[PATCH 6.18.y 3/3] userfaultfd: wait on source PMD during UFFDIO_MOVE

From: Usama Arif <[email protected]>

[ Upstream commit 4165b7d1c45c2da0dfefe528f8d1fb7d79f0d344 ]

move_pages_huge_pmd() snapshots src_pmdval under src_ptl, drops the lock,
and, for migration entries, waits with pmd_migration_entry_wait().

Passing &src_pmdval is wrong.  pmd_migration_entry_wait() must lock and
re-read the real page-table PMD; on split-PMD-lock kernels, a stack
address also resolves to the wrong lock.  softleaf_entry_wait_on_locked()
then waits without a folio reference, which is safe only while serialized
against migration-entry removal by the real PT lock.

Pass src_pmd, matching __handle_mm_fault() and hmm_vma_walk_pmd().

Link: https://lore.kernel.org/[email protected]
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Reported-by: sashiko-bot <[email protected]>
Link: https://sashiko.dev/#/patchset/20260703173903.3789516-1-usama.arif%40linux.dev?part=8
Signed-off-by: Usama Arif <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Reviewed-by: Lance Yang <[email protected]>
Acked-by: David Hildenbrand (Arm) <[email protected]>
Reviewed-by: Lance Yang <[email protected]>
Reviewed-by: Lorenzo Stoakes <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Dev Jain <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Nico Pache <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
---
 mm/huge_memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3e72672b19c69..00f99ec85d8a1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2560,7 +2560,7 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
 	if (!pmd_trans_huge(src_pmdval)) {
 		spin_unlock(src_ptl);
 		if (pmd_is_migration_entry(src_pmdval)) {
-			pmd_migration_entry_wait(mm, &src_pmdval);
+			pmd_migration_entry_wait(mm, src_pmd);
 			return -EAGAIN;
 		}
 		return -ENOENT;
-- 
2.53.0