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, 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 "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 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).