mm/zswap: invalidate old entry when store fail or !zswap_enabled

From: Chengming Zhou <[email protected]>

We may encounter duplicate entry in the zswap_store():

1. swap slot that freed to per-cpu swap cache, doesn't invalidate the zswap entry, then got reused. This has been fixed.

2. !exclusive load mode, swapin folio will leave its zswap entry on the tree, then swapout again. This has been removed.

3. one folio can be dirtied again after zswap_store(), so need to zswap_store() again. This should be handled correctly.

So we must invalidate the old duplicate entry before insert the new one, which actually doesn't have to be done at the beginning of zswap_store(). And this is a normal situation, we shouldn't WARN_ON(1) in this case, so delete it. (The WARN_ON(1) seems want to detect swap entry UAF problem? But not very necessary here.)

The good point is that we don't need to lock tree twice in the store success path.

Note we still need to invalidate the old duplicate entry in the store failure path, otherwise the new data in swapfile could be overwrite by the old data in zswap pool when lru writeback.

We have to do this even when !zswap_enabled since zswap can be disabled anytime. If the folio store success before, then got dirtied again but zswap disabled, we won't invalidate the old duplicate entry in the zswap_store(). So later lru writeback may overwrite the new data in swapfile.

This fix is not good, since we have to grab lock to check everytime even when zswap is disabled, but it's simple.

Signed-off-by Chengming Zhou <[email protected]>

---
 mm/zswap.c | 33 +++++++++++++++------------------
 1 file changed, 15 insertions(+), 18 deletions(-)

mm/zswap.c +1 -13

index cd67f7f6b302..0b7599f4116d 100644
@@ -1518,18 +1518,8 @@ bool zswap_store(struct folio *folio)
 		return false;
 
 	if (!zswap_enabled)
-		return false;
+		goto check_old;
 
-	/*
-	 * If this is a duplicate, it must be removed before attempting to store
-	 * it, otherwise, if the store fails the old page won't be removed from
-	 * the tree, and it might be written back overriding the new data.
-	 */
-	spin_lock(&tree->lock);
-	entry = zswap_rb_search(&tree->rbroot, offset);
-	if (entry)
-		zswap_invalidate_entry(tree, entry);
-	spin_unlock(&tree->lock);
 	objcg = get_obj_cgroup_from_folio(folio);
 	if (objcg && !obj_cgroup_may_zswap(objcg)) {
 		memcg = get_mem_cgroup_from_objcg(objcg);

@@ -1608,15 +1598,11 @@ bool zswap_store(struct folio *folio)
 	/* map */
 	spin_lock(&tree->lock);
 	/*
-	 * A duplicate entry should have been removed at the beginning of this
-	 * function. Since the swap entry should be pinned, if a duplicate is

mm/zswap.c +11 -0

 	if (entry->length) {
 		INIT_LIST_HEAD(&entry->lru);
 		zswap_lru_add(&entry->pool->list_lru, entry);

@@ -1638,6 +1624,17 @@ bool zswap_store(struct folio *folio)
 reject:
 	if (objcg)
 		obj_cgroup_put(objcg);
+check_old:
+	/*
+	 * If zswap store fail or zswap disabled, we must invalidate possible
+	 * old entry which previously stored by this folio. Otherwise, later
+	 * writeback could overwrite the new data in swapfile.
+	 */
+	spin_lock(&tree->lock);
+	entry = zswap_rb_search(&tree->rbroot, offset);
+	if (entry)
+		zswap_invalidate_entry(tree, entry);
+	spin_unlock(&tree->lock);
 	return false;
 
 shrink:

Signature · 1 line

2.40.1

Quoted 19 lines · On Sun, Feb 04, 2024 at 08:34:11AM +0000, [email protected] wrote:

From: Chengming Zhou <[email protected]>

We may encounter duplicate entry in the zswap_store():

1. swap slot that freed to per-cpu swap cache, doesn't invalidate the zswap entry, then got reused. This has been fixed.

2. !exclusive load mode, swapin folio will leave its zswap entry on the tree, then swapout again. This has been removed.

