netfilter: nf_tables: fix oops during rule dump

We can oops in nf_tables_fill_rule_info().

Its not possible to fetch previous element in rcu-protected lists
when deletions are not prevented somehow: list_del_rcu poisons
the ->prev pointer value.

Before rcu-conversion this was safe as dump operations did hold
nfnetlink mutex.

Pass previous rule as argument, obtained by keeping a pointer to
the previous rule during traversal.

Fixes: d9adf22a ("netfilter: nf_tables: use call_rcu in netlink dumps")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -2270,13 +2270,13 @@ static int nf_tables_fill_rule_info(struct sk_buff *skb, struct net *net,
 				    u32 flags, int family,
 				    const struct nft_table *table,
 				    const struct nft_chain *chain,
-				    const struct nft_rule *rule)
+				    const struct nft_rule *rule,
+				    const struct nft_rule *prule)
 {
 	struct nlmsghdr *nlh;
 	struct nfgenmsg *nfmsg;
 	const struct nft_expr *expr, *next;
 	struct nlattr *list;
-	const struct nft_rule *prule;
 	u16 type = nfnl_msg_type(NFNL_SUBSYS_NFTABLES, event);
 
 	nlh = nlmsg_put(skb, portid, seq, type, sizeof(struct nfgenmsg), flags);
@@ -2296,8 +2296,7 @@ static int nf_tables_fill_rule_info(struct sk_buff *skb, struct net *net,
 			 NFTA_RULE_PAD))
 		goto nla_put_failure;
 
-	if ((event != NFT_MSG_DELRULE) && (rule->list.prev != &chain->rules)) {
-		prule = list_prev_entry(rule, list);
+	if (event != NFT_MSG_DELRULE && prule) {
 		if (nla_put_be64(skb, NFTA_RULE_POSITION,
 				 cpu_to_be64(prule->handle),
 				 NFTA_RULE_PAD))
@@ -2344,7 +2343,7 @@ static void nf_tables_rule_notify(const struct nft_ctx *ctx,
 
 	err = nf_tables_fill_rule_info(skb, ctx->net, ctx->portid, ctx->seq,
 				       event, 0, ctx->family, ctx->table,
-				       ctx->chain, rule);
+				       ctx->chain, rule, NULL);
 	if (err < 0) {
 		kfree_skb(skb);
 		goto err;
@@ -2369,12 +2368,13 @@ static int __nf_tables_dump_rules(struct sk_buff *skb,
 				  const struct nft_chain *chain)
 {
 	struct net *net = sock_net(skb->sk);
+	const struct nft_rule *rule, *prule;
 	unsigned int s_idx = cb->args[0];
-	const struct nft_rule *rule;
 
+	prule = NULL;
 	list_for_each_entry_rcu(rule, &chain->rules, list) {
 		if (!nft_is_active(net, rule))
-			goto cont;
+			goto cont_skip;
 		if (*idx < s_idx)
 			goto cont;
 		if (*idx > s_idx) {
@@ -2386,11 +2386,13 @@ static int __nf_tables_dump_rules(struct sk_buff *skb,
 					NFT_MSG_NEWRULE,
 					NLM_F_MULTI | NLM_F_APPEND,
 					table->family,
-					table, chain, rule) < 0)
+					table, chain, rule, prule) < 0)
 			return 1;
 
 		nl_dump_check_consistent(cb, nlmsg_hdr(skb));
 cont:
+		prule = rule;
+cont_skip:
 		(*idx)++;
 	}
 	return 0;
@@ -2546,7 +2548,7 @@ static int nf_tables_getrule(struct net *net, struct sock *nlsk,
 
 	err = nf_tables_fill_rule_info(skb2, net, NETLINK_CB(skb).portid,
 				       nlh->nlmsg_seq, NFT_MSG_NEWRULE, 0,
-				       family, table, chain, rule);
+				       family, table, chain, rule, NULL);
 	if (err < 0)
 		goto err;