Re: [PATCH nf] netfilter: conntrack: fix infinite loop on rmmod

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Sun, May 10, 2020 at 02:28:07PM +0200, Florian Westphal wrote:
> 'rmmod nf_conntrack' can hang forever, because the netns exit
> gets stuck in nf_conntrack_cleanup_net_list():
> 
> i_see_dead_people:
>  busy = 0;
>  list_for_each_entry(net, net_exit_list, exit_list) {
>   nf_ct_iterate_cleanup(kill_all, net, 0, 0);
>   if (atomic_read(&net->ct.count) != 0)
>    busy = 1;
>  }
>  if (busy) {
>   schedule();
>   goto i_see_dead_people;
>  }
> 
> When nf_ct_iterate_cleanup iterates the conntrack table, all nf_conn
> structures can be found twice:
> once for the original tuple and once for the conntracks reply tuple.
> 
> get_next_corpse() only calls the iterator when the entry is
> in original direction -- the idea was to avoid unneeded invocations
> of the iterator callback.
> 
> When support for clashing entries was added, the assumption that
> all nf_conn objects are added twice, once in original, once for reply
> tuple no longer holds -- NF_CLASH_BIT entries are only added in
> the non-clashing reply direction.
> 
> Thus, if at least one NF_CLASH entry is in the list then
> nf_conntrack_cleanup_net_list() always skips it completely.
> 
> During normal netns destruction, this causes a hang of several
> seconds, until the gc worker removes the entry (NF_CLASH entries
> always have a 1 second timeout).
> 
> But in the rmmod case, the gc worker has already been stopped, so
> ct.count never becomes 0.
> 
> We can fix this in two ways:
> 
> 1. Add a second test for CLASH_BIT and call iterator for those
>    entries as well, or:
> 2. Skip the original tuple direction and use the reply tuple.
> 
> 2) is simpler, so do that.

Applied, thanks.