On 08/13, Daniel Borkmann wrote:
> On 8/12/19 7:52 PM, Stanislav Fomichev wrote:
> > On 08/12, Daniel Borkmann wrote:
> > > On 8/9/19 6:10 PM, Stanislav Fomichev wrote:
> > > > Add new helper bpf_sk_storage_clone which optionally clones sk storage
> > > > and call it from sk_clone_lock.
> > > >
> > > > Cc: Martin KaFai Lau <kafai@xxxxxx>
> > > > Cc: Yonghong Song <yhs@xxxxxx>
> > > > Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
> > > [...]
> > > > +int bpf_sk_storage_clone(const struct sock *sk, struct sock *newsk)
> > > > +{
> > > > + struct bpf_sk_storage *new_sk_storage = NULL;
> > > > + struct bpf_sk_storage *sk_storage;
> > > > + struct bpf_sk_storage_elem *selem;
> > > > + int ret;
> > > > +
> > > > + RCU_INIT_POINTER(newsk->sk_bpf_storage, NULL);
> > > > +
> > > > + rcu_read_lock();
> > > > + sk_storage = rcu_dereference(sk->sk_bpf_storage);
> > > > +
> > > > + if (!sk_storage || hlist_empty(&sk_storage->list))
> > > > + goto out;
> > > > +
> > > > + hlist_for_each_entry_rcu(selem, &sk_storage->list, snode) {
> > > > + struct bpf_sk_storage_elem *copy_selem;
> > > > + struct bpf_sk_storage_map *smap;
> > > > + struct bpf_map *map;
> > > > + int refold;
> > > > +
> > > > + smap = rcu_dereference(SDATA(selem)->smap);
> > > > + if (!(smap->map.map_flags & BPF_F_CLONE))
> > > > + continue;
> > > > +
> > > > + map = bpf_map_inc_not_zero(&smap->map, false);
> > > > + if (IS_ERR(map))
> > > > + continue;
> > > > +
> > > > + copy_selem = bpf_sk_storage_clone_elem(newsk, smap, selem);
> > > > + if (!copy_selem) {
> > > > + ret = -ENOMEM;
> > > > + bpf_map_put(map);
> > > > + goto err;
> > > > + }
> > > > +
> > > > + if (new_sk_storage) {
> > > > + selem_link_map(smap, copy_selem);
> > > > + __selem_link_sk(new_sk_storage, copy_selem);
> > > > + } else {
> > > > + ret = sk_storage_alloc(newsk, smap, copy_selem);
> > > > + if (ret) {
> > > > + kfree(copy_selem);
> > > > + atomic_sub(smap->elem_size,
> > > > + &newsk->sk_omem_alloc);
> > > > + bpf_map_put(map);
> > > > + goto err;
> > > > + }
> > > > +
> > > > + new_sk_storage = rcu_dereference(copy_selem->sk_storage);
> > > > + }
> > > > + bpf_map_put(map);
> > >
> > > The map get/put combination /under/ RCU read lock seems a bit odd to me, could
> > > you exactly describe the race that this would be preventing?
> > There is a race between sk storage release and sk storage clone.
> > bpf_sk_storage_map_free uses synchronize_rcu to wait for all existing
> > users to finish and the new ones are prevented via map's refcnt being
> > zero; we need to do something like that for the clone.
> > Martin suggested to use bpf_map_inc_not_zero/bpf_map_put.
> > If I read everythin correctly, I think without map_inc/map_put we
> > get the following race:
> >
> > CPU0 CPU1
> >
> > bpf_map_put
> > bpf_sk_storage_map_free(smap)
> > synchronize_rcu
> >
> > // no more users via bpf or
> > // syscall, but clone
> > // can still happen
> >
> > for each (bucket)
> > selem_unlink
> > selem_unlink_map(smap)
> >
> > // adding anything at
> > // this point to the
> > // bucket will leak
> >
> > rcu_read_lock
> > tcp_v4_rcv
> > tcp_v4_do_rcv
> > // sk is lockless TCP_LISTEN
> > tcp_v4_cookie_check
> > tcp_v4_syn_recv_sock
> > bpf_sk_storage_clone
> > rcu_dereference(sk->sk_bpf_storage)
> > selem_link_map(smap, copy)
> > // adding new element to the
> > // map -> leak
> > rcu_read_unlock
> >
> > selem_unlink_sk
> > sk->sk_bpf_storage = NULL
> >
> > synchronize_rcu
> >
>
> Makes sense, thanks for clarifying. Perhaps a small comment on top of
> the bpf_map_inc_not_zero() would be great as well, so it's immediately
> clear also from this location when reading the code why this is done.
Sure, no problem, will have something similar to what I have before
synchronize_rcu in bpf_sk_storage_map_free.
> Thanks,
> Daniel
