netfilter: nf_tables: Don't allocate nft_rule_dump_ctx

Since struct netlink_callback::args is not used by rule dumpers anymore,
use it to hold nft_rule_dump_ctx. Add a build-time check to make sure it
won't ever exceed the available space.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>

net/netfilter/nf_tables_api.c

@@ -3453,7 +3453,7 @@ static int __nf_tables_dump_rules(struct sk_buff *skb,
 				  const struct nft_table *table,
 				  const struct nft_chain *chain)
 {
-	struct nft_rule_dump_ctx *ctx = cb->data;
+	struct nft_rule_dump_ctx *ctx = (void *)cb->ctx;
 	struct net *net = sock_net(skb->sk);
 	const struct nft_rule *rule, *prule;
 	unsigned int entries = 0;
@@ -3498,7 +3498,7 @@ static int nf_tables_dump_rules(struct sk_buff *skb,
 				struct netlink_callback *cb)
 {
 	const struct nfgenmsg *nfmsg = nlmsg_data(cb->nlh);
-	struct nft_rule_dump_ctx *ctx = cb->data;
+	struct nft_rule_dump_ctx *ctx = (void *)cb->ctx;
 	struct nft_table *table;
 	const struct nft_chain *chain;
 	unsigned int idx = 0;
@@ -3553,42 +3553,35 @@ static int nf_tables_dump_rules(struct sk_buff *skb,
 
 static int nf_tables_dump_rules_start(struct netlink_callback *cb)
 {
+	struct nft_rule_dump_ctx *ctx = (void *)cb->ctx;
 	const struct nlattr * const *nla = cb->data;
-	struct nft_rule_dump_ctx *ctx = NULL;
 
-	ctx = kzalloc(sizeof(*ctx), GFP_ATOMIC);
-	if (!ctx)
-		return -ENOMEM;
+	BUILD_BUG_ON(sizeof(*ctx) > sizeof(cb->ctx));
 
 	if (nla[NFTA_RULE_TABLE]) {
 		ctx->table = nla_strdup(nla[NFTA_RULE_TABLE], GFP_ATOMIC);
-		if (!ctx->table) {
-			kfree(ctx);
+		if (!ctx->table)
 			return -ENOMEM;
-		}
 	}
 	if (nla[NFTA_RULE_CHAIN]) {
 		ctx->chain = nla_strdup(nla[NFTA_RULE_CHAIN], GFP_ATOMIC);
 		if (!ctx->chain) {
 			kfree(ctx->table);
-			kfree(ctx);
 			return -ENOMEM;
 		}
 	}
 	if (NFNL_MSG_TYPE(cb->nlh->nlmsg_type) == NFT_MSG_GETRULE_RESET)
 		ctx->reset = true;
 
-	cb->data = ctx;
 	return 0;
 }
 
 static int nf_tables_dump_rules_done(struct netlink_callback *cb)
 {
-	struct nft_rule_dump_ctx *ctx = cb->data;
+	struct nft_rule_dump_ctx *ctx = (void *)cb->ctx;
 
 	kfree(ctx->table);
 	kfree(ctx->chain);
-	kfree(ctx);
 	return 0;
 }