netfilter: conntrack: do not renew entry stuck in tcp SYN_SENT state

[ Upstream commit da678759a8d1b63e40c595ba81e1f527fe57bdef ]

Consider:
  client -----> conntrack ---> Host

client sends a SYN, but $Host is unreachable/silent.
Client eventually gives up and the conntrack entry will time out.

However, if the client is restarted with same addr/port pair, it
may prevent the conntrack entry from timing out.

This is noticeable when the existing conntrack entry has no NAT
transformation or an outdated one and port reuse happens either
on client or due to a NAT middlebox.

This change prevents refresh of the timeout for SYN retransmits,
so entry is going away after nf_conntrack_tcp_timeout_syn_sent
seconds (default: 60).

Entry will be re-created on next connection attempt, but then
nat rules will be evaluated again.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index b8cc3339a249527c4ea78ef5cc539db80e5eaefc..aed967e2f30fbf74a251ea5330303cdb71e1d0e2 100644 (file)
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -1158,6 +1158,16 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
                        nf_ct_kill_acct(ct, ctinfo, skb);
                        return NF_ACCEPT;
                }
+
+               if (index == TCP_SYN_SET && old_state == TCP_CONNTRACK_SYN_SENT) {
+                       /* do not renew timeout on SYN retransmit.
+                        *
+                        * Else port reuse by client or NAT middlebox can keep
+                        * entry alive indefinitely (including nat info).
+                        */
+                       return NF_ACCEPT;
+               }
+
                /* ESTABLISHED without SEEN_REPLY, i.e. mid-connection
                 * pickup with loose=1. Avoid large ESTABLISHED timeout.
                 */