[PATCH] userfaultfd: preserve write protection across UFFDIO_MOVE

[Gregory Price <gourry@gourry.net>]

move_present_ptes() unconditionally makes the destination PTE writable,
dropping uffd-wp write-protection from the source PTE.

The original intent was to follow mremap() behavior, but mremap()'s
move_ptes() preserves the source write state unconditionally.

Modify uffd to preserve the source write state and check the uffd-wp
condition of the source before setting writable on the destination.

Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Cc: stable@vger.kernel.org
Signed-off-by: Gregory Price <gourry@gourry.net>
---
 mm/userfaultfd.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index e6dfd5f28acd..783ca68aed88 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1123,7 +1123,10 @@ static long move_present_ptes(struct mm_struct *mm,
 			orig_dst_pte = pte_mksoft_dirty(orig_dst_pte);
 		if (pte_dirty(orig_src_pte))
 			orig_dst_pte = pte_mkdirty(orig_dst_pte);
-		orig_dst_pte = pte_mkwrite(orig_dst_pte, dst_vma);
+		if (pte_write(orig_src_pte))
+			orig_dst_pte = pte_mkwrite(orig_dst_pte, dst_vma);
+		if (pte_uffd_wp(orig_src_pte))
+			orig_dst_pte = pte_mkuffd_wp(orig_dst_pte);
 		set_pte_at(mm, dst_addr, dst_pte, orig_dst_pte);
 
 		src_addr += PAGE_SIZE;
-- 
2.52.0