linux-kernel - [PATCH] mm/swap: Fix pte_same_as_swp() not removing uffd-wp bit when compare

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Date:   Thu,  3 Jun 2021 14:05:46 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     peterx@...hat.com, Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: [PATCH] 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.

Cc: Hugh Dickins <hughd@...gle.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 include/linux/swapops.h | 15 +++++++++++----
 mm/swapfile.c           |  2 +-
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index af3d2661e41e..d356ab4047f7 100644
--- a/include/linux/swapops.h
+++ b/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_entry(pte_t pte)
 {
 	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));
 }
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 6212bc033602..1e07d1c776f2 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1887,7 +1887,7 @@ unsigned int count_swap_pages(int type, int free)
 
 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);
 }
 
 /*
-- 
2.31.1