netfilter: nf_conntrack: there maybe a bug in __nf_conntrack_confirm, when it race against get_next_corpse

[billbonaparte]

Hi, all:
	In function __nf_conntrack_confirm, we check the conntrack if it was
alreay dead, before insert it into hash-table. 
	we do this because if we insert an already 'dead' hash,  it will
block further use of that particular connection.
	but we don't do that right.
    let's consider the following case:
	
	cpu1
cpu2
	__nf_conntrack_confirm
get_next_corpse
   		lock corresponding hash-list
....
		check nf_ct_is_dying(ct)
for_each_possible_cpu(cpu) {
		......
spin_lock_bh(&pcpu->lock);
		......
set_bit(IPS_DYING_BIT, &ct->status);
		nf_ct_del_from_dying_or_unconfirmed_list(ct);
spin_unlock_bh(&pcpu_lock);
		add_timer(&ct->timeout);
}	
		ct->status |= IPS_CONFIRMD;
		__nf_conntrack_hash_insert(ct);


	
	The above case reveal two problems:
	1. we may insert a dead conntrack to hash-table, it will block
further use of that particular connection.
	2. operation on ct->status should be atomic, because it race aginst
get_next_corpse.
	  due to this reason, the operation on ct->status in
nf_nat_setup_info should be atomic as well.

	if we want to resolve the first problem, we must delete the
unconfirmed conntrack from unconfirmed-list first, then check if it is
already dead.
	Am I right to do this ?
	Appreciate any comments and reply.

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