[PATCH] mm/swap, PM: hibernate: atomically replace hibernation pin

[David Carlier <devnexen@gmail.com>]

snapshot_set_swap_area() unpins the previously selected swap device
and pins the new one in two separate swap_lock critical sections.
In the gap between them, swapoff() observes SWP_HIBERNATION cleared,
bypasses the guard, and tears down the device, reopening the race
the SWP_HIBERNATION pin was meant to close. The window is reachable
on any SNAPSHOT_SET_SWAP_AREA call after the snapshot device is
opened for hibernation, and on any retry after the resume path's
first selection.

Add repin_hibernation_swap_type(), which looks up the new device,
clears the old SWP_HIBERNATION flag and sets the new one under a
single swap_lock acquisition. The same-device case is short-
circuited so userspace can re-select the same swap area without
tripping WARN_ON_ONCE and -EBUSY. Switch snapshot_set_swap_area()
to the new helper.

A failed lookup now preserves the previous pin instead of dropping
it, so a bad SNAPSHOT_SET_SWAP_AREA leaves the prior selection
intact. The open and release paths keep using
pin_hibernation_swap_type() and unpin_hibernation_swap_type().

The race was identified during AI-assisted review of the
SWP_HIBERNATION pinning series.

Fixes: 8e6e0d845823 ("mm/swap, PM: hibernate: fix swapoff race in uswsusp by pinning swap device")
Assisted-by: Codex (gpt-5-codex)
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 include/linux/swap.h |  2 ++
 kernel/power/user.c  | 17 ++++--------
 mm/swapfile.c        | 61 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 68 insertions(+), 12 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 1930f81e6be4..213ecb627a39 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -436,6 +436,8 @@ static inline long get_nr_swap_pages(void)
 extern void si_swapinfo(struct sysinfo *);
 extern int pin_hibernation_swap_type(dev_t device, sector_t offset);
 extern void unpin_hibernation_swap_type(int type);
+extern int repin_hibernation_swap_type(int old_type, dev_t device,
+                                       sector_t offset);
 extern int find_hibernation_swap_type(dev_t device, sector_t offset);
 int find_first_swap(dev_t *device);
 extern unsigned int count_swap_pages(int, int);
diff --git a/kernel/power/user.c b/kernel/power/user.c
index d0fcfba7ac23..6e4f40e49319 100644
--- a/kernel/power/user.c
+++ b/kernel/power/user.c
@@ -218,6 +218,7 @@ static int snapshot_set_swap_area(struct snapshot_data *data,
 {
 	sector_t offset;
 	dev_t swdev;
+	int new_type;
 
 	if (swsusp_swap_in_use())
 		return -EPERM;
@@ -238,19 +239,11 @@ static int snapshot_set_swap_area(struct snapshot_data *data,
 		offset = swap_area.offset;
 	}
 
-	/*
-	 * Unpin the swap device if a swap area was already
-	 * set by SNAPSHOT_SET_SWAP_AREA.
-	 */
-	unpin_hibernation_swap_type(data->swap);
+	new_type = repin_hibernation_swap_type(data->swap, swdev, offset);
+	if (new_type < 0)
+		return new_type;
 
-	/*
-	 * User space encodes device types as two-byte values,
-	 * so we need to recode them
-	 */
-	data->swap = pin_hibernation_swap_type(swdev, offset);
-	if (data->swap < 0)
-		return swdev ? -ENODEV : -EINVAL;
+	data->swap = new_type;
 	data->dev = swdev;
 	return 0;
 }
diff --git a/mm/swapfile.c b/mm/swapfile.c
index c7e173b93e11..4840fd40f36f 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2219,6 +2219,67 @@ int pin_hibernation_swap_type(dev_t device, sector_t offset)
 	return type;
 }
 
+/**
+ * repin_hibernation_swap_type - Atomically replace the hibernation pin
+ * @old_type: Swap type currently pinned (or < 0 if none).
+ * @device:   Block device of the new resume image.
+ * @offset:   Offset identifying the new swap area.
+ *
+ * Look up the swap device for @device/@offset and atomically transfer
+ * the SWP_HIBERNATION pin from @old_type (if valid) to the new device,
+ * all under a single swap_lock critical section. This closes the
+ * swapoff() window that exists when callers unpin and re-pin in two
+ * separate operations.
+ *
+ * If the new device cannot be located, the existing pin on @old_type
+ * is preserved and an error is returned. If @old_type already refers
+ * to the same swap_info_struct as the new lookup, no flag changes are
+ * made and @old_type is returned.
+ *
+ * Return:
+ * >= 0 on success (new swap type).
+ * -EINVAL if @device is invalid.
+ * -ENODEV if the swap device is not found.
+ * -EBUSY  if the new device is already pinned by another context.
+ */
+int repin_hibernation_swap_type(int old_type, dev_t device, sector_t offset)
+{
+	struct swap_info_struct *old_si, *new_si;
+	int new_type;
+
+	spin_lock(&swap_lock);
+
+	new_type = __find_hibernation_swap_type(device, offset);
+	if (new_type < 0) {
+		spin_unlock(&swap_lock);
+		return new_type;
+	}
+
+	new_si = swap_type_to_info(new_type);
+	if (WARN_ON_ONCE(!new_si)) {
+		spin_unlock(&swap_lock);
+		return -ENODEV;
+	}
+
+	old_si = swap_type_to_info(old_type);
+	if (new_si == old_si) {
+		spin_unlock(&swap_lock);
+		return new_type;
+	}
+
+	if (WARN_ON_ONCE(new_si->flags & SWP_HIBERNATION)) {
+		spin_unlock(&swap_lock);
+		return -EBUSY;
+	}
+
+	if (old_si)
+		old_si->flags &= ~SWP_HIBERNATION;
+	new_si->flags |= SWP_HIBERNATION;
+
+	spin_unlock(&swap_lock);
+	return new_type;
+}
+
 /**
  * unpin_hibernation_swap_type - Unpin the swap device for hibernation
  * @type: Swap type previously returned by pin_hibernation_swap_type()
-- 
2.53.0