userfaultfd: verify VMA state across UFFDIO_COPY retry

userfaultfd: verify VMA state across UFFDIO_COPY retry

Patch series "userfaultfd: verify VMA state across UFFDIO_COPY retry", v2.

... and two more small fixes.

This patch (of 3):

mfill_copy_folio_retry() drops the VMA lock for copy_from_user() and
reacquires it afterwards.  The destination VMA can be replaced during that
window.

The existing check compares vma_uffd_ops() before and after the retry, but
if a shmem VMA with MAP_SHARED is replaced with a shmem VMA with
MAP_PRIVATE (or vice versa) the replacement goes undetected.

The change from MAP_PRIVATE to MAP_SHARED will treat the folio allocated
with shmem_alloc_folio() as anonymous and this will cause BUG() when
mfill_atomic_install_pte() will try to folio_add_new_anon_rmap().

The change from MAP_SHARED to MAP_PRIVATE allows injection of folios into
the page cache of the original VMA.

There is no need to change for hugetlb because it never uses
mfill_copy_folio_retry().

Introduce helpers for more comprehensive comparison of VMA state:
- mfill_retry_state_save() to save the relevant VMA state into a struct
  mfill_retry_state (original uffd_ops, relevant VMA flags, vm_file and
  pgoff) before dropping the lock
- mfill_retry_state_changed() to compare the saved state with the state
  of the VMA acquired after retaking the locks
- mfill_retry_state_put() to release vm_file pinning.

Use DEFINE_FREE() cleanup to wrap mfill_retry_state_put() to avoid
complicating error handling paths in mfill_copy_folio_retry().

Link: https://lore.kernel.org/20260527184751.4147364-1-rppt@kernel.org
Link: https://lore.kernel.org/20260527184751.4147364-2-rppt@kernel.org
Fixes: 292411fda25b ("mm/userfaultfd: detect VMA type change after copy retry in mfill_copy_folio_retry()")
Fixes: 6ab703034f14 ("userfaultfd: mfill_atomic(): remove retry logic")
Co-developed-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Suggested-by: Peter Xu <peterx@redhat.com>
Co-developed-by: David Carlier <devnexen@gmail.com>
Signed-off-by: David Carlier <devnexen@gmail.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>