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Date: Tue, 13 Aug 2019 23:12:50 +0200 From: Daniel Borkmann <daniel@...earbox.net> To: Stanislav Fomichev <sdf@...ichev.me> Cc: Stanislav Fomichev <sdf@...gle.com>, netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net, ast@...nel.org, Martin KaFai Lau <kafai@...com>, Yonghong Song <yhs@...com> Subject: Re: [PATCH bpf-next v2 2/4] bpf: support cloning sk storage on accept() 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@...com> >>> Cc: Yonghong Song <yhs@...com> >>> Signed-off-by: Stanislav Fomichev <sdf@...gle.com> >> [...] >>> +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. Thanks, Daniel
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