netdev - RE: [PATHC bpf v2] tcp_bpf: Fix one concurrency problem in the tcp_bpf_send

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Message-ID: <61781010b20b3_108a220859@john-XPS-13-9370.notmuch>
Date:   Tue, 26 Oct 2021 07:26:24 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Liu Jian <liujian56@...wei.com>, john.fastabend@...il.com,
        daniel@...earbox.net, jakub@...udflare.com, lmb@...udflare.com,
        edumazet@...gle.com, davem@...emloft.net, yoshfuji@...ux-ipv6.org,
        dsahern@...nel.org, kuba@...nel.org, ast@...nel.org,
        andrii@...nel.org, kafai@...com, songliubraving@...com, yhs@...com,
        kpsingh@...nel.org, netdev@...r.kernel.org, bpf@...r.kernel.org
Cc:     liujian56@...wei.com
Subject: RE: [PATHC bpf v2] tcp_bpf: Fix one concurrency problem in the
 tcp_bpf_send_verdict function

Liu Jian wrote:
> With two Msgs, msgA and msgB and a user doing nonblocking sendmsg calls (or
> multiple cores) on a single socket 'sk' we could get the following flow.
> 
>  msgA, sk                               msgB, sk
>  -----------                            ---------------
>  tcp_bpf_sendmsg()
>  lock(sk)
>  psock = sk->psock
>                                         tcp_bpf_sendmsg()
>                                         lock(sk) ... blocking
> tcp_bpf_send_verdict
> if (psock->eval == NONE)
>    psock->eval = sk_psock_msg_verdict
>  ..
>  < handle SK_REDIRECT case >
>    release_sock(sk)                     < lock dropped so grab here >
>    ret = tcp_bpf_sendmsg_redir
>                                         psock = sk->psock
>                                         tcp_bpf_send_verdict
>  lock_sock(sk) ... blocking on B
>                                         if (psock->eval == NONE) <- boom.
>                                          psock->eval will have msgA state
> 
> The problem here is we dropped the lock on msgA and grabbed it with msgB.
> Now we have old state in psock and importantly psock->eval has not been
> cleared. So msgB will run whatever action was done on A and the verdict
> program may never see it.
> 
> Fixes: 604326b41a6fb ("bpf, sockmap: convert to generic sk_msg interface")
> Signed-off-by: Liu Jian <liujian56@...wei.com>

Yep thanks for digging into this. Nice catch. And commit looks good now.

Acked-by: John Fastabend <john.fastabend@...il.com>