CVE-2024-53079

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

mm/thp: fix deferred split unqueue naming and locking

Recent changes are putting more pressure on THP deferred split queues:

under load revealing long-standing races, causing list_del corruptions,

"Bad page state"s and worse (I keep BUGs in both of those, so usually

don't get to see how badly they end up without). The relevant recent

changes being 6.8's mTHP, 6.10's mTHP swapout, and 6.12's mTHP swapin,

improved swap allocation, and underused THP splitting.

Before fixing locking: rename misleading folio_undo_large_rmappable(),

which does not undo large_rmappable, to folio_unqueue_deferred_split(),

which is what it does. But that and its out-of-line __callee are mm

internals of very limited usability: add comment and WARN_ON_ONCEs to

check usage; and return a bool to say if a deferred split was unqueued,

which can then be used in WARN_ON_ONCEs around safety checks (sparing

callers the arcane conditionals in __folio_unqueue_deferred_split()).

Just omit the folio_unqueue_deferred_split() from free_unref_folios(), all

of whose callers now call it beforehand (and if any forget then bad_page()

will tell) - except for its caller put_pages_list(), which itself no

longer has any callers (and will be deleted separately).

Swapout: mem_cgroup_swapout() has been resetting folio->memcg_data 0

without checking and unqueueing a THP folio from deferred split list;

which is unfortunate, since the split_queue_lock depends on the memcg

(when memcg is enabled); so swapout has been unqueueing such THPs later,

when freeing the folio, using the pgdat's lock instead: potentially

corrupting the memcg's list. __remove_mapping() has frozen refcount to 0

here, so no problem with calling folio_unqueue_deferred_split() before

resetting memcg_data.

That goes back to 5.4 commit 87eaceb3faa5 ("mm: thp: make deferred split

shrinker memcg aware"): which included a check on swapcache before adding

to deferred queue, but no check on deferred queue before adding THP to

swapcache. That worked fine with the usual sequence of events in reclaim

(though there were a couple of rare ways in which a THP on deferred queue

could have been swapped out), but 6.12 commit dafff3f4c850 ("mm: split

underused THPs") avoids splitting underused THPs in reclaim, which makes

swapcache THPs on deferred queue commonplace.

Keep the check on swapcache before adding to deferred queue? Yes: it is

no longer essential, but preserves the existing behaviour, and is likely

to be a worthwhile optimization (vmstat showed much more traffic on the

queue under swapping load if the check was removed); update its comment.

Memcg-v1 move (deprecated): mem_cgroup_move_account() has been changing

folio->memcg_data without checking and unqueueing a THP folio from the

deferred list, sometimes corrupting "from" memcg's list, like swapout.

Refcount is non-zero here, so folio_unqueue_deferred_split() can only be

used in a WARN_ON_ONCE to validate the fix, which must be done earlier:

mem_cgroup_move_charge_pte_range() first try to split the THP (splitting

of course unqueues), or skip it if that fails. Not ideal, but moving

charge has been requested, and khugepaged should repair the THP later:

nobody wants new custom unqueueing code just for this deprecated case.

The 87eaceb3faa5 commit did have the code to move from one deferred list

to another (but was not conscious of its unsafety while refcount non-0);

but that was removed by 5.6 commit fac0516b5534 ("mm: thp: don't need care

deferred split queue in memcg charge move path"), which argued that the

existence of a PMD mapping guarantees that the THP cannot be on a deferred

list. As above, false in rare cases, and now commonly false.

Backport to 6.11 should be straightforward. Earlier backports must take

care that other _deferred_list fixes and dependencies are included. There

is not a strong case for backports, but they can fix cornercases.