On Mon, Jan 20, 2020 at 06:00 PM CET, John Fastabend wrote: > Jakub Sitnicki wrote: >> On Sun, Jan 12, 2020 at 12:14 AM CET, John Fastabend wrote: >> > Jakub Sitnicki wrote: >> >> sk_msg and ULP frameworks override protocol callbacks pointer in >> >> sk->sk_prot, while TCP accesses it locklessly when cloning the listening >> >> socket. >> >> >> >> Once we enable use of listening sockets with sockmap (and hence sk_msg), >> >> there can be shared access to sk->sk_prot if socket is getting cloned while >> >> being inserted/deleted to/from the sockmap from another CPU. Mark the >> >> shared access with READ_ONCE/WRITE_ONCE annotations. >> >> >> >> Signed-off-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx> >> > >> > In sockmap side I fixed this by wrapping the access in a lock_sock[0]. So >> > Do you think this is still needed with that in mind? The bpf_clone call >> > is using sk_prot_creater and also setting the newsk's proto field. Even >> > if the listening parent sock was being deleted in parallel would that be >> > a problem? We don't touch sk_prot_creator from the tear down path. I've >> > only scanned the 3..11 patches so maybe the answer is below. If that is >> > the case probably an improved commit message would be helpful. >> >> I think it is needed. Not because of tcp_bpf_clone or that we access >> listener's sk_prot_creator from there, if I'm grasping your question. >> >> Either way I'm glad this came up. Let's go though my reasoning and >> verify it. tcp stack accesses the listener sk_prot while cloning it: >> >> tcp_v4_rcv >> sk = __inet_lookup_skb(...) >> tcp_check_req(sk) >> inet_csk(sk)->icsk_af_ops->syn_recv_sock >> tcp_v4_syn_recv_sock >> tcp_create_openreq_child >> inet_csk_clone_lock >> sk_clone_lock >> READ_ONCE(sk->sk_prot) >> >> It grabs a reference to the listener, but doesn't grab the sk_lock. >> >> On another CPU we can be inserting/removing the listener socket from the >> sockmap and writing to its sk_prot. We have the update and the remove >> path: >> >> sock_map_ops->map_update_elem >> sock_map_update_elem >> sock_map_update_common >> sock_map_link_no_progs >> tcp_bpf_init >> tcp_bpf_update_sk_prot >> sk_psock_update_proto >> WRITE_ONCE(sk->sk_prot, ops) >> >> sock_map_ops->map_delete_elem >> sock_map_delete_elem >> __sock_map_delete >> sock_map_unref >> sk_psock_put >> sk_psock_drop >> sk_psock_restore_proto >> tcp_update_ulp >> WRITE_ONCE(sk->sk_prot, proto) >> >> Following the guidelines from KTSAN project [0], sk_prot looks like a >> candidate for annotating it. At least on these 3 call paths. >> >> If that sounds correct, I can add it to the patch description. >> >> Thanks, >> -jkbs >> >> [0] https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE > > Hi Jakub, can push this to bpf tree as well? There is another case > already in-kernel where this is needed. If the map is removed while > a recvmsg is in flight. > > tcp_bpf_recvmsg() > psock = sk_psock_get(sk) <- refcnt 2 > lock_sock(sk); > ... > sock_map_free() <- refcnt 1 > release_sock(sk) > sk_psock_put() <- refcnt 0 > > Then can you add this diff as well I got a bit too carried away > with that. If your busy I can do it as well if you want. Thanks! Hi John, I get the race between map_free and tcp_bpf_recvmsg, and how we end up dropping psock on a path where we don't hold the sock lock. What a rare case, since we don't destory maps that often usually. However, I'm not sure I follow where shared lockless access to sk->sk_prot is in this case? Perhaps between drop path: sk_psock_put sk_psock_drop sk_psock_restore_proto WRITE_ONCE(sk->sk_prot, proto) ... and update path where we grab sk_callback_lock a little too late, that is after updating the proto? sock_map_update_common sock_map_link tcp_bpf_init tcp_bpf_update_sk_prot sk_psock_update_proto WRITE_ONCE(sk->sk_prot, ops) I'm getting v3 ready to post, so happy to help you spin these bits. I'll need to do it with a fresh head tomorrow, though. If I don't see any patches from you hit the ML, I'll split out the chunks that annotate sk_prot access in sk_psock_{retore,update}_proto and post them together with the revert you suggested below. -jkbs > > diff --git a/net/core/skmsg.c b/net/core/skmsg.c > index 3866d7e20c07..ded2d5227678 100644 > --- a/net/core/skmsg.c > +++ b/net/core/skmsg.c > @@ -594,8 +594,6 @@ EXPORT_SYMBOL_GPL(sk_psock_destroy); > > void sk_psock_drop(struct sock *sk, struct sk_psock *psock) > { > - sock_owned_by_me(sk); > - > sk_psock_cork_free(psock); > sk_psock_zap_ingress(psock);
