[PATCH 7.1.y] userfaultfd: gate must_wait writability check on pte_present()

[Kiryl Shutsemau <kas@kernel.org>]

userfaultfd_must_wait() and userfaultfd_huge_must_wait() read the PTE
without taking the page table lock and then apply pte_write() /
huge_pte_write() to it.  Those accessors decode bits from the present
encoding only; on a swap or migration entry they read the offset bits that
happen to share the same position and return an undefined result.

The intent of the check is "is this fault still WP-blocked?".  A
non-marker swap entry means the page is in transit -- the userfault
context the original fault delivered against is no longer the same, and
the swap-in or migration completion path will re-deliver a fresh fault if
userspace still needs to handle it.  Worst case under the current code the
garbage write bit says "wait", and the thread stays asleep until a
UFFDIO_WAKE that may never arrive.

Gate the writability check on pte_present() so the lockless re-check only
inspects present-PTE bits when the entry is actually present.  The
non-present, non-marker case returns "don't wait" and lets the fault path
retry.

Link: https://lore.kernel.org/20260529172331.356655-6-kas@kernel.org
Fixes: 369cd2121be4 ("userfaultfd: hugetlbfs: userfaultfd_huge_must_wait for hugepmd ranges")
Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reported-by: Sashiko AI review <sashiko-bot@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

(cherry picked from commit 8e80af52db652fbc41320eee45a4f73bc029faf2)
[ kas: apply to fs/userfaultfd.c; these checks moved to mm/userfaultfd.c
  only after 7.1, the change is otherwise identical to upstream ]
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
---
 fs/userfaultfd.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 4b53dc4a3266..648249037731 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -253,6 +253,15 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
 	/* UFFD PTE markers require userspace to resolve the fault. */
 	if (pte_is_uffd_marker(pte))
 		return true;
+	/*
+	 * Concurrent migration may have replaced the present PTE with a
+	 * non-marker swap entry between fault delivery and this lockless
+	 * re-check. huge_pte_write() on a swap entry decodes random offset
+	 * bits, so gate it on pte_present(). The migration completion path
+	 * will re-deliver the fault if it still needs userspace.
+	 */
+	if (!pte_present(pte))
+		return false;
 	/*
 	 * If VMA has UFFD WP faults enabled and WP fault, wait for userspace to
 	 * resolve the fault.
@@ -339,6 +348,17 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
 	/* UFFD PTE markers require userspace to resolve the fault. */
 	if (pte_is_uffd_marker(ptent))
 		goto out;
+	/*
+	 * Concurrent swap-out / migration may have replaced the present PTE
+	 * with a non-marker swap entry between fault delivery and this
+	 * lockless re-check. pte_write() on a swap entry decodes random
+	 * offset bits, so gate it on pte_present(). The page-in path will
+	 * re-deliver the fault if it still needs userspace.
+	 */
+	if (!pte_present(ptent)) {
+		ret = false;
+		goto out;
+	}
 	/*
 	 * If VMA has UFFD WP faults enabled and WP fault, wait for userspace to
 	 * resolve the fault.
-- 
2.54.0