CVE-2024-42234

In the Linux kernel, the following vulnerability has been resolved:

mm: fix crashes from deferred split racing folio migration

Even on 6.10-rc6, Ive been seeing elusive Bad page states (often on flags when freeing, yet the flags shown are not bad: PG_locked had been set and cleared??), and VM_BUG_ON_PAGE(page_ref_count(page) == 0)s from deferred_split_scan()s folio_put(), and a variety of other BUG and WARN symptoms implying double free by deferred split and large folio migration.

6.7 commit 9bcef5973e31 (mm: memcg: fix split queue list crash when large folio migration) was right to fix the memcg-dependent locking broken in 85ce2c517ade (memcontrol: only transfer the memcg data for migration), but missed a subtlety of deferred_split_scan(): it moves folios to its own local list to work on them without split_queue_lock, during which time folio->_deferred_list is not empty, but even the right lock does nothing to secure the folio and the list it is on.

Fortunately, deferred_split_scan() is careful to use folio_try_get(): so folio_migrate_mapping() can avoid the race by folio_undo_large_rmappable() while the old folios reference count is temporarily frozen to 0 - adding such a freeze in the !mapping case too (originally, folio lock and unmapping and no swap cache left an anon folio unreachable, so no freezing was needed there: but the deferred split queue offers a way to reach it).

Weakness

The product calls free() twice on the same memory address.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/be9581ea8c058d81154251cb0695987098996cad
• https://git.kernel.org/stable/c/fc7facce686b64201dbf0b9614cc1d0bfad70010