[PATCH v3 07/13] mm, swap: hold a reference during scan and cleanup flag usage

Kairui Song
2024-12-30T17:47:01Z

From: Kairui Song <kasong@tencent.com>

The flag SWP_SCANNING was used as an indicator of whether a device
is being scanned for allocation, and prevents swapoff. Combined with
SWP_WRITEOK, they work as a set of barriers for a clean swapoff:

1. Swapoff clears SWP_WRITEOK, allocation requests will see
~SWP_WRITEOK and abort as it's serialized by si->lock.
2. Swapoff unuses all allocated entries.
3. Swapoff waits for SWP_SCANNING flag to be cleared, so ongoing
allocations will stop, preventing UAF.
4. Now swapoff can free everything safely.

This will make the allocation path have a hard dependency on
si->lock. Allocation always have to acquire si->lock first for
setting SWP_SCANNING and checking SWP_WRITEOK.

This commit removes this flag, and just uses the existing per-CPU
refcount instead to prevent UAF in step 3, which serves well for
such usage without dependency on si->lock, and scales very well too.
Just hold a reference during the whole scan and allocation process.
Swapoff will kill and wait for the counter.

And for preventing any allocation from happening after step 1 so the
unuse in step 2 can ensure all slots are free, swapoff will acquire
the ci->lock of each cluster one by one to ensure all allocations
see ~SWP_WRITEOK and abort.

This way these dependences on si->lock are gone. And worth noting we
can't kill the refcount as the first step for swapoff as the unuse
process have to acquire the refcount.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
include/linux/swap.h | 1 -
mm/swapfile.c | 90 ++++++++++++++++++++++++++++----------------
2 files changed, 57 insertions(+), 34 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index e1eeea6307cd..02120f1005d5 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -219,7 +219,6 @@ enum {
SWP_STABLE_WRITES = (1 << 11), /* no overwrite PG_writeback pages */
SWP_SYNCHRONOUS_IO = (1 << 12), /* synchronous IO is efficient */
/* add others here before... */
- SWP_SCANNING = (1 << 14), /* refcount in scan_swap_map */
};

#define SWAP_CLUSTER_MAX 32UL
diff --git a/mm/swapfile.c b/mm/swapfile.c
index e6e58cfb5178..99fd0b0d84a2 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -658,6 +658,8 @@ static bool cluster_alloc_range(struct swap_info_struct *si, struct swap_cluster
{
unsigned int nr_pages = 1 << order;

+ lockdep_assert_held(&ci->lock);
+
if (!(si->flags & SWP_WRITEOK))
return false;

@@ -1059,8 +1061,6 @@ static int cluster_alloc_swap(struct swap_info_struct *si,
{
int n_ret = 0;

- si->flags += SWP_SCANNING;
-
while (n_ret < nr) {
unsigned long offset = cluster_alloc_swap_entry(si, order, usage);

@@ -1069,8 +1069,6 @@ static int cluster_alloc_swap(struct swap_info_struct *si,
slots[n_ret++] = swp_entry(si->type, offset);
}

- si->flags -= SWP_SCANNING;
-
return n_ret;
}

@@ -1112,6 +1110,22 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
return cluster_alloc_swap(si, usage, nr, slots, order);
}

+static bool get_swap_device_info(struct swap_info_struct *si)
+{
+ if (!percpu_ref_tryget_live(&si->users))
+ return false;
+ /*
+ * Guarantee the si->users are checked before accessing other
+ * fields of swap_info_struct, and si->flags (SWP_WRITEOK) is
+ * up to dated.
+ *
+ * Paired with the spin_unlock() after setup_swap_info() in
+ * enable_swap_info(), and smp_wmb() in swapoff.
+ */
+ smp_rmb();
+ return true;
+}
+
int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_order)
{
int order = swap_entry_order(entry_order);
@@ -1139,13 +1153,16 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_order)
/* requeue si to after same-priority siblings */
plist_requeue(&si->avail_lists[node], &swap_avail_heads[node]);
spin_unlock(&swap_avail_lock);
- spin_lock(&si->lock);
- n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
- n_goal, swp_entries, order);
- spin_unlock(&si->lock);
- if (n_ret || size > 1)
- goto check_out;
- cond_resched();
+ if (get_swap_device_info(si)) {
+ spin_lock(&si->lock);
+ n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
+ n_goal, swp_entries, order);
+ spin_unlock(&si->lock);
+ put_swap_device(si);
+ if (n_ret || size > 1)
+ goto check_out;
+ cond_resched();
+ }

spin_lock(&swap_avail_lock);
/*
@@ -1296,16 +1313,8 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
si = swp_swap_info(entry);
if (!si)
goto bad_nofile;
- if (!percpu_ref_tryget_live(&si->users))
+ if (!get_swap_device_info(si))
goto out;
- /*
- * Guarantee the si->users are checked before accessing other
- * fields of swap_info_struct.
- *
- * Paired with the spin_unlock() after setup_swap_info() in
- * enable_swap_info().
- */
- smp_rmb();
offset = swp_offset(entry);
if (offset >= si->max)
goto put_out;
@@ -1785,10 +1794,13 @@ swp_entry_t get_swap_page_of_type(int type)
goto fail;

/* This is called for allocating swap entry, not cache */
- spin_lock(&si->lock);
- if ((si->flags & SWP_WRITEOK) && scan_swap_map_slots(si, 1, 1, &entry, 0))
- atomic_long_dec(&nr_swap_pages);
- spin_unlock(&si->lock);
+ if (get_swap_device_info(si)) {
+ spin_lock(&si->lock);
+ if ((si->flags & SWP_WRITEOK) && scan_swap_map_slots(si, 1, 1, &entry, 0))
+ atomic_long_dec(&nr_swap_pages);
+ spin_unlock(&si->lock);
+ put_swap_device(si);
+ }
fail:
return entry;
}
@@ -2562,6 +2574,25 @@ bool has_usable_swap(void)
return ret;
}

+/*
+ * Called after clearing SWP_WRITEOK, ensures cluster_alloc_range
+ * see the updated flags, so there will be no more allocations.
+ */
+static void wait_for_allocation(struct swap_info_struct *si)
+{
+ unsigned long offset;
+ unsigned long end = ALIGN(si->max, SWAPFILE_CLUSTER);
+ struct swap_cluster_info *ci;
+
+ BUG_ON(si->flags & SWP_WRITEOK);
+
+ for (offset = 0; offset < end; offset += SWAPFILE_CLUSTER) {
+ ci = lock_cluster(si, offset);
+ unlock_cluster(ci);
+ offset += SWAPFILE_CLUSTER;
+ }
+}
+
SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
{
struct swap_info_struct *p = NULL;
@@ -2632,6 +2663,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
spin_unlock(&p->lock);
spin_unlock(&swap_lock);

+ wait_for_allocation(p);
+
disable_swap_slots_cache_lock();

set_current_oom_origin();
@@ -2674,15 +2707,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
spin_lock(&p->lock);
drain_mmlist();

- /* wait for anyone still in scan_swap_map_slots */
- while (p->flags >= SWP_SCANNING) {
- spin_unlock(&p->lock);
- spin_unlock(&swap_lock);
- schedule_timeout_uninterruptible(1);
- spin_lock(&swap_lock);
- spin_lock(&p->lock);
- }
-
swap_file = p->swap_file;
p->swap_file = NULL;
p->max = 0;
--
2.47.1