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. Reported-by: syzbot+17a611d10af7d18a7092@xxxxxxxxxxxxxxxxxxxxxxxxx Closes: https://lore.kernel.org/all/000000000000ae47f90610803260@xxxxxxxxxx/ Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure") Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx> --- mm/swap_state.c | 10 ++++++---- mm/zswap.c | 1 + 2 files changed, 7 insertions(+), 4 deletions(-) diff --git a/mm/swap_state.c b/mm/swap_state.c index e671266ad772..7255c01a1e4e 100644 --- a/mm/swap_state.c +++ b/mm/swap_state.c @@ -680,9 +680,10 @@ struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask, /* The page was likely read above, so no need for plugging here */ folio = __read_swap_cache_async(entry, gfp_mask, mpol, ilx, &page_allocated, false); - if (unlikely(page_allocated)) + if (unlikely(page_allocated)) { + zswap_folio_swapin(folio); swap_read_folio(folio, false, NULL); - zswap_folio_swapin(folio); + } return folio; } @@ -855,9 +856,10 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask, /* The folio was likely read above, so no need for plugging here */ folio = __read_swap_cache_async(targ_entry, gfp_mask, mpol, targ_ilx, &page_allocated, false); - if (unlikely(page_allocated)) + if (unlikely(page_allocated)) { + zswap_folio_swapin(folio); swap_read_folio(folio, false, NULL); - zswap_folio_swapin(folio); + } return folio; } diff --git a/mm/zswap.c b/mm/zswap.c index 4aea03285532..8c548f73d52e 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -827,6 +827,7 @@ void zswap_folio_swapin(struct folio *folio) struct lruvec *lruvec; if (folio) { + VM_WARN_ON_ONCE(!folio_test_locked(folio)); lruvec = folio_lruvec(folio); atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected); } base-commit: 91f3daa1765ee4e0c89987dc25f72c40f07af34d -- 2.39.3
