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mm/swap_state: update zswap LRU's protection range with the folio locked

When a folio is swapped in, the protection size of the corresponding zswap
LRU is incremented, so that the zswap shrinker is more conservative with
its reclaiming action. This field is embedded within the struct lruvec,
so updating it requires looking up the folio's memcg and lruvec. However,
currently this lookup can happen after the folio is unlocked, for instance
if a new folio is allocated, and swap_read_folio() unlocks the folio
before returning. In this scenario, there is no stability guarantee for
the binding between a folio and its memcg and lruvec:

* A folio's memcg and lruvec can be freed between the lookup and the
update, leading to a UAF.
* Folio migration can clear the now-unlocked folio's memcg_data, which
directs the zswap LRU protection size update towards the root memcg
instead of the original memcg. This was recently picked up by the
syzbot thanks to a warning in the inlined folio_lruvec() call.

Move the zswap LRU protection range update above the swap_read_folio()
call, and only when a new page is allocated, to prevent this.

[nphamcs@gmail.com: add VM_WARN_ON_ONCE() to zswap_folio_swapin()]
Link: https://lkml.kernel.org/r/20240206180855.3987204-1-nphamcs@gmail.com
[nphamcs@gmail.com: remove unneeded if (folio) checks]
Link: https://lkml.kernel.org/r/20240206191355.83755-1-nphamcs@gmail.com
Link: https://lkml.kernel.org/r/20240205232442.3240571-1-nphamcs@gmail.com
Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
Reported-by: syzbot+17a611d10af7d18a7092@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000ae47f90610803260@google.com/
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

authored by

Nhat Pham and committed by
Andrew Morton 16e96ba5 7efa6f2c

+9 -8
+6 -4
mm/swap_state.c
··· 680 680 /* The page was likely read above, so no need for plugging here */ 681 681 folio = __read_swap_cache_async(entry, gfp_mask, mpol, ilx, 682 682 &page_allocated, false); 683 - if (unlikely(page_allocated)) 683 + if (unlikely(page_allocated)) { 684 + zswap_folio_swapin(folio); 684 685 swap_read_folio(folio, false, NULL); 685 - zswap_folio_swapin(folio); 686 + } 686 687 return folio; 687 688 } 688 689 ··· 856 855 /* The folio was likely read above, so no need for plugging here */ 857 856 folio = __read_swap_cache_async(targ_entry, gfp_mask, mpol, targ_ilx, 858 857 &page_allocated, false); 859 - if (unlikely(page_allocated)) 858 + if (unlikely(page_allocated)) { 859 + zswap_folio_swapin(folio); 860 860 swap_read_folio(folio, false, NULL); 861 - zswap_folio_swapin(folio); 861 + } 862 862 return folio; 863 863 } 864 864
+3 -4
mm/zswap.c
··· 377 377 { 378 378 struct lruvec *lruvec; 379 379 380 - if (folio) { 381 - lruvec = folio_lruvec(folio); 382 - atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected); 383 - } 380 + VM_WARN_ON_ONCE(!folio_test_locked(folio)); 381 + lruvec = folio_lruvec(folio); 382 + atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected); 384 383 } 385 384 386 385 /*********************************