[PATCH v2 2/2] mm/hugetlb: Fix cow where page writtable in child

When rework early cow of pinned hugetlb pages, we moved huge_ptep_get() upper
but overlooked a side effect that the huge_ptep_get() will fetch the pte after
wr-protection.  After moving it upwards, we need explicit wr-protect of child
pte or we will keep the write bit set in the child process, which could cause
data corrution where the child can write to the original page directly.

This issue can also be exposed by "memfd_test hugetlbfs" kselftest.

Cc: stable@vger.kernel.org
Fixes: 4eae4efa2c299 ("hugetlb: do early cow when page pinned on src mm")
Reviewed-by: Mike Kravetz <redacted>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/hugetlb.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index aab3a33214d10..72544ebb24f0e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c

@@ -4076,6 +4076,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 				 * See Documentation/vm/mmu_notifier.rst
 				 */
 				huge_ptep_set_wrprotect(src, addr, src_pte);
+				entry = huge_pte_wrprotect(entry);
 			}
 
 			page_dup_rmap(ptepage, true);

-- 
2.31.1