3. one folio can be dirtied again after zswap_store(), so need to zswap_store() again. This should be handled correctly.

So we must invalidate the old duplicate entry before insert the new one, which actually doesn't have to be done at the beginning of zswap_store(). And this is a normal situation, we shouldn't WARN_ON(1) in this case, so delete it. (The WARN_ON(1) seems want to detect swap entry UAF problem? But not very necessary here.)

Quoted 6 lines

The good point is that we don't need to lock tree twice in the store success path.

Note we still need to invalidate the old duplicate entry in the store failure path, otherwise the new data in swapfile could be overwrite by the old data in zswap pool when lru writeback.

I think this may have been introduced by 42c06a0e8ebe ("mm: kill frontswap"). Frontswap used to check if the page was present in frontswap and invalidate it before calling into zswap, so it would invalidate a previously stored page when it is dirtied and swapped out again, even if zswap is disabled.

Johannes, does this sound correct to you? If yes, I think we need a proper Fixes tag and a stable backport as this may cause data corruption.

Quoted 8 lines

We have to do this even when !zswap_enabled since zswap can be disabled anytime. If the folio store success before, then got dirtied again but zswap disabled, we won't invalidate the old duplicate entry in the zswap_store(). So later lru writeback may overwrite the new data in swapfile.

Quoted 2 lines

This fix is not good, since we have to grab lock to check everytime even when zswap is disabled, but it's simple.

Frontswap had a bitmap that we can query locklessly to find out if there is an outdated stored page. I think we can overcome this with the xarray, we can do a lockless lookup first, and only take the lock if there is an outdated entry to remove.

Meanwhile I am not sure if acquiring the lock on every swapout even with zswap disabled is acceptable, but I think it's the simplest fix for now, unless we revive the bitmap.

Quoted 43 lines

Signed-off-by Chengming Zhou <[email protected]>

---
 mm/zswap.c | 33 +++++++++++++++------------------
 1 file changed, 15 insertions(+), 18 deletions(-)

mm/zswap.c +3 -16

index cd67f7f6b302..0b7599f4116d 100644
@@ -1518,18 +1518,8 @@ bool zswap_store(struct folio *folio)
 		return false;
 
 	if (!zswap_enabled)
-		return false;
+		goto check_old;
 
-	/*
-	 * If this is a duplicate, it must be removed before attempting to store
-	 * it, otherwise, if the store fails the old page won't be removed from
-	 * the tree, and it might be written back overriding the new data.
-	 */
-	spin_lock(&tree->lock);
-	entry = zswap_rb_search(&tree->rbroot, offset);
-	if (entry)
-		zswap_invalidate_entry(tree, entry);
-	spin_unlock(&tree->lock);
 	objcg = get_obj_cgroup_from_folio(folio);
 	if (objcg && !obj_cgroup_may_zswap(objcg)) {
 		memcg = get_mem_cgroup_from_objcg(objcg);

@@ -1608,15 +1598,11 @@ bool zswap_store(struct folio *folio)
 	/* map */
 	spin_lock(&tree->lock);
 	/*
-	 * A duplicate entry should have been removed at the beginning of this
-	 * function. Since the swap entry should be pinned, if a duplicate is
-	 * found again here it means that something went wrong in the swap
-	 * cache.
+	 * The folio could be dirtied again, invalidate the possible old entry
+	 * before insert this new entry.
 	 */
-	while (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {

mm/zswap.c bool zswap_store(struct folio *folio) · line 1619

Quoted 4 lines

mm/zswap.c +1 -2

-		WARN_ON(1);
+	while (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST)
 		zswap_invalidate_entry(tree, dupentry);
-	}

I always thought the loop here was confusing. We are holding the lock, so it should be guaranteed that if we get -EEXIST once and invalidate it, we won't find it the next time around.

This should really be a cmpxchg operation, which is simple with the xarray. We can probably do the same with the rbtree, but perhaps it's not worth it if the xarray change is coming soon.

