mm/hugetlb: fix cow where page writtable in child

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.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 4eae4efa2c299 ("hugetlb: do early cow when page pinned on src mm")
Signed-off-by: Peter Xu <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Joel Fernandes (Google) <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>