netdev - Re: [Patch bpf-next v4 04/11] skmsg: avoid lock_sock() in sk_psock

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Date:   Sat, 13 Mar 2021 09:32:17 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, duanxiongchun@...edance.com,
        Dongdong Wang <wangdongdong.6@...edance.com>,
        Jiang Wang <jiang.wang@...edance.com>,
        Cong Wang <cong.wang@...edance.com>,
        John Fastabend <john.fastabend@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [Patch bpf-next v4 04/11] skmsg: avoid lock_sock() in sk_psock_backlog()

On Fri, Mar 12, 2021 at 4:02 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> On Wed, Mar 10, 2021 at 06:32 AM CET, Cong Wang wrote:
> > diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> > index dd53a7771d7e..26ba47b099f1 100644
> > --- a/net/core/sock_map.c
> > +++ b/net/core/sock_map.c
> > @@ -1540,6 +1540,7 @@ void sock_map_close(struct sock *sk, long timeout)
> >       saved_close = psock->saved_close;
> >       sock_map_remove_links(sk, psock);
> >       rcu_read_unlock();
> > +     sk_psock_purge(psock);
> >       release_sock(sk);
> >       saved_close(sk, timeout);
> >  }
>
> Nothing stops sk_psock_backlog from running after sk_psock_purge:
>
>
> CPU 1                                                   CPU 2
>
> sk_psock_skb_redirect()
>   sk_psock(sk_other)
>   sock_flag(sk_other, SOCK_DEAD)
>   sk_psock_test_state(psock_other,
>                       SK_PSOCK_TX_ENABLED)
>                                                         sk_psock_purge()
>   skb_queue_tail(&psock_other->ingress_skb, skb)
>   schedule_work(&psock_other->work)
>
>
> And sock_orphan can run while we're in sendmsg/sendpage_unlocked:
>
>
> CPU 1                                                   CPU 2
>
> sk_psock_backlog
>   ...
>   sendmsg_unlocked
>     sock = sk->sk_socket
>                                                         tcp_close
>                                                           __tcp_close
>                                                             sock_orphan
>     kernel_sendmsg(sock, msg, vec, num, size)
>
>
> So, after this change, without lock_sock in sk_psock_backlog, we will
> not block tcp_close from running.
>
> This makes me think that the process socket can get released from under
> us, before kernel_sendmsg/sendpage runs.

I think you are right, I thought socket is orphaned in inet_release(), clearly
I was wrong. But, I'd argue in the above scenario, the packet should not
be even queued in the first place, as SK_PSOCK_TX_ENABLED is going
to be cleared, so I think the right fix is probably to make clearing psock
state and queuing the packet under a spinlock.

Thanks.