[PATCH 5.12 259/363] mm/hugetlb: fix cow where page writtable in child

[Greg Kroah-Hartman]

From: Peter Xu <peterx@xxxxxxxxxx>

commit 84894e1c42e9f25c17f2888e0c0e1505cb727538 upstream.

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/20210503234356.9097-3-peterx@xxxxxxxxxx
Fixes: 4eae4efa2c299 ("hugetlb: do early cow when page pinned on src mm")
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 mm/hugetlb.c |    1 +
 1 file changed, 1 insertion(+)

--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3905,6 +3905,7 @@ again:
 				 * See Documentation/vm/mmu_notifier.rst
 				 */
 				huge_ptep_set_wrprotect(src, addr, src_pte);
+				entry = huge_pte_wrprotect(entry);
 			}
 
 			page_dup_rmap(ptepage, true);