Python 3.8.5 处理异常.pyc文件造成拒绝服务

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漏洞简介:

极光无限维阵专家团队在对Python最新版Python 3.8.5的分析中发现了一个空指针引用漏洞，漏洞发生在Python对.pyc文件进行处理时。

下面是崩溃信息：

$ Python-3.8.5/python 00-SEGV-on-unknown-address-Python-3.8.5.pyc

    Could not find platform dependent libraries <exec_prefix>
    Consider setting $PYTHONHOME to <prefix>[:<exec_prefix>]
    AddressSanitizer:DEADLYSIGNAL
    =================================================================
    ==8079==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000018 (pc 0x0000008aa86b bp 0x000000000000 sp 0x7ffe2a3f5bf0 T0)
    ==8079==The signal is caused by a READ memory access.
    ==8079==Hint: address points to the zero page.
        #0 0x8aa86a in _PyEval_EvalCodeWithName /home/test/Python-3.8.5/Python/ceval.c:4266:23
        #1 0x866d0f in PyEval_EvalCodeEx /home/test/Python-3.8.5/Python/ceval.c:4327:12
        #2 0x866d0f in PyEval_EvalCode /home/test/Python-3.8.5/Python/ceval.c:718:12
        #3 0x9f7355 in run_eval_code_obj /home/test/Python-3.8.5/Python/pythonrun.c:1125:9
        #4 0x9e682d in run_pyc_file /home/test/Python-3.8.5/Python/pythonrun.c:1184:9
        #5 0x9e682d in PyRun_SimpleFileExFlags /home/test/Python-3.8.5/Python/pythonrun.c:419:13
        #6 0x9e4ca5 in PyRun_AnyFileExFlags /home/test/Python-3.8.5/Python/pythonrun.c:86:16
        #7 0x5108db in pymain_run_file /home/test/Python-3.8.5/Modules/main.c:381:15
        #8 0x5108db in pymain_run_python /home/test/Python-3.8.5/Modules/main.c:606:21
        #9 0x5108db in Py_RunMain /home/test/Python-3.8.5/Modules/main.c:685:5
        #10 0x5129d6 in pymain_main /home/test/Python-3.8.5/Modules/main.c:715:12
        #11 0x512dd7 in Py_BytesMain /home/test/Python-3.8.5/Modules/main.c:739:12
        #12 0x7f8316d4b82f in __libc_start_main /build/glibc-LK5gWL/glibc-2.23/csu/../csu/libc-start.c:291
        #13 0x438888 in _start (/home/test/Python-3.8.5-Fuzz/python+0x438888)

    AddressSanitizer can not provide additional info.
    SUMMARY: AddressSanitizer: SEGV /home/test/Python-3.8.5/Python/ceval.c:4266:23 in _PyEval_EvalCodeWithName
    ==8079==ABORTING

 

漏洞分析:

在函数PyEval_EvalCode(Python-3.8.5/Python/ceval.c)中调用PyEval_EvalCodeEx函数。

PyObject *
    PyEval_EvalCode(PyObject *co, PyObject *globals, PyObject *locals)
{
        return PyEval_EvalCodeEx(co,
                        globals, locals,
                        (PyObject **)NULL, 0,
                        (PyObject **)NULL, 0,
                        (PyObject **)NULL, 0,
                        NULL, NULL);
    }

传递给PyEval_EvalCodeEx函数的参数中closure设置为NULL。

    PyObject *
    PyEval_EvalCodeEx(PyObject *_co, PyObject *globals, PyObject *locals,
                    PyObject *const *args, int argcount,
                    PyObject *const *kws, int kwcount,
                    PyObject *const *defs, int defcount,
                    PyObject *kwdefs, PyObject *closure)
{
        return _PyEval_EvalCodeWithName(_co, globals, locals,
                                        args, argcount,
                                        kws, kws != NULL ? kws + 1 : NULL,
                                        kwcount, 2,
                                        defs, defcount,
                                        kwdefs, closure,
                                        NULL, NULL);
    }

PyEval_EvalCodeEx函数继续调用_PyEval_EvalCodeWithName函数，closure值不变依旧为NULL。

PyObject *
_PyEval_EvalCodeWithName(PyObject *_co, PyObject *globals, PyObject *locals,
        PyObject *const *args, Py_ssize_t argcount,
        PyObject *const *kwnames, PyObject *const *kwargs,
        Py_ssize_t kwcount, int kwstep,
        PyObject *const *defs, Py_ssize_t defcount,
        PyObject *kwdefs, PyObject *closure,
        PyObject *name, PyObject *qualname)
{
    ******
    /* Copy closure variables to free variables */
    for (i = 0; i < PyTuple_GET_SIZE(co->co_freevars); ++i) {
        PyObject *o = PyTuple_GET_ITEM(closure, i);   <----------------------------- crash
        Py_INCREF(o);
        freevars[PyTuple_GET_SIZE(co->co_cellvars) + i] = o;
    }
    ******
}

在_PyEval_EvalCodeWithName函数中对closure进行解引用，此时closure值为NULL，触发空指针引用漏洞。

 

修复建议:

在 _PyEval_EvalCodeWithName函数中引用closure前，对closure的值进行判断。

（完）