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SubjectRe: [PATCH] [RFC] netfilter: nf_conntrack: don't relase a conntrack with non-zero refcnt
On Tue, Jan 14, 2014 at 09:35:48PM +0400, Andrey Vagin wrote:
> ----
> Eric and Florian, could you look at this patch. When you say,
> that it looks good, I will ask the user to validate it.
> I can't reorder these actions, because it's reproduced on a real host
> with real users. Thanks.
> ----
>
> nf_conntrack_free can't be called for a conntract with non-zero ref-counter,
> because it can race with nf_conntrack_find_get().
>
> A conntrack slab is created with SLAB_DESTROY_BY_RCU. Non-zero
> ref-conunter says that this conntrack is used now. So when we release a
> conntrack with non-zero counter, we break this assumption.
>
> CPU1 CPU2
> ____nf_conntrack_find()
> nf_ct_put()
> destroy_conntrack()
> ...
> init_conntrack
> __nf_conntrack_alloc (set use = 1)
> atomic_inc_not_zero(&ct->use) (use = 2)
> if (!l4proto->new(ct, skb, dataoff, timeouts))
> nf_conntrack_free(ct); (use = 2 !!!)
> ...
> __nf_conntrack_alloc (set use = 1)
> if (!nf_ct_key_equal(h, tuple, zone))
> nf_ct_put(ct); (use = 0)
> destroy_conntrack()
> /* continue to work with CT */

If I didn't miss something obvious this looks like a pretty possible
scenario. Thanks!


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