[Python-Dev] Hash randomization for which types? (original) (raw)

Shell Xu shell909090 at gmail.com
Tue Feb 16 22:45:57 EST 2016

I thought you are right. Here is the source code in python 2.7.11:

long PyObject_Hash(PyObject *v) { PyTypeObject *tp = v->ob_type; if (tp->tp_hash != NULL) return (tp->tp_hash)(v); / To keep to the general practice that inheriting * solely from object in C code should work without * an explicit call to PyType_Ready, we implicitly call * PyType_Ready here and then check the tp_hash slot again */ if (tp->tp_dict == NULL) { if (PyType_Ready(tp) < 0) return -1; if (tp->tp_hash != NULL) return (tp->tp_hash)(v); } if (tp->tp_compare == NULL && RICHCOMPARE(tp) == NULL) { return _Py_HashPointer(v); / Use address as hash value / } / If there's a cmp but no hash defined, the object can't be hashed */ return PyObject_HashNotImplemented(v); }

If object has hash function, it will be used. If not, _Py_HashPointer will be used. Which _Py_HashSecret are not used. And I checked reference of _Py_HashSecret. Only bufferobject, unicodeobject and stringobject use _Py_HashSecret.

On Wed, Feb 17, 2016 at 9:54 AM, Steven D'Aprano <steve at pearwood.info> wrote:

On Tue, Feb 16, 2016 at 11:56:55AM -0800, Glenn Linderman wrote: > On 2/16/2016 1:48 AM, Christoph Groth wrote: > >Hello, > > > >Recent Python versions randomize the hashes of str, bytes and datetime > >objects. I suppose that the choice of these three types is the result > >of a compromise. Has this been discussed somewhere publicly? > > Search archives of this list... it was discussed at length.

There's a lot of discussion on the mailing list. I think that this is the very start of it, in Dec 2011: https://mail.python.org/pipermail/python-dev/2011-December/115116.html and continuing into 2012, for example: https://mail.python.org/pipermail/python-dev/2012-January/115577.html https://mail.python.org/pipermail/python-dev/2012-January/115690.html and a LOT more, spread over many different threads and subject lines. You should also read the issue on the bug tracker: http://bugs.python.org/issue13703

My recollection is that it was decided that only strings and bytes need to have their hashes randomized, because only strings and bytes can be used directly from user-input without first having a conversion step with likely input range validation. In addition, changing the hash for ints would break too much code for too little benefit: unlike strings, where hash collision attacks on web apps are proven and easy, hash collision attacks based on ints are more difficult and rare. See also the comment here: http://bugs.python.org/issue13703#msg151847

> >I'm not a web programmer, but don't web applications also use > >dictionaries that are indexed by, say, tuples of integers? > > Sure, and that is the biggest part of the reason they were randomized. But they aren't, as far as I can see: [steve at ando 3.6]$ ./python -c "print(hash((23, 42, 99, 100)))" 1071302475 [steve at ando 3.6]$ ./python -c "print(hash((23, 42, 99, 100)))" 1071302475 Web apps can use dicts indexed by anything that they like, but unless there is an actual attack, what does it matter? Guido makes a good point about security here: https://mail.python.org/pipermail/python-dev/2013-October/129181.html > I think hashes of all types have been randomized, not just the list > you mentioned. I'm pretty sure that's not actually the case. Using 3.6 from the repo (admittedly not fully up to date though), I can see hash randomization working for strings: [steve at ando 3.6]$ ./python -c "print(hash('abc'))" 11601873 [steve at ando 3.6]$ ./python -c "print(hash('abc'))" -2009889747 but not for ints: [steve at ando 3.6]$ ./python -c "print(hash(42))" 42 [steve at ando 3.6]$ ./python -c "print(hash(42))" 42 which agrees with my recollection that only strings and bytes would be randomized. -- Steve

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