CVE-2026-74482

In the Linux kernel, the following vulnerability has been resolved:

mm/huge_memory: unlock i_mmap_rwsem before releasing after-split folios

__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.