[PATCH v2] mm/memory-failure: fix concurrent access issue in min_order_for_split()

From: liyouhong <[email protected]>

min_order_for_split() accesses folio->mapping without proper
synchronization. While the compiler typically caches the value
in a register making a NULL deref unlikely in practice, the
real issue is that the callers in memory-failure.c do not hold
the folio lock at the time of the call:

- memory_failure() explicitly drops the folio lock before calling
  min_order_for_split().
- soft_offline_in_use_page() has not yet acquired the folio lock
  when calling min_order_for_split().

This means the value of folio->mapping may be modified by a
truncate or invalidate operation while min_order_for_split() is
executing, leading to a torn read or use of a stale mapping value.

Fixes: 689b8986776c ("mm/memory-failure: improve large block size folio handling")
Reported-by: Sashiko <[email protected]>
Closes: https://sashiko.dev/#/patchset/[email protected]
Cc: [email protected]
Signed-off-by: liyouhong <[email protected]>
---
v2:
- Dropped the approach of caching folio->mapping inside min_order_for_split() in favor of adding the folio lock at the callers.
- Added VM_WARN_ON_ONCE_FOLIO() in min_order_for_split().
- Updated the commit message to clarify that the real issue is the callers not holding the folio lock, rather than a TOCTOU race.
v1: https://lore.kernel.org/all/[email protected]/

---
 mm/huge_memory.c    |  2 ++
 mm/memory-failure.c | 12 ++++++++++--
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 58cabe6af33d..e3f16dadc1d4 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4300,6 +4300,8 @@ int folio_split(struct folio *folio, unsigned int new_order,
  */
 unsigned int min_order_for_split(struct folio *folio)
 {
+	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
+
 	if (folio_test_anon(folio))
 		return 0;
 
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 3b1e6946821b..7391524b5046 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -2437,12 +2437,17 @@ int memory_failure(unsigned long pfn, int flags)
 		res = -EOPNOTSUPP;
 		goto unlock_mutex;
 	}
+
 	folio_unlock(folio);
 
 	if (folio_test_large(folio)) {
-		const int new_order = min_order_for_split(folio);
+		const int new_order;
 		int err;
 
+		folio_lock(folio);
+		new_order = min_order_for_split(folio);
+		folio_unlock(folio);
+
 		/*
 		 * The flag must be set after the refcount is bumped
 		 * otherwise it may race with THP split.
@@ -2796,8 +2801,11 @@ static int soft_offline_in_use_page(struct page *page)
 	};
 
 	if (!huge && folio_test_large(folio)) {
-		const int new_order = min_order_for_split(folio);
+		const int new_order;
 
+		folio_lock(folio);
+		new_order = min_order_for_split(folio);
+		folio_unlock(folio);
 		/*
 		 * If new_order (target split order) is not 0, do not split the
 		 * folio at all to retain the still accessible large folio.
-- 
2.25.1