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

[Kuniyuki Iwashima]

From: Wang Hai <wanghai38@xxxxxxxxxx>
Date: Mon, 24 Feb 2025 17:00:47 +0800
> 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@xxxxxxxxxx>
> Reviewed-by: Jason Xing <kerneljasonxing@xxxxxxxxx>

Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>