netdev - Re: [PATCH nf v2] netfilter: conntrack: connection timeout after re-register

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Message-ID: <CA+HUmGhBxBHU85oFfvoAyP=hG17DG2kgO67eawk1aXmSjehOWQ@mail.gmail.com>
Date:   Thu, 8 Oct 2020 16:41:30 -0700
From:   Francesco Ruggeri <fruggeri@...sta.com>
To:     open list <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, coreteam@...filter.org,
        netfilter-devel@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>, fw@...len.org,
        kadlec@...filter.org, Pablo Neira Ayuso <pablo@...filter.org>,
        Francesco Ruggeri <fruggeri@...sta.com>
Subject: Re: [PATCH nf v2] netfilter: conntrack: connection timeout after re-register

On Wed, Oct 7, 2020 at 12:32 PM Francesco Ruggeri <fruggeri@...sta.com> wrote:
>
> If the first packet conntrack sees after a re-register is an outgoing
> keepalive packet with no data (SEG.SEQ = SND.NXT-1), td_end is set to
> SND.NXT-1.
> When the peer correctly acknowledges SND.NXT, tcp_in_window fails
> check III (Upper bound for valid (s)ack: sack <= receiver.td_end) and
> returns false, which cascades into nf_conntrack_in setting
> skb->_nfct = 0 and in later conntrack iptables rules not matching.
> In cases where iptables are dropping packets that do not match
> conntrack rules this can result in idle tcp connections to time out.
>
> v2: adjust td_end when getting the reply rather than when sending out
>     the keepalive packet.
>

Any comments?
Here is a simple reproducer.
The idea is to show that keepalive packets in an idle tcp
connection will be dropped (and the connection will time out)
if conntrack hooks are de-registered and then re-registered.
The reproducer has two files.
client_server.py creates both ends of a tcp connection, bounces
a few packets back and forth, and then blocks on a recv on the
client side. The client's keepalive is configured to time out in
20 seconds. This connection should not time out.
test is a bash script that creates a net namespace where it sets
iptables rules for the connection, starts client_server.py, and
then clears and restores the iptables rules (which causes
conntrack hooks to be de-registered and re-registered).

================ file client_server.py
#!/usr/bin/python

import socket

PORT=4446

# create server socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.bind(('localhost', PORT))
sock.listen(1)

# create client socket
cl_sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
cl_sock.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
cl_sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPIDLE, 2)
cl_sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPINTVL, 2)
cl_sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPCNT, 10)
cl_sock.connect(('localhost', PORT))

srv_sock, _ = sock.accept()

# Bounce a packet back and forth a few times
buf = 'aaaaaaaaaaaa'
for i in range(5):
   cl_sock.send(buf)
   buf = srv_sock.recv(100)
   srv_sock.send(buf)
   buf = cl_sock.recv(100)
   print buf

# idle the connection
try:
   buf = cl_sock.recv(100)
except socket.error, e:
   print "Error: %s" % e

sock.close()
cl_sock.close()
srv_sock.close()

============== file test
#!/bin/bash

ip netns add dummy
ip netns exec dummy ip link set lo up
echo "Created namespace"

ip netns exec dummy iptables-restore <<END
*filter
:INPUT DROP [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 4446 -j ACCEPT
COMMIT
END
echo "Installed iptables rules"

ip netns exec dummy ./client_server.py &
echo "Created tcp connection"
sleep 2

ip netns exec dummy iptables-restore << END
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
COMMIT
END
echo "Cleared iptables rules"
sleep 4

ip netns exec dummy iptables-restore << END
*filter
:INPUT DROP [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 4446 -j ACCEPT
COMMIT
END
echo "Restored original iptables rules"

wait
ip netns del dummy
exit 0