[New-bugs-announce] [issue25769] Crash due to using weakref referent without acquiring a strong reference

[lplatypus]

New submission from lplatypus:

I have encountered some crashes in a multithreaded application which appear to be due to a bug in weakref_richcompare in Objects/weakref.c

(I am using Python 2.7.9, but the same weakref code exists in 3.5 and hg default branch too)

weakref_richcompare ends with the statement:

    return PyObject_RichCompare(PyWeakref_GET_OBJECT(self),
                                PyWeakref_GET_OBJECT(other), op);

At this point the code has established that the referents of "self" and "other" are still alive, and it is trying to compare the referents.  However it has not acquired a strong reference to the referents, so I think it is possible for one of them to be deleted half way through this comparison.  This can lead to a crash, because PyObject_RichCompare assumes that the PyObject*’s it was passed will remain usable for the duration of the call.

The crash dumps I have seen involve data corruption consistent with one of these PyObject's being deleted and the memory used for something else, eg:

00 python27!try_3way_compare+0x15 [objects\object.c @ 712]
01 python27!try_3way_to_rich_compare+0xb [objects\object.c @ 901]
02 python27!do_richcmp+0x2c [objects\object.c @ 935]
03 python27!PyObject_RichCompare+0x99 [objects\object.c @ 982]
04 python27!weakref_richcompare+0x6a [objects\weakrefobject.c @ 212]

In this example, in try_3way_compare the value of v->ob_type was 0x5f637865, which is ASCII "exc_" and not a valid pointer at all.

Other places in weakrefobject.c seem to have a similar weakness, eg in weakref_hash and weakref_repr.

I have not been successful in producing a small test case to demonstrate this crash.

----------
components: Interpreter Core
messages: 255608
nosy: ldeller
priority: normal
severity: normal
status: open
title: Crash due to using weakref referent without acquiring a strong reference
type: crash
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4, Python 3.5, Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25769>
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