netdev - Re: [PATCH net] sctp: not allow transport timeout value less than HZ/5 for hb

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Date:   Tue, 5 Jun 2018 06:27:49 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        davem@...emloft.net, Eric Dumazet <edumazet@...gle.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, syzkaller@...glegroups.com
Subject: Re: [PATCH net] sctp: not allow transport timeout value less than
 HZ/5 for hb_timer

On Tue, Jun 05, 2018 at 12:16:58PM +0800, Xin Long wrote:
> syzbot reported a rcu_sched self-detected stall on CPU which is caused
> by too small value set on rto_min with SCTP_RTOINFO sockopt. With this
> value, hb_timer will get stuck there, as in its timer handler it starts
> this timer again with this value, then goes to the timer handler again.
> 
> This problem is there since very beginning, and thanks to Eric for the
> reproducer shared from a syzbot mail.
> 
> This patch fixes it by not allowing sctp_transport_timeout to return a
> smaller value than HZ/5 for hb_timer, which is based on TCP's min rto.
> 
> Note that it doesn't fix this issue by limiting rto_min, as some users
> are still using small rto and no proper value was found for it yet.
> 
> Reported-by: syzbot+3dcd59a1f907245f891f@...kaller.appspotmail.com
> Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> Signed-off-by: Xin Long <lucien.xin@...il.com>
> ---
>  net/sctp/transport.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sctp/transport.c b/net/sctp/transport.c
> index 47f82bd..03fc2c4 100644
> --- a/net/sctp/transport.c
> +++ b/net/sctp/transport.c
> @@ -634,7 +634,7 @@ unsigned long sctp_transport_timeout(struct sctp_transport *trans)
>  	    trans->state != SCTP_PF)
>  		timeout += trans->hbinterval;
>  
> -	return timeout;
> +	return max_t(unsigned long, timeout, HZ / 5);
>  }
>  
>  /* Reset transport variables to their initial values */
> -- 
> 2.1.0
> 
> 
Acked-by: Neil Horman <nhorman@...driver.com>