netfilter: nf_tables: prepare set element accounting for async updates

Use atomic operations for the element count to avoid races with async
updates.

To properly handle the transactional semantics during netlink updates,
deleted but not yet committed elements are accounted for seperately and
are treated as being already removed. This means for the duration of
a netlink transaction, the limit might be exceeded by the amount of
elements deleted. Set implementations must be prepared to handle this.

Signed-off-by: Patrick McHardy <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/net/netfilter/nf_tables.h

···
  * 	@dtype: data type (verdict or numeric type defined by userspace)
  * 	@size: maximum set size
  * 	@nelems: number of elements
+ * 	@ndeact: number of deactivated elements queued for removal
  * 	@timeout: default timeout value in msecs
  * 	@gc_int: garbage collection interval in msecs
  *	@policy: set parameterization (see enum nft_set_policies)
···
 	u32				ktype;
 	u32				dtype;
 	u32				size;
-	u32				nelems;
+	atomic_t			nelems;
+	u32				ndeact;
 	u64				timeout;
 	u32				gc_int;
 	u16				policy;

net/netfilter/nf_tables_api.c

···
 	u32 flags;
 	int err;
 
-	if (set->size && set->nelems == set->size)
-		return -ENFILE;
-
 	err = nla_parse_nested(nla, NFTA_SET_ELEM_MAX, attr,
 			       nft_set_elem_policy);
 	if (err < 0)
···
 		return -EBUSY;
 
 	nla_for_each_nested(attr, nla[NFTA_SET_ELEM_LIST_ELEMENTS], rem) {
-		err = nft_add_set_elem(&ctx, set, attr);
-		if (err < 0)
-			break;
+		if (set->size &&
+		    !atomic_add_unless(&set->nelems, 1, set->size + set->ndeact))
+			return -ENFILE;
 
-		set->nelems++;
+		err = nft_add_set_elem(&ctx, set, attr);
+		if (err < 0) {
+			atomic_dec(&set->nelems);
+			break;
+		}
 	}
 	return err;
 }
···
 		if (err < 0)
 			break;
 
-		set->nelems--;
+		set->ndeact++;
 	}
 	return err;
 }
···
 						 &te->elem,
 						 NFT_MSG_DELSETELEM, 0);
 			te->set->ops->remove(te->set, &te->elem);
+			atomic_dec(&te->set->nelems);
+			te->set->ndeact--;
 			break;
 		}
 	}
···
 			nft_trans_destroy(trans);
 			break;
 		case NFT_MSG_NEWSETELEM:
-			nft_trans_elem_set(trans)->nelems--;
 			te = (struct nft_trans_elem *)trans->data;
 
 			te->set->ops->remove(te->set, &te->elem);
+			atomic_dec(&te->set->nelems);
 			break;
 		case NFT_MSG_DELSETELEM:
 			te = (struct nft_trans_elem *)trans->data;
 
-			nft_trans_elem_set(trans)->nelems++;
 			te->set->ops->activate(te->set, &te->elem);
+			te->set->ndeact--;
 
 			nft_trans_destroy(trans);
 			break;

net/netfilter/nft_hash.c

···
 
 static void nft_hash_gc(struct work_struct *work)
 {
-	const struct nft_set *set;
+	struct nft_set *set;
 	struct nft_hash_elem *he;
 	struct nft_hash *priv;
 	struct nft_set_gc_batch *gcb = NULL;
···
 		if (gcb == NULL)
 			goto out;
 		rhashtable_remove_fast(&priv->ht, &he->node, nft_hash_params);
+		atomic_dec(&set->nelems);
 		nft_set_gc_batch_add(gcb, he);
 	}
 out: