netdev - Re: [PATCH v3 net] tcp: Defer ts_recent changes until req is owned

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Message-ID: <CANn89iLXxRKH_tQCggxn=yuNk=+QEZ+uJuxYozxO7TEpkJHB0w@mail.gmail.com>
Date: Mon, 24 Feb 2025 10:22:23 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Wang Hai <wanghai38@...wei.com>
Cc: kerneljasonxing@...il.com, ncardwell@...gle.com, kuniyu@...zon.com, 
	davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org, pabeni@...hat.com, 
	horms@...nel.org, zhangchangzhong@...wei.com, liujian56@...wei.com, 
	yuehaibing@...wei.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 net] tcp: Defer ts_recent changes until req is owned

On Mon, Feb 24, 2025 at 10:03 AM Wang Hai <wanghai38@...wei.com> wrote:
>
> Recently a bug was discovered where the server had entered TCP_ESTABLISHED
> state, but the upper layers were not notified.
>
> The same 5-tuple packet may be processed by different CPUSs, so two
> CPUs may receive different ack packets at the same time when the
> state is TCP_NEW_SYN_RECV.
>
> In that case, req->ts_recent in tcp_check_req may be changed concurrently,
> which will probably cause the newsk's ts_recent to be incorrectly large.
> So that tcp_validate_incoming will fail. At this point, newsk will not be
> able to enter the TCP_ESTABLISHED.
>
> cpu1                                    cpu2
> tcp_check_req
>                                         tcp_check_req
>  req->ts_recent = rcv_tsval = t1
>                                          req->ts_recent = rcv_tsval = t2
>
>  syn_recv_sock
>   tcp_sk(child)->rx_opt.ts_recent = req->ts_recent = t2 // t1 < t2
> tcp_child_process
>  tcp_rcv_state_process
>   tcp_validate_incoming
>    tcp_paws_check
>     if ((s32)(rx_opt->ts_recent - rx_opt->rcv_tsval) <= paws_win)
>         // t2 - t1 > paws_win, failed
>                                         tcp_v4_do_rcv
>                                          tcp_rcv_state_process
>                                          // TCP_ESTABLISHED
>
> The cpu2's skb or a newly received skb will call tcp_v4_do_rcv to get
> the newsk into the TCP_ESTABLISHED state, but at this point it is no
> longer possible to notify the upper layer application. A notification
> mechanism could be added here, but the fix is more complex, so the
> current fix is used.
>
> In tcp_check_req, req->ts_recent is used to assign a value to
> tcp_sk(child)->rx_opt.ts_recent, so removing the change in req->ts_recent
> and changing tcp_sk(child)->rx_opt.ts_recent directly after owning the
> req fixes this bug.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Wang Hai <wanghai38@...wei.com>
> Reviewed-by: Jason Xing <kerneljasonxing@...il.com>
> ---

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Thanks for the fix !