mm/migrate: fix shmem xarray update during migration

Vulnerability Description

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/Z8idPCkaJW1IChjT@casper.infradead.org/ 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.

References

• https://git.kernel.org/stable/c/49100c0b070e900f87c8fac3be9b9ef8a30fa673
• https://git.kernel.org/stable/c/29124ae980e2860f0eec7355949d3d3292ee81da
• https://git.kernel.org/stable/c/c057ee03f751d6cecf7ee64f52f6545d94082aaa
• https://git.kernel.org/stable/c/75cfb92eb63298d717b6b0118f91ba12c4fcfeb5
• https://git.kernel.org/stable/c/60cf233b585cdf1f3c5e52d1225606b86acd08b0