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

Pablo Neira Ayuso
2020-10-14T08:11:50Z

On Wed, Oct 14, 2020 at 02:06:28AM +0200, Pablo Neira Ayuso wrote:
> On Fri, Oct 09, 2020 at 10:05:48PM +0200, Florian Westphal wrote:
> > Jozsef Kadlecsik <kadlec@netfilter.org> wrote:
> > > > The "delay unregister" remark was wrt. the "all rules were deleted"
> > > > case, i.e. add a "grace period" rather than acting right away when
> > > > conntrack use count did hit 0.
> > >
> > > Now I understand it, thanks really. The hooks are removed, so conntrack
> > > cannot "see" the packets and the entries become stale.
> >
> > Yes.
> >
> > > What is the rationale behind "remove the conntrack hooks when there are no
> > > rule left referring to conntrack"? Performance optimization? But then the
> > > content of the whole conntrack table could be deleted too... ;-)
> >
> > Yes, this isn't the case at the moment -- only hooks are removed,
> > entries will eventually time out.
> >
> > > > Conntrack entries are not removed, only the base hooks get unregistered.
> > > > This is a problem for tcp window tracking.
> > > >
> > > > When re-register occurs, kernel is supposed to switch the existing
> > > > entries to "loose" mode so window tracking won't flag packets as
> > > > invalid, but apparently this isn't enough to handle keepalive case.
> > >
> > > "loose" (nf_ct_tcp_loose) mode doesn't disable window tracking, it
> > > enables/disables picking up already established connections.
> > >
> > > nf_ct_tcp_be_liberal would disable TCP window checking (but not tracking)
> > > for non RST packets.
> >
> > You are right, mixup on my part.
> >
> > > But both seems to be modified only via the proc entries.
> >
> > Yes, we iterate table on re-register and modify the existing entries.
>
> For iptables-nft, it might be possible to avoid this deregister +
> register ct hooks in the same transaction: Maybe add something like
> nf_ct_netns_get_all() to bump refcounters by one _iff_ they are > 0
> before starting the transaction processing, then call
> nf_ct_netns_put_all() which decrements refcounters and unregister
> hooks if they reach 0.

Hm, scratch that, put_all() would create an imbalance with this
conditional increment.

> The only problem with this approach is that this pulls in the
> conntrack module, to solve that, struct nf_ct_hook in
> net/netfilter/core.c could be used to store the reference to
> ->netns_get_all and ->net_put_all.
>
> Legacy would still be flawed though.