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

[Florian Westphal]

Hangyu Hua <hbh25y@xxxxxxxxx> wrote:
> On 13/2/2023 16:17, Florian Westphal wrote:
> > Hangyu Hua <hbh25y@xxxxxxxxx> wrote:
> > > On 12/2/2023 20:53, Florian Westphal wrote:
> > > > Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx> wrote:
> > > > > > 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?
> > > > 
> > > > Seems we first need to add a "bool *inserted" so we know when the ct
> > > > entry went public.
> > > > 
> > > I don't think so.
> > > 
> > > nf_conntrack_hash_check_insert(struct nf_conn *ct)
> > > {
> > > ...
> > > 	/* The caller holds a reference to this object */
> > > 	refcount_set(&ct->ct_general.use, 2);			// [1]
> > > 	__nf_conntrack_hash_insert(ct, hash, reply_hash);
> > > 	nf_conntrack_double_unlock(hash, reply_hash);
> > > 	NF_CT_STAT_INC(net, insert);
> > > 	local_bh_enable();
> > > 
> > > 	if (!nf_ct_ext_valid_post(ct->ext)) {
> > > 		nf_ct_kill(ct);					// [2]
> > > 		NF_CT_STAT_INC_ATOMIC(net, drop);
> > > 		return -ETIMEDOUT;
> > > 	}
> > > ...
> > > }
> > > 
> > > We set ct->ct_general.use to 2 in nf_conntrack_hash_check_insert()([1]).
> > > nf_ct_kill willn't put the last refcount. So ct->master will not be freed in
> > > this way. But this means the situation not only causes ct->master's refcount
> > > leak but also releases ct whose refcount is still 1 in nf_conntrack_free()
> > > (in ctnetlink_create_conntrack() err1).
> > 
> > at [2] The refcount could be > 1, as entry became public.  Other CPU
> > might have obtained a reference.
> > 
> > > I think it may be a good idea to set ct->ct_general.use to 0 after
> > > nf_ct_kill() ([2]) to put the caller's reference. What do you think?
> > 
> > We can't, see above.  We need something similar to this (not even compile
> > tested):
> > 
> 
> I see. This patch look good to me. Do I need to make a v2 like this one? Or
> you guys can handle this.

No, I think its best if your patch is applied as-is because it fixes a
real bug.   Mixing both bug fixes in one fix makes it harder for
-stable.