Re: [bug report] one possible out-of-order issue in sockmap

[Cong Wang <xiyou.wangcong@gmail.com>]

On Sat, Sep 24, 2022 at 07:59:15AM +0000, liujian (CE) wrote:
> Hello,
> 
> I had a scp failure problem here. I analyze the code, and the reasons may be as follows:
> 
> From commit e7a5f1f1cd00 ("bpf/sockmap: Read psock ingress_msg before
>  sk_receive_queue", if we use sockops (BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB
> and BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB) to enable socket's sockmap
> function, and don't enable strparse and verdict function, the out-of-order
> problem may occur in the following process.
> 
> client SK                                   server SK
> --------------------------------------------------------------------------
> tcp_rcv_synsent_state_process
>   tcp_finish_connect
>     tcp_init_transfer
>       tcp_set_state(sk, TCP_ESTABLISHED);
>       // insert SK to sockmap
>     wake up waitter
>     tcp_send_ack
> 
> tcp_bpf_sendmsg(msgA)
> // msgA will go tcp stack
>                                             tcp_rcv_state_process
>                                               tcp_init_transfer
>                                                 //insert SK to sockmap
>                                               tcp_set_state(sk,
>                                                      TCP_ESTABLISHED)
>                                               wake up waitter

Here after the socket is inserted to a sockmap, its ->sk_data_ready() is
already replaced with sk_psock_verdict_data_ready(), so msgA should go
to sockmap, not TCP stack?

> tcp_bpf_sendmsg(msgB)
> // msgB go sockmap
>                                               tcp_bpf_recvmsg
>                                                 //msgB, out-of-order
>                                               tcp_bpf_recvmsg
>                                                 //msgA, out-of-order
> 
> 
> Even if msgA arrives earlier than msgB (in most cases), tcp_bpf_recvmsg receives msg from the psock queue first.
> The worst case is that msgA waits for serverSK to change to TCP_ESTABLISHED in the protocol stack. msgA may arrive at the serverSK receive queue later than msgB.
> If msgA befor than msgB, 
> 
> If the ACK packets of the three-way TCP handshake are dropped for a period of time, the OOO problem is easily reproduced.
> 
> iptables -A INPUT -p tcp -m tcp --dport 5006 --tcp-flags SYN,RST,ACK,FIN ACK -j DROP
> ...
> iptables -D INPUT -p tcp -m tcp --dport 5006 --tcp-flags SYN,RST,ACK,FIN ACK -j DROP
> 
> Best Wishes
> Liu Jian