[PATCH 6.6.y] mm/huge_memory: unlock i_mmap_rwsem before releasing after-split folios

From: "Kiryl Shutsemau (Meta)" <[email protected]>

__folio_split() keeps dereferencing the mapping after the split:
shmem_uncharge(mapping->host) and remap_page() while the folios are still
frozen/locked, and i_mmap_unlock_read(mapping) at the very end, after the
after-split folios have been unlocked and freed.

Nothing holds an inode reference across that.  The split relies on @folio
-- which the beyond-EOF drop loop never removes, as it starts at
folio_next(folio) -- staying locked and in the page cache to hold off
eviction.  But the unlock loop unlocks @folio before i_mmap_unlock_read()
runs.  If the caller's @lock_at is a tail beyond EOF, as memory_failure()
passes when splitting a poisoned tail of a shmem THP that reaches past
i_size during truncation, it too is gone from the page cache; so once
@folio is unlocked no locked, in-cache folio pins the inode, and a
concurrent final iput() can evict and RCU-free it before
i_mmap_unlock_read() touches i_mmap_rwsem:

  BUG: KASAN: slab-use-after-free in __up_read+0x634/0x790
   i_mmap_unlock_read include/linux/fs.h:537 [inline]
   __folio_split+0x732/0x1640 mm/huge_memory.c:4100
   try_to_split_thp_page+0xab/0x390 mm/memory-failure.c:1675
   memory_failure+0x1394/0x26e0 mm/memory-failure.c:2470

  Freed by task 4601:
   shmem_free_in_core_inode+0x54/0xb0 mm/shmem.c:5177
   evict+0x57f/0xac0 fs/inode.c:870

Do every mapping dereference while @folio still pins the inode: drop
i_mmap_rwsem right after remap_page(), before the loop that unlocks and
frees the after-split folios, and clear @mapping so the exit path does not
unlock it again.  shmem_uncharge() and remap_page() already run before
that point, so after this nothing past the unlock loop touches the inode
or the mapping.

This is now a rule the split depends on, alongside keeping @folio frozen
until the page cache is updated: no inode or mapping dereference once the
after-split folios start being unlocked.

Link: https://lore.kernel.org/[email protected]
Fixes: baa355fd3314 ("thp: file pages support for split_huge_page()")
Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]>
Reported-by: Hao Zhang <[email protected]>
Closes: https://lore.kernel.org/linux-mm/20260710071344.GA106129@zh-pc
Co-developed-by: Hao Zhang <[email protected]>
Signed-off-by: Hao Zhang <[email protected]>
Acked-by: David Hildenbrand (Arm) <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Dev Jain <[email protected]>
Cc: Lance Yang <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Nico Pache <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

(cherry picked from commit e923bd21058ea02fd0dcd3549d151d143fd036e5)
[ kas: adapt to the __split_huge_page()/split_huge_page_to_list()
  two-function split: pass @mapping into __split_huge_page() and drop it
  there, before the loop that frees the after-split subpages while the
  head is still locked; the caller then skips its own i_mmap unlock ]
Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]>
---
 mm/huge_memory.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 4443cc44cbf9..ff95a802d158 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2489,7 +2489,7 @@ static void __split_huge_page_tail(struct folio *folio, int tail,
 }
 
 static void __split_huge_page(struct page *page, struct list_head *list,
-		pgoff_t end)
+		pgoff_t end, struct address_space *mapping)
 {
 	struct folio *folio = page_folio(page);
 	struct page *head = &folio->page;
@@ -2564,6 +2564,16 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 	if (folio_test_swapcache(folio))
 		split_swap_cluster(folio->swap);
 
+	/*
+	 * Drop the mapping while the head page is still locked and thus pins
+	 * the inode. The loop below may free the after-split subpages --
+	 * including the head, when @page is a tail beyond EOF that the split
+	 * dropped from the page cache -- which could otherwise let the inode,
+	 * and @mapping, be freed before this unlock.
+	 */
+	if (mapping)
+		i_mmap_unlock_read(mapping);
+
 	for (i = 0; i < nr; i++) {
 		struct page *subpage = head + i;
 		if (subpage == page)
@@ -2745,7 +2755,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 			}
 		}
 
-		__split_huge_page(page, list, end);
+		__split_huge_page(page, list, end, mapping);
+		/* __split_huge_page() dropped the i_mmap lock */
+		mapping = NULL;
 		ret = 0;
 	} else {
 		spin_unlock(&ds_queue->split_queue_lock);
-- 
2.54.0