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.

[[email protected]: add VM_WARN_ON_ONCE() to zswap_folio_swapin()]
  Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: remove unneeded if (folio) checks]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Nhat Pham <[email protected]>
Reviewed-by: Chengming Zhou <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Yosry Ahmed <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

2 files changed, 9 insertions, 8 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 @@ skip:
 	/* 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 @@ skip:
 	/* 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 d2423247acfd..36903d938c15 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -377,10 +377,9 @@ void zswap_folio_swapin(struct folio *folio)
 {
 	struct lruvec *lruvec;
 
-	if (folio) {
-		lruvec = folio_lruvec(folio);
-		atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected);
-	}
+	VM_WARN_ON_ONCE(!folio_test_locked(folio));
+	lruvec = folio_lruvec(folio);
+	atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected);
 }
 
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