netfilter: nf_nat: fix NAT issue in 2.6.30.4+

Vitezslav Samel discovered that since 2.6.30.4+ active FTP can not work
over NAT. The "cause" of the problem was a fix of unacknowledged data
detection with NAT (commit a3a9f79e361e864f0e9d75ebe2a0cb43d17c4272).
However, actually, that fix uncovered a long standing bug in TCP conntrack:
when NAT was enabled, we simply updated the max of the right edge of
the segments we have seen (td_end), by the offset NAT produced with
changing IP/port in the data. However, we did not update the other parameter
(td_maxend) which is affected by the NAT offset. Thus that could drift
away from the correct value and thus resulted breaking active FTP.

The patch below fixes the issue by *not* updating the conntrack parameters
from NAT, but instead taking into account the NAT offsets in conntrack in a
consistent way. (Updating from NAT would be more harder and expensive because
it'd need to re-calculate parameters we already calculated in conntrack.)

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 0 deletions

diff --git a/net/ipv4/netfilter/nf_nat_core.c b/net/ipv4/netfilter/nf_nat_core.c
index 68afc6ecd343..fe1a64479dd0 100644
--- a/net/ipv4/netfilter/nf_nat_core.c
+++ b/net/ipv4/netfilter/nf_nat_core.c
@@ -750,6 +750,8 @@ static int __init nf_nat_init(void)
 	BUG_ON(nfnetlink_parse_nat_setup_hook != NULL);
 	rcu_assign_pointer(nfnetlink_parse_nat_setup_hook,
 			   nfnetlink_parse_nat_setup);
+	BUG_ON(nf_ct_nat_offset != NULL);
+	rcu_assign_pointer(nf_ct_nat_offset, nf_nat_get_offset);
 	return 0;
 
  cleanup_extend:
@@ -764,6 +766,7 @@ static void __exit nf_nat_cleanup(void)
 	nf_ct_extend_unregister(&nat_extend);
 	rcu_assign_pointer(nf_nat_seq_adjust_hook, NULL);
 	rcu_assign_pointer(nfnetlink_parse_nat_setup_hook, NULL);
+	rcu_assign_pointer(nf_ct_nat_offset, NULL);
 	synchronize_net();
 }