static PyObject *
partial_setstate(partialobject *pto, PyObject *state)
{
    PyObject *fn, *fnargs, *kw, *dict;
    if (!PyArg_ParseTuple(state, "OOOO",
                          &fn, &fnargs, &kw, &dict))
        return NULL;
    Py_XDECREF(pto->fn);
    Py_XDECREF(pto->args);
    Py_XDECREF(pto->kw);
    Py_XDECREF(pto->dict);
    pto->fn = fn;
    pto->args = fnargs; //we control pto->args here

`partial_setstate` performs no checks on the objects
it is passed as an argument.

static PyObject *
partial_call(partialobject *pto, PyObject *args, PyObject *kw)
{
    PyObject *ret;
    PyObject *argappl = NULL, *kwappl = NULL;

    assert (PyCallable_Check(pto->fn));
    assert (PyTuple_Check(pto->args)); //assume pto->args is a tuple
                                       //assertion not present in release build
    assert (pto->kw == Py_None  ||  PyDict_Check(pto->kw));

    if (PyTuple_GET_SIZE(pto->args) == 0) {
        argappl = args;
        Py_INCREF(args);
    } else if (PyTuple_GET_SIZE(args) == 0) {
        argappl = pto->args; //partial function called with no arguments
        Py_INCREF(pto->args);
    } else {
        argappl = PySequence_Concat(pto->args, args);
        if (argappl == NULL)
            return NULL;
    }

    if (pto->kw == Py_None) {
        kwappl = kw;
        Py_XINCREF(kw);
    } else {
        kwappl = PyDict_Copy(pto->kw);
        if (kwappl == NULL) {
            Py_DECREF(argappl);
            return NULL;
        }
        if (kw != NULL) {
            if (PyDict_Merge(kwappl, kw, 1) != 0) {
                Py_DECREF(argappl);
                Py_DECREF(kwappl);
                return NULL;
            }
        }
    }
    ret = PyObject_Call(pto->fn, argappl, kwappl); //pto->fn called with non-tuple argappl

We can see that in the provided POC there is an increment on a user-controlled address (in this case, the literal refcount of a given "argument" is interpreted as a pointer), as `_PyEval_EvalCodeWithName` does not validate the type of `PyObject **args` either (I assume this is a fair assumption for `_PyEval_EvalCodeWithName`, and the bug simply lies in the unsafe partial code.

vagrant@vagrant-ubuntu-wily-64:/vagrant/Python-3.5.1$ gdb -q ./python.exe
...
(gdb) r partialpoc2.py
Starting program: /vagrant/Python-3.5.1/python.exe partialpoc2.py
...
Program received signal SIGSEGV, Segmentation fault.
_PyEval_EvalCodeWithName (_co=0x7ffff7045ae0, globals=<optimized out>, locals=locals@entry=0x0, args=args@entry=0x7ffff6fbc520, argcount=1280, kws=kws@entry=0x0, kwcount=0, defs=0x0, defcount=0,
    kwdefs=0x0, closure=0x0, name=0x0, qualname=0x0) at Python/ceval.c:3793
3793	            Py_INCREF(x);
(gdb) i r
rax            0x9b4b68	10177384
rbx            0x7ffff6fbc520	140737337083168
rcx            0x1	1
rdx            0x2	2
rsi            0x500	1280
rdi            0x0	0
rbp            0x0	0x0
rsp            0x7fffffffdb30	0x7fffffffdb30
r8             0x500	1280
r9             0x0	0
r10            0x7ffff74a6c58	140737342237784
r11            0x9b4b40	10177344
r12            0x0	0
r13            0x0	0
r14            0x7ffff6fb91e0	140737337070048
r15            0x7ffff7e1a048	140737352147016
rip            0x4fc771	0x4fc771 <_PyEval_EvalCodeWithName+961>
eflags         0x10202	[ IF RF ]
cs             0x33	51
ss             0x2b	43
ds             0x0	0
es             0x0	0
fs             0x0	0
gs             0x0	0
(gdb) x/3i $pc
=> 0x4fc771 <_PyEval_EvalCodeWithName+961>:	addq   $0x1,(%rsi)
   0x4fc775 <_PyEval_EvalCodeWithName+965>:	cmp    %edx,%r8d
   0x4fc778 <_PyEval_EvalCodeWithName+968>:	mov    %rsi,0x18(%rax,%rcx,8)

There are other bugs in partial() that lead to crash, leak, or invalid behavior. Proposed patch fixes these bugs.

I didn't do a full review of the C code changes, but the new test cases look good to me, and the changes specifically to partial_setstate also look good.

This looks correct.

Thank you Nick and Raymond for your reviews.

New changeset 542b5744ddc3 by Serhiy Storchaka in branch '3.5':
Issue #25945: Fixed bugs in functools.partial.
https://hg.python.org/cpython/rev/542b5744ddc3

New changeset 33109176538d by Serhiy Storchaka in branch 'default':
Issue #25945: Fixed bugs in functools.partial.
https://hg.python.org/cpython/rev/33109176538d

New changeset 628ce2975e29 by Serhiy Storchaka in branch '2.7':
Issue #25945: Fixed bugs in functools.partial.
https://hg.python.org/cpython/rev/628ce2975e29