An iptable rule like the following on a multicore systems will result in accepting more connections than set in the rule.
iptables -A INPUT -p tcp -m tcp --syn --dport 7777 -m connlimit \ --connlimit-above 2000 --connlimit-mask 0 -j DROP
In check_hlist function, connections that are found in saved connections but not in netfilter conntrack are deleted, assuming that those connections do not exist anymore. But for multi core systems, there exists a small time window, when a connection has been added to the xt_connlimit maintained rb-tree but has not yet made to netfilter conntrack table. This causes concurrent connections to return incorrect counts and go over limit set in iptable rule.
Connection 1 on Core 1 Connection 2 on Core 2
list_length = N conntrack_table_len = N spin_lock_bh() In check_hlist() function a. loop over saved connections 1. call nf_conntrack_find_get() 2. If not found in 1, i. call hlist_del() b. return total count to caller c. connection gets added to list of saved connections. spin_unlock_bh() list_length = N + 1 spin_lock_bh() on core 2 In check_hlist() function a. loop over saved connection. 1. call nf_conntrack_find_get() 2. If not found in 1. i. call hlist_del() [Connection 1 was in the but not in nf_conntrack yet] ii. connection 1 gets deleted
list_length = N conntrack_table_len = N b. return total count to caller c. connection 2 gets added to list of saved connections spin_unlock_bh()
d. connection 1 gets added to nf_conntrack list_length = N + 1 conntrack_table_len = N + 1 e. connection 2 gets added to nf_conntrack list_length = N + 1 conntrack_table_len = N + 2
So we end up with N + 1 connections in the list but N + 2 in nf_conntrack, allowing more number of connections eventually than set in the rule.
This fix adds an additional field to track such pending connections and prevent them from being deleted by another execution thread on a different core and returns correct count.
Signed-off-by: Alakesh Haloi alakeshh@amazon.com Cc: Pablo Neira Ayuso pablo@netfilter.org Cc: Jozsef Kadlecsik kadlec@blackhole.kfki.hu Cc: Florian Westphal fw@strlen.de Cc: "David S. Miller" davem@davemloft.net Cc: stable@vger.kernel.org # v4.15 and before --- net/netfilter/xt_connlimit.c | 24 +++++++++++++++++++++--- 1 file changed, 21 insertions(+), 3 deletions(-)
diff --git a/net/netfilter/xt_connlimit.c b/net/netfilter/xt_connlimit.c index ffa8eec980e9..bd7563c209a4 100644 --- a/net/netfilter/xt_connlimit.c +++ b/net/netfilter/xt_connlimit.c @@ -47,6 +47,7 @@ struct xt_connlimit_conn { struct hlist_node node; struct nf_conntrack_tuple tuple; union nf_inet_addr addr; + bool pending_add; };
struct xt_connlimit_rb { @@ -126,6 +127,7 @@ static bool add_hlist(struct hlist_head *head, return false; conn->tuple = *tuple; conn->addr = *addr; + conn->pending_add = true; hlist_add_head(&conn->node, head); return true; } @@ -144,15 +146,31 @@ static unsigned int check_hlist(struct net *net,
*addit = true;
- /* check the saved connections */ + /* check the saved connections + */ hlist_for_each_entry_safe(conn, n, head, node) { found = nf_conntrack_find_get(net, zone, &conn->tuple); if (found == NULL) { - hlist_del(&conn->node); - kmem_cache_free(connlimit_conn_cachep, conn); + /* It could be an already deleted connection or + * a new connection that is not there in conntrack + * yet. If former delete it from the list, else + * increase count and move on. + */ + if (conn->pending_add) { + length++; + } else { + hlist_del(&conn->node); + kmem_cache_free(connlimit_conn_cachep, conn); + } continue; }
+ /* If it is a connection that was pending insertion to + * connection tracking table before, then it's time to clear + * the flag. + */ + conn->pending_add = false; + found_ct = nf_ct_tuplehash_to_ctrack(found);
if (nf_ct_tuple_equal(&conn->tuple, tuple)) {