Re: [PATCH v2 2/3] mm/zswap: fix race between lru writeback and swapoff

[Chengming Zhou]

On 2024/1/30 08:22, Yosry Ahmed wrote:
> On Sun, Jan 28, 2024 at 01:28:50PM +0000, Chengming Zhou wrote:
>> LRU writeback has race problem with swapoff, as spotted by Yosry [1]:
>>
>> CPU1			CPU2
>> shrink_memcg_cb		swap_off
>>   list_lru_isolate	  zswap_invalidate
>> 			  zswap_swapoff
>> 			    kfree(tree)
>>   // UAF
>>   spin_lock(&tree->lock)
>>
>> The problem is that the entry in lru list can't protect the tree from
>> being swapoff and freed, and the entry also can be invalidated and freed
>> concurrently after we unlock the lru lock.
>>
>> We can fix it by moving the swap cache allocation ahead before
>> referencing the tree, then check invalidate race with tree lock,
>> only after that we can safely deref the entry. Note we couldn't
>> deref entry or tree anymore after we unlock the folio, since we
>> depend on this to hold on swapoff.
>>
>> So this patch moves all tree and entry usage to zswap_writeback_entry(),
>> we only use the copied swpentry on the stack to allocate swap cache
>> and if returned with folio locked we can reference the tree safely.
>> Then we can check invalidate race with tree lock, the following things
>> is much the same like zswap_load().
>>
>> Since we can't deref the entry after zswap_writeback_entry(), we
>> can't use zswap_lru_putback() anymore, instead we rotate the entry
>> in the beginning. And it will be unlinked and freed when invalidated
>> if writeback success.
> 
> You are also removing one redundant lookup from the zswap writeback path
> to check for the invalidation race, and simplifying the code. Give
> yourself full credit :)

Ah right, forgot to mention it, I will add this part in the commit message.
Thanks for your reminder!

> 
>>
>> Another change is we don't update the memcg nr_zswap_protected in
>> the -ENOMEM and -EEXIST cases anymore. -EEXIST case means we raced
>> with swapin or concurrent shrinker action, since swapin already
>> have memcg nr_zswap_protected updated, don't need double counts here.
>> For concurrent shrinker, the folio will be writeback and freed anyway.
>> -ENOMEM case is extremely rare and doesn't happen spuriously either,
>> so don't bother distinguishing this case.
>>
>> [1] https://lore.kernel.org/all/CAJD7tkasHsRnT_75-TXsEe58V9_OW6m3g6CF7Kmsvz8CKRG_EA@xxxxxxxxxxxxxx/
>>
>> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
>> Acked-by: Nhat Pham <nphamcs@xxxxxxxxx>
>> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>> ---
>>  mm/zswap.c | 114 ++++++++++++++++++++++++++-----------------------------------
>>  1 file changed, 49 insertions(+), 65 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 81cb3790e0dd..f5cb5a46e4d7 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -277,7 +277,7 @@ static inline struct zswap_tree *swap_zswap_tree(swp_entry_t swp)
>>  		 zpool_get_type((p)->zpools[0]))
>>  
>>  static int zswap_writeback_entry(struct zswap_entry *entry,
>> -				 struct zswap_tree *tree);
>> +				 swp_entry_t swpentry);
>>  static int zswap_pool_get(struct zswap_pool *pool);
>>  static void zswap_pool_put(struct zswap_pool *pool);
>>  
>> @@ -445,27 +445,6 @@ static void zswap_lru_del(struct list_lru *list_lru, struct zswap_entry *entry)
>>  	rcu_read_unlock();
>>  }
>>  
>> -static void zswap_lru_putback(struct list_lru *list_lru,
>> -		struct zswap_entry *entry)
>> -{
>> -	int nid = entry_to_nid(entry);
>> -	spinlock_t *lock = &list_lru->node[nid].lock;
>> -	struct mem_cgroup *memcg;
>> -	struct lruvec *lruvec;
>> -
>> -	rcu_read_lock();
>> -	memcg = mem_cgroup_from_entry(entry);
>> -	spin_lock(lock);
>> -	/* we cannot use list_lru_add here, because it increments node's lru count */
>> -	list_lru_putback(list_lru, &entry->lru, nid, memcg);
>> -	spin_unlock(lock);
>> -
>> -	lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(entry_to_nid(entry)));
>> -	/* increment the protection area to account for the LRU rotation. */
>> -	atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected);
>> -	rcu_read_unlock();
>> -}
>> -
>>  /*********************************
>>  * rbtree functions
>>  **********************************/
>> @@ -860,40 +839,47 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
>>  {
>>  	struct zswap_entry *entry = container_of(item, struct zswap_entry, lru);
>>  	bool *encountered_page_in_swapcache = (bool *)arg;
>> -	struct zswap_tree *tree;
>> -	pgoff_t swpoffset;
>> +	swp_entry_t swpentry;
>>  	enum lru_status ret = LRU_REMOVED_RETRY;
>>  	int writeback_result;
>>  
>> +	/*
>> +	 * Rotate the entry to the tail before unlocking the LRU,
>> +	 * so that in case of an invalidation race concurrent
>> +	 * reclaimers don't waste their time on it.
>> +	 *
>> +	 * If writeback succeeds, or failure is due to the entry
>> +	 * being invalidated by the swap subsystem, the invalidation
>> +	 * will unlink and free it.
>> +	 *
>> +	 * Temporary failures, where the same entry should be tried
>> +	 * again immediately, almost never happen for this shrinker.
>> +	 * We don't do any trylocking; -ENOMEM comes closest,
>> +	 * but that's extremely rare and doesn't happen spuriously
>> +	 * either. Don't bother distinguishing this case.
>> +	 *
>> +	 * But since they do exist in theory, the entry cannot just
>> +	 * be unlinked, or we could leak it. Hence, rotate.
> 
> The entry cannot be unlinked because we cannot get a ref on it without
> holding the tree lock, and we cannot deref the tree before we acquire a
> swap cache ref in zswap_writeback_entry() -- or if
> zswap_writeback_entry() fails. This should be called out explicitly
> somewhere. Perhaps we can describe this whole deref dance with added
> docs to shrink_memcg_cb().

Maybe we should add some comments before or after zswap_writeback_entry()?
Or do you have some suggestions? I'm not good at this. :)

> 
> We could also use a comment in zswap_writeback_entry() (or above it) to
> state that we only deref the tree *after* we get the swapcache ref.

I just notice there are some comments in zswap_writeback_entry(), should
we add more comments here?

	/*
	 * folio is locked, and the swapcache is now secured against
	 * concurrent swapping to and from the slot. Verify that the
	 * swap entry hasn't been invalidated and recycled behind our
	 * backs (our zswap_entry reference doesn't prevent that), to
	 * avoid overwriting a new swap folio with old compressed data.
	 */