netfilter: nf_nat: avoid double seq_adjust for loopback

	Avoid double seq adjustment for loopback traffic
because it causes silent repetition of TCP data. One
example is passive FTP with DNAT rule and difference in the
length of IP addresses.

	This patch adds check if packet is sent and
received via loopback device. As the same conntrack is
used both for outgoing and incoming direction, we restrict
seq adjustment to happen only in POSTROUTING.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Patrick McHardy <kaber@trash.net>

1 files changed, 3 insertions, 1 deletions

diff --git a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
index db10075dd88e..de9da21113a1 100644
--- a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
+++ b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
@@ -121,7 +121,9 @@ static unsigned int ipv4_confirm(unsigned int hooknum,
                 return ret;
         }
 
-        if (test_bit(IPS_SEQ_ADJUST_BIT, &ct->status)) {
+        /* adjust seqs for loopback traffic only in outgoing direction */
+        if (test_bit(IPS_SEQ_ADJUST_BIT, &ct->status) &&
+            !nf_is_loopback_packet(skb)) {
                 typeof(nf_nat_seq_adjust_hook) seq_adjust;
 
                 seq_adjust = rcu_dereference(nf_nat_seq_adjust_hook);