Description:
------------
Null dereference in function set_conversion_mode due uncheck _ctypes_conversion_errors:

static PyObject *
set_conversion_mode(PyObject *self, PyObject *args)
{
    char *coding, *mode;
    PyObject *result;
...
    PyMem_Free(_ctypes_conversion_errors);
    _ctypes_conversion_errors = PyMem_Malloc(strlen(mode) + 1); //if memory is not enough, _ctypes_conversion_errors will be null
    strcpy(_ctypes_conversion_errors, mode); // crash here
    return result;
}



Test script:
---------------

import ctypes

s = 'a'*(0xffffffff/2-0xffff)
sss = 'a'*(0xffffffff/4)
ctypes.set_conversion_mode('a', s)


Expected result:
----------------
No Crash

Actual result:
--------------
Starting program: /home/minhrau/cpython-2.7/python ~/pythontestcase/test.py
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".

Program received signal SIGSEGV, Segmentation fault.
0xf7def209 in __strcpy_sse2 () from /usr/lib32/libc.so.6
(gdb) bt
#0  0xf7def209 in __strcpy_sse2 () from /usr/lib32/libc.so.6
#1  0xf7fba5f2 in set_conversion_mode (self=0x0, args=0xf7cd602c) at /home/minhrau/cpython-2.7/Modules/_ctypes/callproc.c:1706
#2  0x080f6dfc in call_function (oparg=<optimized out>, pp_stack=0xffffd45c) at Python/ceval.c:4350
#3  PyEval_EvalFrameEx (f=<optimized out>, throwflag=<optimized out>) at Python/ceval.c:2987
#4  0x080f964e in PyEval_EvalCodeEx (co=0xf7cc94e8, globals=0xf7d5b714, locals=0xf7d5b714, args=0x0, argcount=0, kws=0x0, kwcount=0, defs=0x0, defcount=0, closure=0x0) at Python/ceval.c:3582
#5  0x080f9942 in PyEval_EvalCode (co=0xf7cc94e8, globals=0xf7d5b714, locals=0xf7d5b714) at Python/ceval.c:669
#6  0x0811e928 in run_mod (arena=0x8264f18, flags=0xffffd64c, locals=0xf7d5b714, globals=0xf7d5b714, filename=0xffffd96e "/home/minhrau/pythontestcase/test.py", mod=0x826daa0) at Python/pythonrun.c:1376
#7  PyRun_FileExFlags (fp=0x826c788, filename=0xffffd96e "/home/minhrau/pythontestcase/test.py", start=257, globals=0xf7d5b714, locals=0xf7d5b714, closeit=1, flags=0xffffd64c) at Python/pythonrun.c:1362
#8  0x081202f4 in PyRun_SimpleFileExFlags (fp=0x826c788, filename=0xffffd96e "/home/minhrau/pythontestcase/test.py", closeit=1, flags=0xffffd64c) at Python/pythonrun.c:948
#9  0x0805a37d in Py_Main (argc=2, argv=0xffffd794) at Modules/main.c:640
#10 0x080594cb in main (argc=2, argv=0xffffd794) at ./Modules/python.c:20
(gdb) 


Patch:
--------------
file: cpython-2.7/Modules/_ctypes/callproc.c
1706,1707d1705
<     if (_ctypes_conversion_errors == NULL)
<         return PyErr_NoMemory();

Hmm, I tested the example snippet but it works. And PyMem_Free shouldn't fail when encounter NULL. The doc explicitly says "If p is NULL, no operation is performed".

Ooh, I treat dereference as decref, sorry. Then I think it may happen when there is not enough memory. But your patch is not complete, at least you should Py_DECREF(result). But this function may need more care since the malloc failure in it will alter states.

issue27963.patch tries to fix the crash caused by not enough memory and avoid inconsistent state when failure.

the patch look good. Thank Xiang

New changeset 637ce96423ef by Serhiy Storchaka in branch '2.7':
Issue #27963: Fixed possible null pointer dereference in ctypes.set_conversion_mode().
https://hg.python.org/cpython/rev/637ce96423ef