netfilter: nft_hash: support deletion of inactive elements

New elements are inactive in the preparation phase, and its
NFT_SET_ELEM_BUSY_MASK flag is set on.

This busy flag doesn't allow us to delete it from the same transaction,
following a sequence like:

	begin transaction
	add element X
	delete element X
	end transaction

This sequence is valid and may be triggered by robots. To resolve this
problem, allow deactivating elements that are active in the current
generation (ie. those that has been just added in this batch).
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_hash.c

@@ -153,9 +153,10 @@ static void *nft_hash_deactivate(const struct nft_set *set,
 				 const struct nft_set_elem *elem)
 {
 	struct nft_hash *priv = nft_set_priv(set);
+	struct net *net = read_pnet(&set->pnet);
 	struct nft_hash_elem *he;
 	struct nft_hash_cmp_arg arg = {
-		.genmask = nft_genmask_next(read_pnet(&set->pnet)),
+		.genmask = nft_genmask_next(net),
 		.set	 = set,
 		.key	 = elem->key.val.data,
 	};
@@ -163,7 +164,8 @@ static void *nft_hash_deactivate(const struct nft_set *set,
 	rcu_read_lock();
 	he = rhashtable_lookup_fast(&priv->ht, &arg, nft_hash_params);
 	if (he != NULL) {
-		if (!nft_set_elem_mark_busy(&he->ext))
+		if (!nft_set_elem_mark_busy(&he->ext) ||
+		    !nft_is_active(net, &he->ext))
 			nft_set_elem_change_active(set, &he->ext);
 		else
 			he = NULL;