CVE-2026-74482: mm/huge_memory: unlock i_mmap_rwsem before releasing after-split folios

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-74482 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.8 with commit baa355fd331424526e742d41d9b90d5f9d10f716 and fixed in 6.6.151 with commit be106f7855f03d3128ed0ce70ba74b484a90b473
	Issue introduced in 4.8 with commit baa355fd331424526e742d41d9b90d5f9d10f716 and fixed in 6.12.103 with commit e3dd774dbfd0b5bc2dbd0995221751b1234f8205
	Issue introduced in 4.8 with commit baa355fd331424526e742d41d9b90d5f9d10f716 and fixed in 6.18.44 with commit 10065fb891651d9541e7a5a2db84c1e656ece4f9
	Issue introduced in 4.8 with commit baa355fd331424526e742d41d9b90d5f9d10f716 and fixed in 7.1.8 with commit d640efe94d86d3be893d4c19220362546a637e90
	Issue introduced in 4.8 with commit baa355fd331424526e742d41d9b90d5f9d10f716 and fixed in 7.2-rc6 with commit e923bd21058ea02fd0dcd3549d151d143fd036e5

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74482
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	mm/huge_memory.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/be106f7855f03d3128ed0ce70ba74b484a90b473
	https://git.kernel.org/stable/c/e3dd774dbfd0b5bc2dbd0995221751b1234f8205
	https://git.kernel.org/stable/c/10065fb891651d9541e7a5a2db84c1e656ece4f9
	https://git.kernel.org/stable/c/d640efe94d86d3be893d4c19220362546a637e90
	https://git.kernel.org/stable/c/e923bd21058ea02fd0dcd3549d151d143fd036e5