netfilter: nf_tables: can't fail after linking rule into active rule list

rules in nftables a free'd using kfree, but protected by rcu, i.e. we
must wait for a grace period to elapse.

Normal removal patch does this, but nf_tables_newrule() doesn't obey
this rule during error handling.

It calls nft_trans_rule_add() *after* linking rule, and, if that
fails to allocate memory, it unlinks the rule and then kfree() it --
this is unsafe.

Switch order -- first add rule to transaction list, THEN link it
to public list.

Note: nft_trans_rule_add() uses GFP_KERNEL; it will not fail so this
is not a problem in practice (spotted only during code review).

Fixes: 0628b123c96d12 ("netfilter: nfnetlink: add batch support and use it from nf_tables")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -2361,41 +2361,46 @@ static int nf_tables_newrule(struct net *net, struct sock *nlsk,
 	}
 
 	if (nlh->nlmsg_flags & NLM_F_REPLACE) {
-		if (nft_is_active_next(net, old_rule)) {
-			trans = nft_trans_rule_add(&ctx, NFT_MSG_DELRULE,
-						   old_rule);
-			if (trans == NULL) {
-				err = -ENOMEM;
-				goto err2;
-			}
-			nft_deactivate_next(net, old_rule);
-			chain->use--;
-			list_add_tail_rcu(&rule->list, &old_rule->list);
-		} else {
+		if (!nft_is_active_next(net, old_rule)) {
 			err = -ENOENT;
 			goto err2;
 		}
-	} else if (nlh->nlmsg_flags & NLM_F_APPEND)
-		if (old_rule)
-			list_add_rcu(&rule->list, &old_rule->list);
-		else
-			list_add_tail_rcu(&rule->list, &chain->rules);
-	else {
-		if (old_rule)
-			list_add_tail_rcu(&rule->list, &old_rule->list);
-		else
-			list_add_rcu(&rule->list, &chain->rules);
-	}
+		trans = nft_trans_rule_add(&ctx, NFT_MSG_DELRULE,
+					   old_rule);
+		if (trans == NULL) {
+			err = -ENOMEM;
+			goto err2;
+		}
+		nft_deactivate_next(net, old_rule);
+		chain->use--;
 
-	if (nft_trans_rule_add(&ctx, NFT_MSG_NEWRULE, rule) == NULL) {
-		err = -ENOMEM;
-		goto err3;
+		if (nft_trans_rule_add(&ctx, NFT_MSG_NEWRULE, rule) == NULL) {
+			err = -ENOMEM;
+			goto err2;
+		}
+
+		list_add_tail_rcu(&rule->list, &old_rule->list);
+	} else {
+		if (nft_trans_rule_add(&ctx, NFT_MSG_NEWRULE, rule) == NULL) {
+			err = -ENOMEM;
+			goto err2;
+		}
+
+		if (nlh->nlmsg_flags & NLM_F_APPEND) {
+			if (old_rule)
+				list_add_rcu(&rule->list, &old_rule->list);
+			else
+				list_add_tail_rcu(&rule->list, &chain->rules);
+		 } else {
+			if (old_rule)
+				list_add_tail_rcu(&rule->list, &old_rule->list);
+			else
+				list_add_rcu(&rule->list, &chain->rules);
+		}
 	}
 	chain->use++;
 	return 0;
 
-err3:
-	list_del_rcu(&rule->list);
 err2:
 	nf_tables_rule_destroy(&ctx, rule);
 err1: