mm/migrate: snapshot the source nid before the misplaced migration

A successful migration frees the source folio before the promotion
counter update, so the folio_nid() read there operates on a freed
folio — which may already have been reallocated. It happens to work
today, as Zi explained:

  : Strictly speaking, it can be seen as a benign UAF. It works, because
  : 1. folio->flags happens to be the same as page->flags,
  : 2. folio/page nid is stored in the ->flags and never changed.
  :
  : In memdesc world, after a folio is freed, I assume folio->flags would be
  : zeroed since struct folio is freed and calling folio_nid() on a freed
  : folio would give a bogus result (yeah, it can be mitigated by not
  : zeroing node field and each node has its own struct folio pool, but that
  : makes folio_nid() implementation dependent).

and Matthew explained further:

  : once struct folio is separately allocated, it's a UAF.  It could be
  : recycled to a different NID or maybe the slab containing the folio could
  : be freed (assuming we're not holding the RCU read lock at this point in
  : migration, and I don't see why we would be).

A separate concern is memory hot-removal.  With memory-tiering mode
enabled, successful migration can release the source before the
promotion-accounting folio_nid() read.  If the task is preempted there,
an otherwise-free source block can be offlined and removed first.  On
the x86 path, removal can unmap the source's vmemmap, so the later read
can fault.  The snapshot avoids that read.

Link: https://lore.kernel.org/20261027021721.95781-1-laoar.shao@gmail.com
Fixes: e39bb6be9f2b ("NUMA Balancing: add page promotion counter")
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Zi Yan <ziy@nvidia.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: <stable@vger.kernel.org>

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