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

[Jason Xing <kerneljasonxing@xxxxxxxxx>]

Hello Florian,

On Thu, Mar 7, 2024 at 5:33 PM Florian Westphal <fw@xxxxxxxxx> wrote:
>
> Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
> > client_ip:client_port <--> server_ip:b_port
> >
> > Then, some strange skbs from client or gateway, say, out-of-window
> > skbs are sent to the server_ip:a_port (not b_port) due to DNAT
> > 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.
>
> So far everything is as intended.
>
> > I think, even we have set DNAT policy, it would be better if the
> > whole process/behaviour adheres to the original TCP behaviour.
> >
> > Signed-off-by: Jason Xing <kernelxing@xxxxxxxxxxx>
> > ---
> >  net/netfilter/nf_conntrack_proto_tcp.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
> > index ae493599a3ef..3f3e620f3969 100644
> > --- a/net/netfilter/nf_conntrack_proto_tcp.c
> > +++ b/net/netfilter/nf_conntrack_proto_tcp.c
> > @@ -1253,13 +1253,11 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
> >       res = tcp_in_window(ct, dir, index,
> >                           skb, dataoff, th, state);
> >       switch (res) {
> > -     case NFCT_TCP_IGNORE:
> > -             spin_unlock_bh(&ct->lock);
> > -             return NF_ACCEPT;
> >       case NFCT_TCP_INVALID:
> >               nf_tcp_handle_invalid(ct, dir, index, skb, state);
> > +     case NFCT_TCP_IGNORE:
> >               spin_unlock_bh(&ct->lock);
> > -             return -NF_ACCEPT;
> > +             return NF_ACCEPT;
>
> This looks wrong.  -NF_ACCEPT means 'pass packet, but its not part
> of the connection' (packet will match --ctstate INVALID check).
>
> This change disables most of the tcp_in_window() test, this will
> pretend everything is fine even though tcp_in_window says otherwise.

Thanks for the information. It does make sense.

What I've done is quite similar to nf_conntrack_tcp_be_liberal sysctl
knob which you also pointed out. It also pretends to ignore those
out-of-window skbs.

>
> You could:
>  - drop invalid tcp packets in input hook

How about changing the return value only as below? Only two cases will
be handled:

diff --git a/net/netfilter/nf_conntrack_proto_tcp.c
b/net/netfilter/nf_conntrack_proto_tcp.c
index ae493599a3ef..c88ce4cd041e 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -1259,7 +1259,7 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
        case NFCT_TCP_INVALID:
                nf_tcp_handle_invalid(ct, dir, index, skb, state);
                spin_unlock_bh(&ct->lock);
-               return -NF_ACCEPT;
+               return -NF_DROP;
        case NFCT_TCP_ACCEPT:
                break;
        }

Then it will be dropped naturally in nf_hook_slow().

>  - set nf_conntrack_tcp_be_liberal=1

Sure, it can workaround this case, but I would like to refuse the
out-of-window in netfilter or TCP layer as default instead of turning
on this sysctl knob. If I understand wrong, please correct me.

Thanks,
Jason

>
> both will avoid this 'rst' issue.