Andrey Vagin <avagin@xxxxxxxxxx> wrote:
> Lets look at destroy_conntrack:
>
> hlist_nulls_del_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode);
> ...
> nf_conntrack_free(ct)
> kmem_cache_free(net->ct.nf_conntrack_cachep, ct);
>
> The hash is protected by rcu, so readers look up conntracks without
> locks.
> A conntrack is removed from the hash, but in this moment a few readers
> still can use the conntrack, so if we call kmem_cache_free now, all
> readers will read released object.
>
> Bellow you can find more tricky race condition of three tasks.
>
> 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
> memset(&ct->tuplehash[IP_CT_DIR_MAX],
> if (nf_ct_is_dying(ct))
>
> In this case the task 2 will not understand, that it uses a wrong
> conntrack.
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;
}
}
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