mm/mglru: fix and remove redundant unevictable folio handling

Summary

In the Linux kernel, the following vulnerability has been resolved: mm/mglru: fix and remove redundant unevictable folio handling sort_folio() has a shortcut for moving folios that are no longer evictable but are still sitting on a generation list. However, this shortcut is buggy. It does not follow the PG_lru usage convention, and it has a more serious issue. Unevictable folios are not threaded on lists[LRU_UNEVICTABLE], so that folio->lru can be reused to hold folio->mlock_count (see the comment in lruvec_init()). Hence lruvec_add_folio() skips the list_add() for them, and every other place that turns a folio unevictable initialises mlock_count explicitly: lru_add() sets it to 0, __mlock_folio() and __mlock_new_folio() set it to !!folio_test_mlocked(folio). sort_folio() sets nothing, and the lru_gen_del_folio() right above it may have already poisoned folio->lru via list_del(), so mlock_count ends up aliasing LIST_POISON2, which reads as 0x122, i.e. 290. The result is user visible. On munlock, __munlock_folio() decrements that bogus count, finds it still non-zero and bails out before clearing PG_mlocked, so the folio remains unevictable and the Mlocked accounting stays inflated until the folio is freed. The shortcut also touches the LRU flags in the wrong order. It calls lru_gen_del_folio() while PG_lru is still set, so a concurrent folio_test_clear_lru() (e.g. compaction, folio_isolate_lru()) can succeed on a folio that has already been taken off the generation list, which may lead to unexpected behavior. So fix it by isolating them as common folios and letting the generic shrink path cull them. This matches the classical LRU behavior, and there should be no visible effect on the generic eviction or isolation behavior. There is no performance concern either, such a folio goes through this once, and then it is off the generation lists for good.

Timeline

1. Sep 11, 2026, 7:46 PM UTCCVE published
2. Oct 21, 2026, 12:48 PM UTCRecord last updated

Affected versions

• Linux Linux

  From ac35a490237446b71e3b4b782b1596967edd0aa8 · fixed in b1d7c69e6c7d3ec47513b911ce7529e56d183dad; From ac35a490237446b71e3b4b782b1596967edd0aa8 · fixed in 49193dac48d30eee2a21da88a7262d52a7feefae; From ac35a490237446b71e3b4b782b1596967edd0aa8 · fixed in 54a58d6656dd403b686b247f0bad8507cbfb45df; From ac35a490237446b71e3b4b782b1596967edd0aa8 · fixed in 15d3a2a71c8bf7333d019d249ee33669e69e4275; From ac35a490237446b71e3b4b782b1596967edd0aa8 · fixed in f7e698e326b239a91ea15844817551921209e826

• Linux Linux

  6.1

• KernelLinux

  From 6.1.0 · fixed in 6.6.160; From 6.7.0 · fixed in 6.12.114; From 6.13.0 · fixed in 6.18.51; From 6.19.0 · fixed in 7.2.4

References 5