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

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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).

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?

Thanks,
Hangyu

I'll also have a look at switching to a refcount based model for
all extensions that reference external objects, this would avoid
the entire problem, but thats likely more intrusive.



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