netdev - Re: [PATCH nf-next v2] netfilter: conntrack: avoid sending RST to reply out-of-window skb

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Message-ID: <CAL+tcoCDs+0OJ3VE59KSyvvyzOxqf0SW-hojDeccwdB=PazwqA@mail.gmail.com>
Date: Tue, 19 Mar 2024 10:52:44 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Simon Horman <horms@...nel.org>
Cc: edumazet@...gle.com, pablo@...filter.org, kadlec@...filter.org, 
	fw@...len.de, kuba@...nel.org, pabeni@...hat.com, davem@...emloft.net, 
	netfilter-devel@...r.kernel.org, coreteam@...filter.org, 
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH nf-next v2] netfilter: conntrack: avoid sending RST to
 reply out-of-window skb

Hello Simon,

On Tue, Mar 19, 2024 at 4:16 AM Simon Horman <horms@...nel.org> wrote:
>
> On Mon, Mar 11, 2024 at 03:05:50PM +0800, Jason Xing wrote:
> > From: Jason Xing <kernelxing@...cent.com>
> >
> > Supposing we set DNAT policy converting a_port to b_port on the
> > server at the beginning, the socket is set up by using 4-tuple:
> >
> > client_ip:client_port <--> server_ip:b_port
> >
> > Then, some strange skbs from client or gateway, say, out-of-window
> > skbs are eventually sent to the server_ip:a_port (not b_port)
> > in TCP layer due to netfilter clearing skb->_nfct value in
> > nf_conntrack_in() function. Why? Because the tcp_in_window()
> > considers the incoming skb as an invalid skb by returning
> > NFCT_TCP_INVALID.
> >
> > At last, the TCP layer process the out-of-window
> > skb (client_ip,client_port,server_ip,a_port) and try to look up
> > such an socket in tcp_v4_rcv(), as we can see, it will fail for sure
> > because the port is a_port not our expected b_port and then send
> > back an RST to the client.
> >
> > The detailed call graphs go like this:
> > 1)
> > nf_conntrack_in()
> >   -> nf_conntrack_handle_packet()
> >     -> nf_conntrack_tcp_packet()
> >       -> tcp_in_window() // tests if the skb is out-of-window
> >       -> return -NF_ACCEPT;
> >   -> skb->_nfct = 0; // if the above line returns a negative value
> > 2)
> > tcp_v4_rcv()
> >   -> __inet_lookup_skb() // fails, then jump to no_tcp_socket
> >   -> tcp_v4_send_reset()
> >
> > The moment the client receives the RST, it will drop. So the RST
> > skb doesn't hurt the client (maybe hurt some gateway which cancels
> > the session when filtering the RST without validating
> > the sequence because of performance reason). Well, it doesn't
> > matter. However, we can see many strange RST in flight.
> >
> > The key reason why I wrote this patch is that I don't think
> > the behaviour is expected because the RFC 793 defines this
> > case:
> >
> > "If the connection is in a synchronized state (ESTABLISHED,
> >  FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),
> >  any unacceptable segment (out of window sequence number or
> >  unacceptible acknowledgment number) must elicit only an empty
>
> Not for those following along, it appears that RFC 793 does misspell
> unacceptable as above. Perhaps spelling was different in 1981 :)

Thanks for the check. Yes, it did misspell that word. Should I correct
that word in my quotation?

>
> >  acknowledgment segment containing the current send-sequence number
> >  and an acknowledgment..."
> >
> > I think, even we have set DNAT policy, it would be better if the
> > whole process/behaviour adheres to the original TCP behaviour as
> > default.
> >
> > Suggested-by: Florian Westphal <fw@...len.de>
> > Signed-off-by: Jason Xing <kernelxing@...cent.com>
>
> ...