CVE-2024-42234 Information

Description

In the Linux kernel the following vulnerability has been resolved:

mm: fix crashes from deferred split racing folio migration

Even on 6.10-rc6 I’ve been seeing elusive \Bad page state\s (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 ight\ 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 folio’s 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).

Reference

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