gh-149816: Fix UAF on concurrent dict resize in free-threaded builds#153712
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gh-149816: Fix UAF on concurrent dict resize in free-threaded builds#153712sourabhligade wants to merge 1 commit into
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…uilds When a shared dict is resized, the old keys/values tables are retained via QSBR so concurrent lock-free readers can still touch the memory. Those tables previously kept live me_key/me_value pointers after ownership was transferred or released, so a later update could free the objects while a reader (including LOAD_ATTR_WITH_HINT) still held a stale table — a use-after-free. Clear entry slots before QSBR-delayed free, null before decref in dictkeys_decref, re-validate ma_keys after acquiring a value in the hint fast path and threadsafe unicode lookup, and treat a NULL me_key under an active index as KEY_CHANGED so poisoned tables force a locked retry rather than a false miss. This addresses finding (124) from the free-threading race audit.
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Summary
Fixes finding (124) from #149816: a use-after-free when a free-threaded dict is resized while another thread does lock-free lookups (including specialized
LOAD_ATTR_WITH_HINT).Root cause
On resize of a shared dict, the old
PyDictKeysObject/ values array is retained via QSBR so concurrent lock-free readers can still touch the table memory. Those tables previously kept liveme_key/me_valuepointers after ownership was transferred to the new table (or released on clear). A later update of the dict can free those objects while a reader still holds a stale table pointer and tries to incref them — UAF / ABA.Fix
Structural hardening of the reclaim path, not a one-line band-aid:
dictkeys_clear_entries/free_keys_object/free_values: clear entry slots before QSBR-delayed free so stale readers cannot revive transferred objects.dictkeys_decref: snapshot then null slots beforeXDECREF, so concurrent readers cannot follow a pointer we are about to free.NULLme_keyunder an active index is treated asDKIX_KEY_CHANGED(force locked retry) rather than a false miss — necessary so poisoning does not surface as spuriousKeyError/AttributeError.lookup_threadsafe_unicodeand_LOAD_ATTR_WITH_HINT: re-validatema_keys(and the slot value) after acquiring a reference, matching_Py_dict_lookup_threadsafe. Covers the deferred-refcount try-incref path that skips slot re-validation.Tests
test_racing_load_attr_with_hint_and_resizetest_racing_dict_resize_and_lookupBoth stress concurrent resize + lock-free / specialized reads under free-threading (and TSAN in local builds).
Test plan
./python.exe -m unittest test.test_free_threading.test_dict.TestDict test.test_free_threading.test_races.TestRaces(×5, free-threaded + TSAN)