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[backport to 3.5] bpo-29438: Fixed use-after-free in key sharing dict #40

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Feb 13, 2017
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[backport to 3.5] bpo-29438: Fixed use-after-free in key sharing dict #40

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2 changes: 2 additions & 0 deletions Misc/NEWS
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,8 @@ Release date: XXXX-XX-XX
Core and Builtins
-----------------

- bpo-29438: Fixed use-after-free problem in key sharing dict.

- Issue #29319: Prevent RunMainFromImporter overwriting sys.path[0].

- Issue #29337: Fixed possible BytesWarning when compare the code objects.
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10 changes: 4 additions & 6 deletions Objects/dictobject.c
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Expand Up @@ -3893,20 +3893,18 @@ _PyObjectDict_SetItem(PyTypeObject *tp, PyObject **dictptr,
}
if (value == NULL) {
res = PyDict_DelItem(dict, key);
if (cached != ((PyDictObject *)dict)->ma_keys) {
CACHED_KEYS(tp) = NULL;
DK_DECREF(cached);
}
}
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Why these lines are deleted?

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In Python 3.5, PyDict_DelItem won't resize dict. So cached == dict->ma_keys in most cases.

One exception is callback called by weakref or __del__ inserts some items to the dict and resize happened.
In this case, cached != CACHED_KEYS(tp), so DK_DECREF(cached) will be "use-after-free".

In such case, the insertion from the callback would update CACHED_KEYS(tp) correctly.
So clearing CACHED_KEYS(tp) doesn't make sense for most case.

Even when the callback inserts items through __dict__ (not regular attribute access), skipping CACHED_KEYS(tp) = NULL
doesn't cause uncontrolled memory growth. And it happens very rarely.
So I think this code is not worth enough.

else {
int was_shared = cached == ((PyDictObject *)dict)->ma_keys;
int was_shared = (cached == ((PyDictObject *)dict)->ma_keys);
res = PyDict_SetItem(dict, key, value);
/* PyDict_SetItem() may call dictresize() and convert split table
* into combined table. In such case, convert it to split
* table again and update type's shared key only when this is
* the only dict sharing key with the type.
*/
if (was_shared && cached != ((PyDictObject *)dict)->ma_keys) {
if (was_shared &&
(cached = CACHED_KEYS(tp)) != NULL &&
cached != ((PyDictObject *)dict)->ma_keys) {
if (cached->dk_refcnt == 1) {
CACHED_KEYS(tp) = make_keys_shared(dict);
} else {
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