Re: [PATCH] net: netfilter: fix possible refcount leak in ctnetlink_create_conntrack()

[Pablo Neira Ayuso]

Hi Florian,

On Fri, Feb 10, 2023 at 11:32:50AM +0100, Florian Westphal wrote:
> Hangyu Hua <hbh25y@xxxxxxxxx> wrote:
> > nf_ct_put() needs to be called to put the refcount got by
> > nf_conntrack_find_get() to avoid refcount leak when
> > nf_conntrack_hash_check_insert() fails.
> > 
> > Fixes: 7d367e06688d ("netfilter: ctnetlink: fix soft lockup when netlink adds new entries (v2)")
> > Signed-off-by: Hangyu Hua <hbh25y@xxxxxxxxx>
> > ---
> >  net/netfilter/nf_conntrack_netlink.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
> > index 1286ae7d4609..ca4d5bb1ea52 100644
> > --- a/net/netfilter/nf_conntrack_netlink.c
> > +++ b/net/netfilter/nf_conntrack_netlink.c
> > @@ -2375,12 +2375,15 @@ ctnetlink_create_conntrack(struct net *net,
> >  
> >  	err = nf_conntrack_hash_check_insert(ct);
> >  	if (err < 0)
> > -		goto err2;
> > +		goto err3;
> 
> Ouch, looks like this is broken in more than one way?
> 
> nf_conntrack_hash_check_insert() can call nf_ct_kill()
> and return an error, in that case ct->master reference
> is already dropped for us.
> 
> One way would be to return 0 in that case (in
> nf_conntrack_hash_check_insert()).  What do you think?

This is misleading to the user that adds an entry via ctnetlink?

ETIMEDOUT also looks a bit confusing to report to userspace.
Rewinding: if the intention is to deal with stale conntrack extension,
for example, helper module has been removed while this entry was
added. Then, probably call EAGAIN so nfnetlink has a chance to retry
transparently?

BTW, I think we should remove:

NF_CT_STAT_INC_ATOMIC(net, drop);

that is under nf_ct_ext_valid_post(), no packet is dropped in this
path.

Thanks.