Subject: Re: [PATCH bpf-next v2 2/4] bpf: support cloning sk storage on accept()

From: Stanislav Fomichev <[email-masked]>
Date: Tue, 13 Aug 2019 14:28:47 -0700

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 <[email-masked]>
> > > > Cc: Yonghong Song <[email-masked]>
> > > > Signed-off-by: Stanislav Fomichev <[email-masked]>
> > > [...]
> > > > +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