[PATCH v8 1/2] mm/swap, PM: hibernate: fix swapoff race in uswsusp by pinning swap device

Youngjun Park
2026-03-23T16:08:33Z

Hibernation via uswsusp (/dev/snapshot ioctls) has a race window:
after selecting the resume swap area but before user space is frozen,
swapoff may run and invalidate the selected swap device.

Fix this by pinning the swap device with SWP_HIBERNATION while it is
in use. The pin is exclusive, which is sufficient since
hibernate_acquire() already prevents concurrent hibernation sessions.

The kernel swsusp path (sysfs-based hibernate/resume) uses
find_hibernation_swap_type() which is not affected by the pin. It
freezes user space before touching swap, so swapoff cannot race.

Introduce dedicated helpers:
- pin_hibernation_swap_type(): Look up and pin the swap device.
Used by the uswsusp path.
- find_hibernation_swap_type(): Lookup without pinning.
Used by the kernel swsusp path.
- unpin_hibernation_swap_type(): Clear the hibernation pin.

While a swap device is pinned, swapoff is prevented from proceeding.

Signed-off-by: Youngjun Park <youngjun.park@lge.com>
---
include/linux/swap.h | 5 +-
kernel/power/swap.c | 2 +-
kernel/power/user.c | 15 ++++-
mm/swapfile.c | 135 ++++++++++++++++++++++++++++++++++++++-----
4 files changed, 136 insertions(+), 21 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 7a09df6977a5..1930f81e6be4 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -213,6 +213,7 @@ enum {
SWP_PAGE_DISCARD = (1 << 10), /* freed swap page-cluster discards */
SWP_STABLE_WRITES = (1 << 11), /* no overwrite PG_writeback pages */
SWP_SYNCHRONOUS_IO = (1 << 12), /* synchronous IO is efficient */
+ SWP_HIBERNATION = (1 << 13), /* pinned for hibernation */
/* add others here before... */
};

@@ -433,7 +434,9 @@ static inline long get_nr_swap_pages(void)
}

extern void si_swapinfo(struct sysinfo *);
-int swap_type_of(dev_t device, sector_t offset);
+extern int pin_hibernation_swap_type(dev_t device, sector_t offset);
+extern void unpin_hibernation_swap_type(int type);
+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);
extern sector_t swapdev_block(int, pgoff_t);
diff --git a/kernel/power/swap.c b/kernel/power/swap.c
index 2e64869bb5a0..cc4764149e8f 100644
--- a/kernel/power/swap.c
+++ b/kernel/power/swap.c
@@ -341,7 +341,7 @@ static int swsusp_swap_check(void)
* This is called before saving the image.
*/
if (swsusp_resume_device)
- res = swap_type_of(swsusp_resume_device, swsusp_resume_block);
+ res = find_hibernation_swap_type(swsusp_resume_device, swsusp_resume_block);
else
res = find_first_swap(&swsusp_resume_device);
if (res < 0)
diff --git a/kernel/power/user.c b/kernel/power/user.c
index 4401cfe26e5c..4406f5644a56 100644
--- a/kernel/power/user.c
+++ b/kernel/power/user.c
@@ -71,7 +71,7 @@ static int snapshot_open(struct inode *inode, struct file *filp)
memset(&data->handle, 0, sizeof(struct snapshot_handle));
if ((filp->f_flags & O_ACCMODE) == O_RDONLY) {
/* Hibernating. The image device should be accessible. */
- data->swap = swap_type_of(swsusp_resume_device, 0);
+ data->swap = pin_hibernation_swap_type(swsusp_resume_device, 0);
data->mode = O_RDONLY;
data->free_bitmaps = false;
error = pm_notifier_call_chain_robust(PM_HIBERNATION_PREPARE, PM_POST_HIBERNATION);
@@ -90,8 +90,10 @@ static int snapshot_open(struct inode *inode, struct file *filp)
data->free_bitmaps = !error;
}
}
- if (error)
+ if (error) {
+ unpin_hibernation_swap_type(data->swap);
hibernate_release();
+ }

data->frozen = false;
data->ready = false;
@@ -115,6 +117,7 @@ static int snapshot_release(struct inode *inode, struct file *filp)
data = filp->private_data;
data->dev = 0;
free_all_swap_pages(data->swap);
+ unpin_hibernation_swap_type(data->swap);
if (data->frozen) {
pm_restore_gfp_mask();
free_basic_memory_bitmaps();
@@ -235,11 +238,17 @@ 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);
+
/*
* User space encodes device types as two-byte values,
* so we need to recode them
*/
- data->swap = swap_type_of(swdev, offset);
+ data->swap = pin_hibernation_swap_type(swdev, offset);
if (data->swap < 0)
return swdev ? -ENODEV : -EINVAL;
data->dev = swdev;
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 802332850e24..c5b459a18f43 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -133,7 +133,7 @@ static DEFINE_PER_CPU(struct percpu_swap_cluster, percpu_swap_cluster) = {
/* May return NULL on invalid type, caller must check for NULL return */
static struct swap_info_struct *swap_type_to_info(int type)
{
- if (type >= MAX_SWAPFILES)
+ if (type < 0 || type >= MAX_SWAPFILES)
return NULL;
return READ_ONCE(swap_info[type]); /* rcu_dereference() */
}
@@ -2138,22 +2138,15 @@ void swap_free_hibernation_slot(swp_entry_t entry)
put_swap_device(si);
}

