netdev - Re: [PATCH net] sctp: not allow to set rto

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Date:   Sat, 26 May 2018 17:42:22 +0200
From:   Michael Tuexen <michael.tuexen@...chi.franken.de>
To:     Neil Horman <nhorman@...driver.com>
Cc:     Xin Long <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>,
        linux-sctp@...r.kernel.org, davem@...emloft.net,
        David Ahern <dsa@...ulusnetworks.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        syzkaller@...glegroups.com
Subject: Re: [PATCH net] sctp: not allow to set rto_min with a value below 200
 msecs

> On 25. May 2018, at 21:13, Neil Horman <nhorman@...driver.com> wrote:
> 
> On Sat, May 26, 2018 at 01:41:02AM +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 to set rto_min with a value below
>> 200 msecs, which is based on TCP's, by either setsockopt or sysctl.
>> 
>> 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>
>> ---
>> include/net/sctp/constants.h |  1 +
>> net/sctp/socket.c            | 10 +++++++---
>> net/sctp/sysctl.c            |  3 ++-
>> 3 files changed, 10 insertions(+), 4 deletions(-)
>> 
>> diff --git a/include/net/sctp/constants.h b/include/net/sctp/constants.h
>> index 20ff237..2ee7a7b 100644
>> --- a/include/net/sctp/constants.h
>> +++ b/include/net/sctp/constants.h
>> @@ -277,6 +277,7 @@ enum { SCTP_MAX_GABS = 16 };
>> #define SCTP_RTO_INITIAL	(3 * 1000)
>> #define SCTP_RTO_MIN		(1 * 1000)
>> #define SCTP_RTO_MAX		(60 * 1000)
>> +#define SCTP_RTO_HARD_MIN	200
>> 
>> #define SCTP_RTO_ALPHA          3   /* 1/8 when converted to right shifts. */
>> #define SCTP_RTO_BETA           2   /* 1/4 when converted to right shifts. */
>> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
>> index ae7e7c6..6ef12c7 100644
>> --- a/net/sctp/socket.c
>> +++ b/net/sctp/socket.c
>> @@ -3029,7 +3029,8 @@ static int sctp_setsockopt_nodelay(struct sock *sk, char __user *optval,
>>  * be changed.
>>  *
>>  */
>> -static int sctp_setsockopt_rtoinfo(struct sock *sk, char __user *optval, unsigned int optlen)
>> +static int sctp_setsockopt_rtoinfo(struct sock *sk, char __user *optval,
>> +				   unsigned int optlen)
>> {
>> 	struct sctp_rtoinfo rtoinfo;
>> 	struct sctp_association *asoc;
>> @@ -3056,10 +3057,13 @@ static int sctp_setsockopt_rtoinfo(struct sock *sk, char __user *optval, unsigne
>> 	else
>> 		rto_max = asoc ? asoc->rto_max : sp->rtoinfo.srto_max;
>> 
>> -	if (rto_min)
>> +	if (rto_min) {
>> +		if (rto_min < SCTP_RTO_HARD_MIN)
>> +			return -EINVAL;
>> 		rto_min = asoc ? msecs_to_jiffies(rto_min) : rto_min;
>> -	else
>> +	} else {
>> 		rto_min = asoc ? asoc->rto_min : sp->rtoinfo.srto_min;
>> +	}
>> 
>> 	if (rto_min > rto_max)
>> 		return -EINVAL;
>> diff --git a/net/sctp/sysctl.c b/net/sctp/sysctl.c
>> index 33ca5b7..7ec854a 100644
>> --- a/net/sctp/sysctl.c
>> +++ b/net/sctp/sysctl.c
>> @@ -52,6 +52,7 @@ static int rto_alpha_min = 0;
>> static int rto_beta_min = 0;
>> static int rto_alpha_max = 1000;
>> static int rto_beta_max = 1000;
>> +static int rto_hard_min = SCTP_RTO_HARD_MIN;
>> 
>> static unsigned long max_autoclose_min = 0;
>> static unsigned long max_autoclose_max =
>> @@ -116,7 +117,7 @@ static struct ctl_table sctp_net_table[] = {
>> 		.maxlen		= sizeof(unsigned int),
>> 		.mode		= 0644,
>> 		.proc_handler	= proc_sctp_do_rto_min,
>> -		.extra1         = &one,
>> +		.extra1         = &rto_hard_min,
>> 		.extra2         = &init_net.sctp.rto_max
>> 	},
>> 	{
>> -- 
>> 2.1.0
>> 
>> --
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>> 
> Patch looks fine, you probably want to note this hard minimum in man(7) sctp as
> well
> 
I'm aware of some signalling networks which use RTO.min of smaller values than 200ms.
So could this be reduced?

Best regards
Michael
> Acked-by: Neil Horman <nhorman@...driver.com>
> 
> --
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