CVE-2025-22015

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

mm/migrate: fix shmem xarray update during migration

A shmem folio can be either in page cache or in swap cache, but not at the

same time. Namely, once it is in swap cache, folio->mapping should be

NULL, and the folio is no longer in a shmem mapping.

In __folio_migrate_mapping(), to determine the number of xarray entries to

update, folio_test_swapbacked() is used, but that conflates shmem in page

cache case and shmem in swap cache case. It leads to xarray multi-index

entry corruption, since it turns a sibling entry to a normal entry during

xas_store() (see [1] for a userspace reproduction). Fix it by only using

folio_test_swapcache() to determine whether xarray is storing swap cache

entries or not to choose the right number of xarray entries to update.

[1] https://lore.kernel.org/linux-mm/[email protected]/

Note:

In __split_huge_page(), folio_test_anon() && folio_test_swapcache() is

used to get swap_cache address space, but that ignores the shmem folio in

swap cache case. It could lead to NULL pointer dereferencing when a

in-swap-cache shmem folio is split at __xa_store(), since

!folio_test_anon() is true and folio->mapping is NULL. But fortunately,

its caller split_huge_page_to_list_to_order() bails out early with EBUSY

when folio->mapping is NULL. So no need to take care of it here.