netfilter: nf_tables: check NFT_SET_CONCAT flag if field_count is specified

Since f3a2181e ("netfilter: nf_tables: Support for sets with
multiple ranged fields"), it possible to combine intervals and
concatenations. Later on, ef516e86 ("netfilter: nf_tables:
reintroduce the NFT_SET_CONCAT flag") provides the NFT_SET_CONCAT flag
for userspace to report that the set stores a concatenation.

Make sure NFT_SET_CONCAT is set on if field_count is specified for
consistency. Otherwise, if NFT_SET_CONCAT is specified with no
field_count, bail out with EINVAL.

Fixes: ef516e86 ("netfilter: nf_tables: reintroduce the NFT_SET_CONCAT flag")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -4451,6 +4451,11 @@ static int nf_tables_newset(struct sk_buff *skb, const struct nfnl_info *info,
 		err = nf_tables_set_desc_parse(&desc, nla[NFTA_SET_DESC]);
 		if (err < 0)
 			return err;
+
+		if (desc.field_count > 1 && !(flags & NFT_SET_CONCAT))
+			return -EINVAL;
+	} else if (flags & NFT_SET_CONCAT) {
+		return -EINVAL;
 	}
 
 	if (nla[NFTA_SET_EXPR] || nla[NFTA_SET_EXPRESSIONS])