[02/18] mm/swap: fix pte_same_as_swp() not removing uffd-wp bit when compare

Commit Message

Andrew Morton June 16, 2021, 1:23 a.m. UTC

From: Peter Xu <[e-mail]>
Subject: mm/swap: fix pte_same_as_swp() not removing uffd-wp bit when compare

I found it by pure code review, that pte_same_as_swp() of unuse_vma()
didn't take uffd-wp bit into account when comparing ptes. 
pte_same_as_swp() returning false negative could cause failure to swapoff
swap ptes that was wr-protected by userfaultfd.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration")
Signed-off-by: Peter Xu <[e-mail]>
Acked-by: Hugh Dickins <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: <[e-mail]>	[5.7+]
Signed-off-by: Andrew Morton <[e-mail]>
---

 include/linux/swapops.h |   15 +++++++++++----
 mm/swapfile.c           |    2 +-
 2 files changed, 12 insertions(+), 5 deletions(-)

Patch

--- a/include/linux/swapops.h~mm-swap-fix-pte_same_as_swp-not-removing-uffd-wp-bit-when-compare
+++ a/include/linux/swapops.h
@@ -23,6 +23,16 @@ 
 #define SWP_TYPE_SHIFT	(BITS_PER_XA_VALUE - MAX_SWAPFILES_SHIFT)
 #define SWP_OFFSET_MASK	((1UL << SWP_TYPE_SHIFT) - 1)
 
+/* Clear all flags but only keep swp_entry_t related information */
+static inline pte_t pte_swp_clear_flags(pte_t pte)
+{
+	if (pte_swp_soft_dirty(pte))
+		pte = pte_swp_clear_soft_dirty(pte);
+	if (pte_swp_uffd_wp(pte))
+		pte = pte_swp_clear_uffd_wp(pte);
+	return pte;
+}
+
 /*
  * Store a type+offset into a swp_entry_t in an arch-independent format
  */
@@ -66,10 +76,7 @@  static inline swp_entry_t pte_to_swp_ent
 {
 	swp_entry_t arch_entry;
 
-	if (pte_swp_soft_dirty(pte))
-		pte = pte_swp_clear_soft_dirty(pte);
-	if (pte_swp_uffd_wp(pte))
-		pte = pte_swp_clear_uffd_wp(pte);
+	pte = pte_swp_clear_flags(pte);
 	arch_entry = __pte_to_swp_entry(pte);
 	return swp_entry(__swp_type(arch_entry), __swp_offset(arch_entry));
 }
--- a/mm/swapfile.c~mm-swap-fix-pte_same_as_swp-not-removing-uffd-wp-bit-when-compare
+++ a/mm/swapfile.c
@@ -1900,7 +1900,7 @@  unsigned int count_swap_pages(int type,
 
 static inline int pte_same_as_swp(pte_t pte, pte_t swp_pte)
 {
-	return pte_same(pte_swp_clear_soft_dirty(pte), swp_pte);
+	return pte_same(pte_swp_clear_flags(pte), swp_pte);
 }
 
 /*