[PATCH 5.15.y] mm/hugetlb: fix use-after-free in hugetlb_mfill_atomic_pte()

[Sasha Levin <sashal@kernel.org>]

From: "David Hildenbrand (Arm)" <david@kernel.org>

[ Upstream commit d8a9cd7fd2fb01211430c933932da58839b8ca65 ]

Let's fix another instance where we use a folio after dropping the PTL,
where someone else could e.g., MADV_DONTNEED it once we drop the PTL,
making us trigger a use-after-free on the folio.

Link: https://lore.kernel.org/20261015-urgent_fixes-v1-3-42796911043b@kernel.org
Fixes: cb6acd01e2e4 ("hugetlbfs: fix races and page leaks during migration")
Fixes: 1c9e8def43a3 ("userfaultfd: hugetlbfs: add UFFDIO_COPY support for shared mappings")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Adam Litke <agl@us.ibm.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Rik van Riel <riel@surriel.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: <stable@vger.kernel.org>
[ sashal: adapted folio-based code and comments to page-based APIs in hugetlb_mcopy_atomic_pte() ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/hugetlb.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 947610e37a33c..e8ee7ef7fe03b 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5390,11 +5390,16 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	/* No need to invalidate - it was non-present before */
 	update_mmu_cache(dst_vma, dst_addr, dst_pte);
 
-	spin_unlock(ptl);
 	if (!is_continue)
 		SetHPageMigratable(page);
 	if (vm_shared || is_continue)
 		unlock_page(page);
+
+	/*
+	 * Hold the PTL until we're done with the page: the page table owns the
+	 * page reference now.
+	 */
+	spin_unlock(ptl);
 	ret = 0;
 out:
 	return ret;
-- 
2.53.0