-bit box, therefore it had a small address space.)

Here's the crash and backtrace:

(gdb) r ../poc10.py
Starting program: /home/ubuntu32/python3/Python-3.5.2/python ../poc10.py

Program received signal SIGSEGV, Segmentation fault.
0x081d4255 in methodcaller_dealloc (mc=mc@entry=0xb7c31b94) at ./Modules/_operator.c:976
976 Py_XDECREF(mc->name);
(gdb) p mc->name
$3 = (PyObject *) 0xcbcbcbcb
(gdb) bt
#0 0x081d4255 in methodcaller_dealloc (mc=mc@entry=0xb7c31b94) at ./Modules/_operator.c:976
#1 0x080e4bff in _Py_Dealloc (op=op@entry=0xb7c31b94) at Objects/object.c:1786
#2 0x081d361a in methodcaller_new (type=0x82f0200 <methodcaller_type>, args=0x37c2d024, kwds=0x0) at ./Modules/_operator.c:956
...

New changeset 11a9bca71528 by Benjamin Peterson in branch '2.7':
rearrange methodcaller_new so that the main error case does not cause uninitialized memory usage (closes bpo-27783)
https://hg.python.org/cpython/rev/11a9bca71528

New changeset 8e3b9bf917a7 by Benjamin Peterson in branch '3.3':
rearrange methodcaller_new so that the main error case does not cause uninitialized memory usage (closes bpo-27783)
https://hg.python.org/cpython/rev/8e3b9bf917a7

New changeset 675e20c38fda by Benjamin Peterson in branch '3.4':
merge 3.3 (bpo-27783)
https://hg.python.org/cpython/rev/675e20c38fda

New changeset d1b93ce7dad8 by Benjamin Peterson in branch '3.5':
merge 3.4 (bpo-27783)
https://hg.python.org/cpython/rev/d1b93ce7dad8

New changeset 0f0a040d45b2 by Benjamin Peterson in branch 'default':
merge 3.5 (bpo-27783)
https://hg.python.org/cpython/rev/0f0a040d45b2