On Mon, Jan 20, 2020 at 07:11 PM CET, Jakub Sitnicki wrote: > 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. I've sent out the partial revert you wanted: https://lore.kernel.org/netdev/20200121123147.706666-1-jakub@xxxxxxxxxxxxxx/T/#u But otherwise didn't manage to convince myself that we need to annotate access to sk_prot with READ/WRITE_ONCE in sk_psock_update/restore_proto. Instead, I believe we might need to the extend critical section that grabs sk_callback_lock in sock_map_link over tcp_bpf_init/reinit() to serialize the writers. Unless I'm missing the point here and you had some other race in mind? -jkbs