-/*
- * Find the swap type that corresponds to given device (if any).
- *
- * @offset - number of the PAGE_SIZE-sized block of the device, starting
- * from 0, in which the swap header is expected to be located.
- *
- * This is needed for the suspend to disk (aka swsusp).
- */
-int swap_type_of(dev_t device, sector_t offset)
+static int __find_hibernation_swap_type(dev_t device, sector_t offset)
{
int type;

+ lockdep_assert_held(&swap_lock);
+
if (!device)
- return -1;
+ return -EINVAL;

- spin_lock(&swap_lock);
for (type = 0; type < nr_swapfiles; type++) {
struct swap_info_struct *sis = swap_info[type];

@@ -2163,16 +2156,118 @@ int swap_type_of(dev_t device, sector_t offset)
if (device == sis->bdev->bd_dev) {
struct swap_extent *se = first_se(sis);

- if (se->start_block == offset) {
- spin_unlock(&swap_lock);
+ if (se->start_block == offset)
return type;
- }
}
}
- spin_unlock(&swap_lock);
return -ENODEV;
}

+/**
+ * pin_hibernation_swap_type - Pin the swap device for hibernation
+ * @device: Block device containing the resume image
+ * @offset: Offset identifying the swap area
+ *
+ * Locate the swap device for @device/@offset and mark it as pinned
+ * for hibernation. While pinned, swapoff() is prevented.
+ *
+ * Only one uswsusp context may pin a swap device at a time.
+ * If already pinned, this function returns -EBUSY.
+ *
+ * Return:
+ * >= 0 on success (swap type).
+ * -EINVAL if @device is invalid.
+ * -ENODEV if the swap device is not found.
+ * -EBUSY if the device is already pinned for hibernation.
+ */
+int pin_hibernation_swap_type(dev_t device, sector_t offset)
+{
+ int type;
+ struct swap_info_struct *si;
+
+ spin_lock(&swap_lock);
+
+ type = __find_hibernation_swap_type(device, offset);
+ if (type < 0) {
+ spin_unlock(&swap_lock);
+ return type;
+ }
+
+ si = swap_type_to_info(type);
+ if (WARN_ON_ONCE(!si)) {
+ spin_unlock(&swap_lock);
+ return -ENODEV;
+ }
+
+ /*
+ * hibernate_acquire() prevents concurrent hibernation sessions.
+ * This check additionally guards against double-pinning within
+ * the same session.
+ */
+ if (WARN_ON_ONCE(si->flags & SWP_HIBERNATION)) {
+ spin_unlock(&swap_lock);
+ return -EBUSY;
+ }
+
+ si->flags |= SWP_HIBERNATION;
+
+ spin_unlock(&swap_lock);
+ return type;
+}
+
+/**
+ * unpin_hibernation_swap_type - Unpin the swap device for hibernation
+ * @type: Swap type previously returned by pin_hibernation_swap_type()
+ *
+ * Clear the hibernation pin on the given swap device, allowing
+ * swapoff() to proceed normally.
+ *
+ * If @type does not refer to a valid swap device, this function
+ * does nothing.
+ */
+void unpin_hibernation_swap_type(int type)
+{
+ struct swap_info_struct *si;
+
+ spin_lock(&swap_lock);
+ si = swap_type_to_info(type);
+ if (!si) {
+ spin_unlock(&swap_lock);
+ return;
+ }
+ si->flags &= ~SWP_HIBERNATION;
+ spin_unlock(&swap_lock);
+}
+
+/**
+ * find_hibernation_swap_type - Find swap type for hibernation
+ * @device: Block device containing the resume image
+ * @offset: Offset within the device identifying the swap area
+ *
+ * Locate the swap device corresponding to @device and @offset.
+ *
+ * Unlike pin_hibernation_swap_type(), this function only performs a
+ * lookup and does not mark the swap device as pinned for hibernation.
+ *
+ * This is safe in the sysfs-based hibernation path where user space
+ * is already frozen and swapoff() cannot run concurrently.
+ *
+ * Return:
+ * A non-negative swap type on success.
+ * -EINVAL if @device is invalid.
+ * -ENODEV if no matching swap device is found.
+ */
+int find_hibernation_swap_type(dev_t device, sector_t offset)
+{
+ int type;
+
+ spin_lock(&swap_lock);
+ type = __find_hibernation_swap_type(device, offset);
+ spin_unlock(&swap_lock);
+
+ return type;
+}
+
int find_first_swap(dev_t *device)
{
int type;
@@ -2936,6 +3031,14 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
spin_unlock(&swap_lock);
goto out_dput;
}
+
+ /* Refuse swapoff while the device is pinned for hibernation */
+ if (p->flags & SWP_HIBERNATION) {
+ err = -EBUSY;
+ spin_unlock(&swap_lock);
+ goto out_dput;
+ }
+
if (!security_vm_enough_memory_mm(current->mm, p->pages))
vm_unacct_memory(p->pages);
else {
--
2.34.1