For now, I think an if condition is clearer:

if (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
	zswap_invalidate_entry(tree, dupentry);
	/* Must succeed, we just removed the dup under the lock */
	WARN_ON(zswap_rb_insert(&tree->rbroot, entry, &dupentry));
}

Quoted 25 lines

mm/zswap.c +11 -0

 	if (entry->length) {
 		INIT_LIST_HEAD(&entry->lru);
 		zswap_lru_add(&entry->pool->list_lru, entry);

@@ -1638,6 +1624,17 @@ bool zswap_store(struct folio *folio)
 reject:
 	if (objcg)
 		obj_cgroup_put(objcg);
+check_old:
+	/*
+	 * If zswap store fail or zswap disabled, we must invalidate possible
+	 * old entry which previously stored by this folio. Otherwise, later
+	 * writeback could overwrite the new data in swapfile.
+	 */
+	spin_lock(&tree->lock);
+	entry = zswap_rb_search(&tree->rbroot, offset);
+	if (entry)
+		zswap_invalidate_entry(tree, entry);
+	spin_unlock(&tree->lock);
 	return false;
 
 shrink:

Signature · 1 line

2.40.1

Quoted 33 lines · Yosry Ahmed

Quoted 19 lines · On Sun, Feb 04, 2024 at 08:34:11AM +0000, [email protected] wrote:

From: Chengming Zhou <[email protected]>

We may encounter duplicate entry in the zswap_store():

1. swap slot that freed to per-cpu swap cache, doesn't invalidate the zswap entry, then got reused. This has been fixed.

2. !exclusive load mode, swapin folio will leave its zswap entry on the tree, then swapout again. This has been removed.

3. one folio can be dirtied again after zswap_store(), so need to zswap_store() again. This should be handled correctly.

So we must invalidate the old duplicate entry before insert the new one, which actually doesn't have to be done at the beginning of zswap_store(). And this is a normal situation, we shouldn't WARN_ON(1) in this case, so delete it. (The WARN_ON(1) seems want to detect swap entry UAF problem? But not very necessary here.)

Quoted 6 lines

The good point is that we don't need to lock tree twice in the store success path.

Note we still need to invalidate the old duplicate entry in the store failure path, otherwise the new data in swapfile could be overwrite by the old data in zswap pool when lru writeback.

I think this may have been introduced by 42c06a0e8ebe ("mm: kill frontswap"). Frontswap used to check if the page was present in frontswap and invalidate it before calling into zswap, so it would invalidate a previously stored page when it is dirtied and swapped out again, even if zswap is disabled.

Quoted 3 lines

Johannes, does this sound correct to you? If yes, I think we need a proper Fixes tag and a stable backport as this may cause data corruption.

I haven't looked into that commit. If this is true, will add:

Fixes 42c06a0e8ebe ("mm: kill frontswap")

Quoted 6 lines

Quoted 6 lines

We have to do this even when !zswap_enabled since zswap can be disabled anytime. If the folio store success before, then got dirtied again but zswap disabled, we won't invalidate the old duplicate entry in the zswap_store(). So later lru writeback may overwrite the new data in swapfile.

Quoted 7 lines

Quoted 2 lines

This fix is not good, since we have to grab lock to check everytime even when zswap is disabled, but it's simple.

Frontswap had a bitmap that we can query locklessly to find out if there is an outdated stored page. I think we can overcome this with the xarray, we can do a lockless lookup first, and only take the lock if there is an outdated entry to remove.

Yes, agree! We can lockless lookup once xarray lands in.

Quoted 3 lines

Meanwhile I am not sure if acquiring the lock on every swapout even with zswap disabled is acceptable, but I think it's the simplest fix for now, unless we revive the bitmap.

Yeah, it's simple. I think bitmap is not needed if we will use xarray.

Quoted 9 lines

Quoted 1 line

Signed-off-by Chengming Zhou <[email protected]>

For now, I think an if condition is clearer:

if (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
	zswap_invalidate_entry(tree, dupentry);
	/* Must succeed, we just removed the dup under the lock */
	WARN_ON(zswap_rb_insert(&tree->rbroot, entry, &dupentry));
}

This is clearer, will change to this version.