Re: [PATCH] netfilter: nf_conntrack: fix RCU race innf_conntrack_find_get

[Eric Dumazet]

On Thu, 2014-01-09 at 09:24 +0400, Andrew Vagin wrote:

> I have one more question. Looks like I found another problem.
> 
> init_conntrack:
>         hlist_nulls_add_head_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode,
>                       &net->ct.unconfirmed);
> 
> __nf_conntrack_hash_insert:
> 	hlist_nulls_add_head_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode,
> 		   &net->ct.hash[hash]);
> 
> We use one hlist_nulls_node to add a conntrack into two different lists.
> As far as I understand, it's unacceptable in case of
> SLAB_DESTROY_BY_RCU.

I guess you missed :

net/netfilter/nf_conntrack_core.c:1598: INIT_HLIST_NULLS_HEAD(&net->ct.unconfirmed, UNCONFIRMED_NULLS_VAL);


> 
> Lets imagine that we have two threads. The first one enumerates objects
> from a first list, the second one recreates an object and insert it in a
> second list.  If a first thread in this moment stays on the object, it
> can read "next", when it's in the second list, so it will continue
> to enumerate objects from the second list. It is not what we want, isn't
> it?
> 
> cpu1				cpu2
> 				hlist_nulls_for_each_entry_rcu(ct)
> destroy_conntrack
>  kmem_cache_free
> 
> init_conntrack
>  hlist_nulls_add_head_rcu
> 				ct = ct->next
> 

This will be fine.

I think we even have a counter to count number of occurrence of this
rare event. (I personally never read a non null search_restart value)

 NF_CT_STAT_INC(net, search_restart); 



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