[PATCH v3 25/28] userfaultfd: wp: fixup swap entries in change_pte_range

[Peter Xu]

In change_pte_range() we do nothing for uffd if the PTE is a swap
entry.  That can lead to data mismatch if the page that we are going
to write protect is swapped out when sending the UFFDIO_WRITEPROTECT.
This patch applies/removes the uffd-wp bit even for the swap entries.

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---

I kept this patch a standalone one majorly to make review easier.  The
patch can be considered as standalone or to squash into the patch
"userfaultfd: wp: support swap and page migration".
---
 mm/mprotect.c | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 96c0f521099d..a23e03053787 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -183,11 +183,11 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 			}
 			ptep_modify_prot_commit(mm, addr, pte, ptent);
 			pages++;
-		} else if (IS_ENABLED(CONFIG_MIGRATION)) {
+		} else if (is_swap_pte(oldpte)) {
 			swp_entry_t entry = pte_to_swp_entry(oldpte);
+			pte_t newpte;
 
 			if (is_write_migration_entry(entry)) {
-				pte_t newpte;
 				/*
 				 * A protection check is difficult so
 				 * just be safe and disable write
@@ -198,22 +198,24 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 					newpte = pte_swp_mksoft_dirty(newpte);
 				if (pte_swp_uffd_wp(oldpte))
 					newpte = pte_swp_mkuffd_wp(newpte);
-				set_pte_at(mm, addr, pte, newpte);
-
-				pages++;
-			}
-
-			if (is_write_device_private_entry(entry)) {
-				pte_t newpte;
-
+			} else if (is_write_device_private_entry(entry)) {
 				/*
 				 * We do not preserve soft-dirtiness. See
 				 * copy_one_pte() for explanation.
 				 */
 				make_device_private_entry_read(&entry);
 				newpte = swp_entry_to_pte(entry);
-				set_pte_at(mm, addr, pte, newpte);
+			} else {
+				newpte = oldpte;
+			}
 
+			if (uffd_wp)
+				newpte = pte_swp_mkuffd_wp(newpte);
+			else if (uffd_wp_resolve)
+				newpte = pte_swp_clear_uffd_wp(newpte);
+
+			if (!pte_same(oldpte, newpte)) {
+				set_pte_at(mm, addr, pte, newpte);
 				pages++;
 			}
 		}
-- 
2.17.1