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Date: Sat, 26 May 2018 21:01:00 -0400
From: Neil Horman <nhorman@...driver.com>
To: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Michael Tuexen <michael.tuexen@...chi.franken.de>,
Xin Long <lucien.xin@...il.com>,
network dev <netdev@...r.kernel.org>,
linux-sctp@...r.kernel.org, David Miller <davem@...emloft.net>,
David Ahern <dsa@...ulusnetworks.com>,
Eric Dumazet <edumazet@...gle.com>,
Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] sctp: not allow to set rto_min with a value below
200 msecs
On Sat, May 26, 2018 at 05:50:39PM +0200, Dmitry Vyukov wrote:
> On Sat, May 26, 2018 at 5:42 PM, Michael Tuexen
> <michael.tuexen@...chi.franken.de> wrote:
> >> 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
> >>>
> >>> --
> >>> To unsubscribe from this list: send the line "unsubscribe linux-sctp" in
> >>> the body of a message to majordomo@...r.kernel.org
> >>> More majordomo info at http://vger.kernel.org/majordomo-info.html
> >>>
> >> 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?
>
> Hi Michael,
>
> What value do they use?
>
> Xin, Neil, is there more principled way of ensuring that a timer won't
> cause a hard CPU stall? There are slow machines and there are slow
> kernels (in particular syzbot kernel has tons of debug configs
> enabled). 200ms _should_ not cause problems because we did not see
> them with tcp. But it's hard to say what's the low limit as we are
> trying to put a hard upper bound on execution time of a complex
> section of code. Is there something like cond_resched for timers?
Unfortunately, Theres not really a way to do conditional rescheduling of timers,
additionally, we have a problem because the timer is reset as a side effect of
the SCTP state machine, and so the execution time between timer updates has a
signifcant amount of jitter (meaning its a pretty hard value to calibrate,
unless you just select a 'safe' large value for the floor).
What we might could do (though this might impact the protocol function is change
the timer update side effects to simply set a flag, and consistently update the
timers on exit from sctp_do_sm, so they don't re-arm until all state machine
processing is complete. Anyone have any thoughts on that?
Neil
>
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