Re: [PATCH net] net/smc: fix panic smc_tcp_syn_recv_sock() while closing listen socket

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On 05.10.23 20:14, Wenjia Zhang wrote:


On 26.09.23 11:06, D. Wythe wrote:


On 9/26/23 3:18 PM, Alexandra Winter wrote:

On 26.09.23 05:00, D. Wythe wrote:
You are right. The key point is how to ensure the valid of smc sock during the life time of clc sock, If so, READ_ONCE is good enough. Unfortunately, I found  that there are no such guarantee, so it's still a life-time problem.
Did you discover a scenario, where clc sock could live longer than smc sock? Wouldn't that be a dangerous scenario in itself? I still have some hope that the lifetime of an smc socket is by design longer
than that of the corresponding tcp socket.


Hi Alexandra,

Yes there is. Considering scenario:

tcp_v4_rcv(skb)

/* req sock */
reqsk = _inet_lookup_skb(skb)

/* listen sock */
sk = reqsk(reqsk)->rsk_listener;
sock_hold(sk);
tcp_check_req(sk)


                                                 smc_release /* release smc listen sock */
                                                 __smc_release
smc_close_active()         /*  smc_sk->sk_state = SMC_CLOSED; */
                                                     if (smc_sk->sk_state == SMC_CLOSED)
smc_clcsock_release();
sock_release(clcsk);        /* close clcsock */
     sock_put(sk);              /* might not  the final refcnt */

sock_put(smc_sk)    /* might be the final refcnt of smc_sock  */

syn_recv_sock(sk...)
/* might be the final refcnt of tcp listen sock */
sock_put(sk);

Fortunately, this scenario only affects smc_syn_recv_sock and smc_hs_congested, as other callbacks already have locks to protect smc, which can guarantee that the sk_user_data is either NULL (set in smc_close_active) or valid under the lock. I'm kind of confused with this scenario. How could the
smc_clcsock_release()->sock_release(clcsk) happen?
Because the syn_recv_sock happens short prior to accept(), that means that the &smc->tcp_listen_work is already triggered but the real accept() is still not happening. At this moment, the incoming connection is being added into the accept queue. Thus, if the sk->sk_state is changed from SMC_LISTEN to SMC_CLOSED in smc_close_active(), there is still "flush_work(&smc->tcp_listen_work);" after that. That ensures the smc_clcsock_release() should not happen, if smc_clcsock_accept() is not finished. Do you think that the execution of the &smc->tcp_listen_work is already done? Or am I missing something?
 > Hi wenjia,
>
> Sorry for late reply, we have just returned from vacation.
>
> The smc_clcsock_release here release the listen clcsock rather than
> the child clcsock.
> So the flush_work might not be helpful for this scenario.
>
> Best wishes,
> D. Wythe

It seems like that I lost some mails these days :-( Just saw your answer.

Maybe I didn't describe my thought clearly. Following data flow is your scenario, right?
			–
(sk_state == SMC_LISTEN)|
tcp_check_req()		| smc_release()
			| ->__smc_release()
			|   -> smc_close_active()
			|     -> sk->sk_state = SMC_CLOSED;
			|     -> ...
			|     -> smc->clcsock->sk->sk_user_data = NULL;
			|     -> ...
			|*1)  -> flush_work(&smc->tcp_listen_work);
			|*4)
			|	-> smc_clcsock_accept()
	    		|         -> kernel_accept()
			| 	    -> inet_csk_accept()
			|*5)
			|   if (sk->sk_state == SMC_CLOSED)
			|*3)-> smc_clcsock_release()
-> syn_recv_sock()   *2)|
			|
			v
My question is how the smc_clcsock_release() could happen after the syn_recv_sock()? IMO, the syn_recv_sock() should be called during the &smc->tcp_listen_work, which is corresponding to lsmc (listen smc). And in smc_clcsock_accept(), the lsmc->clcsock as the listening socket goes on to be used to accept a new connection. If the &smc->tcp_listen_work is not finished, *1) will wait for its finishing. It can only happen in following situation:
*4) sk_state is SMC_CLOSED, then no connection is accepted.
*5) old sk_state is SMC_LISTEN, TCP accept is successful. But current sk_state is SMC_CLOSED. Thus, no new smc connection.

What do you think? Please let me know if I have any lapse of thought.

Thanks,
Wenjia



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