netfilter: nf_tables: reject hook configuration updates on existing chains

Currently, if you add a base chain whose name clashes with an existing
non-base chain, nf_tables doesn't complain about this. Similarly, if you
update the chain type, the hook number and priority.

With this patch, nf_tables bails out in case any of this unsupported
operations occur by returning EBUSY.

 # nft add table x
 # nft add chain x y
 # nft add chain x y { type nat hook input priority 0\; }
 <cmdline>:1:1-49: Error: Could not process rule: Device or resource busy
 add chain x y { type nat hook input priority 0; }
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -1348,6 +1348,37 @@ static int nf_tables_newchain(struct net *net, struct sock *nlsk,
 		if (nlh->nlmsg_flags & NLM_F_REPLACE)
 			return -EOPNOTSUPP;
 
+		if (nla[NFTA_CHAIN_HOOK]) {
+			struct nft_base_chain *basechain;
+			struct nft_chain_hook hook;
+			struct nf_hook_ops *ops;
+
+			if (!(chain->flags & NFT_BASE_CHAIN))
+				return -EBUSY;
+
+			err = nft_chain_parse_hook(net, nla, afi, &hook,
+						   create);
+			if (err < 0)
+				return err;
+
+			basechain = nft_base_chain(chain);
+			if (basechain->type != hook.type) {
+				nft_chain_release_hook(&hook);
+				return -EBUSY;
+			}
+
+			for (i = 0; i < afi->nops; i++) {
+				ops = &basechain->ops[i];
+				if (ops->hooknum != hook.num ||
+				    ops->priority != hook.priority ||
+				    ops->dev != hook.dev) {
+					nft_chain_release_hook(&hook);
+					return -EBUSY;
+				}
+			}
+			nft_chain_release_hook(&hook);
+		}
+
 		if (nla[NFTA_CHAIN_HANDLE] && name) {
 			struct nft_chain *chain2;