netfilter: nf_tables: split chain policy validation from actually setting it

Currently nf_tables_newchain() atomicity is broken because of having
validation of some netlink attributes performed after changing attributes
of the chain. The chain policy is (currently) fine, but split it up as
preparation for the following fixes and to avoid future mistakes.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -760,22 +760,6 @@ static int nf_tables_getchain(struct sock *nlsk, struct sk_buff *skb,
 	return err;
 }
 
-static int
-nf_tables_chain_policy(struct nft_base_chain *chain, const struct nlattr *attr)
-{
-	switch (ntohl(nla_get_be32(attr))) {
-	case NF_DROP:
-		chain->policy = NF_DROP;
-		break;
-	case NF_ACCEPT:
-		chain->policy = NF_ACCEPT;
-		break;
-	default:
-		return -EINVAL;
-	}
-	return 0;
-}
-
 static const struct nla_policy nft_counter_policy[NFTA_COUNTER_MAX + 1] = {
 	[NFTA_COUNTER_PACKETS]	= { .type = NLA_U64 },
 	[NFTA_COUNTER_BYTES]	= { .type = NLA_U64 },
@@ -834,6 +818,7 @@ static int nf_tables_newchain(struct sock *nlsk, struct sk_buff *skb,
 	struct nlattr *ha[NFTA_HOOK_MAX + 1];
 	struct net *net = sock_net(skb->sk);
 	int family = nfmsg->nfgen_family;
+	u8 policy = NF_ACCEPT;
 	u64 handle = 0;
 	unsigned int i;
 	int err;
@@ -869,6 +854,22 @@ static int nf_tables_newchain(struct sock *nlsk, struct sk_buff *skb,
 		}
 	}
 
+	if (nla[NFTA_CHAIN_POLICY]) {
+		if ((chain != NULL &&
+		    !(chain->flags & NFT_BASE_CHAIN)) ||
+		    nla[NFTA_CHAIN_HOOK] == NULL)
+			return -EOPNOTSUPP;
+
+		policy = nla_get_be32(nla[NFTA_CHAIN_POLICY]);
+		switch (policy) {
+		case NF_DROP:
+		case NF_ACCEPT:
+			break;
+		default:
+			return -EINVAL;
+		}
+	}
+
 	if (chain != NULL) {
 		if (nlh->nlmsg_flags & NLM_F_EXCL)
 			return -EEXIST;
@@ -879,15 +880,8 @@ static int nf_tables_newchain(struct sock *nlsk, struct sk_buff *skb,
 		    !IS_ERR(nf_tables_chain_lookup(table, nla[NFTA_CHAIN_NAME])))
 			return -EEXIST;
 
-		if (nla[NFTA_CHAIN_POLICY]) {
-			if (!(chain->flags & NFT_BASE_CHAIN))
-				return -EOPNOTSUPP;
-
-			err = nf_tables_chain_policy(nft_base_chain(chain),
-						     nla[NFTA_CHAIN_POLICY]);
-			if (err < 0)
-				return err;
-		}
+		if (nla[NFTA_CHAIN_POLICY])
+			nft_base_chain(chain)->policy = policy;
 
 		if (nla[NFTA_CHAIN_COUNTERS]) {
 			if (!(chain->flags & NFT_BASE_CHAIN))
@@ -958,17 +952,7 @@ static int nf_tables_newchain(struct sock *nlsk, struct sk_buff *skb,
 		}
 
 		chain->flags |= NFT_BASE_CHAIN;
-
-		if (nla[NFTA_CHAIN_POLICY]) {
-			err = nf_tables_chain_policy(basechain,
-						     nla[NFTA_CHAIN_POLICY]);
-			if (err < 0) {
-				free_percpu(basechain->stats);
-				kfree(basechain);
-				return err;
-			}
-		} else
-			basechain->policy = NF_ACCEPT;
+		basechain->policy = policy;
 
 		if (nla[NFTA_CHAIN_COUNTERS]) {
 			err = nf_tables_counters(basechain,