Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> On Mon, Jan 06, 2014 at 06:02:35PM +0100, Florian Westphal wrote:
> >
> > Can you elaborate?
> > Yes, nf_ct_is_dying(ct) might be called for the wrong conntrack.
> >
> > But, in case we _think_ that its the right one we call
> > nf_ct_tuple_equal() to verify we indeed found the right one:
> >
> > h = ____nf_conntrack_find(net, zone, tuple, hash);
> > if (h) { // might be released right now, but page won't go away (SLAB_BY_RCU)
> > ct = nf_ct_tuplehash_to_ctrack(h);
> > if (unlikely(nf_ct_is_dying(ct) ||
> > !atomic_inc_not_zero(&ct->ct_general.use)))
> > // which means we should hit this path (0 ref).
> > h = NULL;
> > else {
> > // otherwise, it cannot go away from under us, since
> > // we own a reference now.
> > if (unlikely(!nf_ct_tuple_equal(tuple, &h->tuple) ||
> > nf_ct_zone(ct) != zone)) {
> > // if we get here, the entry got recycled on other cpu
> > // for a different tuple, we can bail out and drop
> > // the reference safely and re-try the lookup
> > nf_ct_put(ct);
> > goto begin;
> > }
> > }
>
> I think tuple may match if
>
> task 1 task 2 task 3
> nf_conntrack_find_get
> ____nf_conntrack_find
> destroy_conntrack
> hlist_nulls_del_rcu
> nf_conntrack_free
> kmem_cache_free
> __nf_conntrack_alloc
> kmem_cache_alloc
> if (nf_ct_is_dying(ct))
>
> data is not yet cleaned
>
> memset(&ct->tuplehash[IP_CT_DIR_MAX],
>
> No? Or there something obvious I'm missing?
IMHO this isn't obvious at all :-)
But, in the example above, the atomic_inc_not_zero() should fail
until after __nf_conntrack_alloc() re-inits the refcount to 1.
The mb there should make sure ____nf_conntrack_find() doesn't
find an outdated tuple before this.
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