Re: [PATCH] sctp: Don't add the shutdown timer if its already been added

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Tue, Apr 28, 2020 at 12:58:48PM -0400, Neil Horman wrote:
> This BUG halt was reported a while back, but the patch somehow got
> missed:
> 
> PID: 2879   TASK: c16adaa0  CPU: 1   COMMAND: "sctpn"
>  #0 [f418dd28] crash_kexec at c04a7d8c
>  #1 [f418dd7c] oops_end at c0863e02
>  #2 [f418dd90] do_invalid_op at c040aaca
>  #3 [f418de28] error_code (via invalid_op) at c08631a5
>     EAX: f34baac0  EBX: 00000090  ECX: f418deb0  EDX: f5542950  EBP: 00000000
>     DS:  007b      ESI: f34ba800  ES:  007b      EDI: f418dea0  GS:  00e0
>     CS:  0060      EIP: c046fa5e  ERR: ffffffff  EFLAGS: 00010286
>  #4 [f418de5c] add_timer at c046fa5e
>  #5 [f418de68] sctp_do_sm at f8db8c77 [sctp]
>  #6 [f418df30] sctp_primitive_SHUTDOWN at f8dcc1b5 [sctp]
>  #7 [f418df48] inet_shutdown at c080baf9
>  #8 [f418df5c] sys_shutdown at c079eedf
>  #9 [f418df70] sys_socketcall at c079fe88
>     EAX: ffffffda  EBX: 0000000d  ECX: bfceea90  EDX: 0937af98
>     DS:  007b      ESI: 0000000c  ES:  007b      EDI: b7150ae4
>     SS:  007b      ESP: bfceea7c  EBP: bfceeaa8  GS:  0033
>     CS:  0073      EIP: b775c424  ERR: 00000066  EFLAGS: 00000282
> 
> It appears that the side effect that starts the shutdown timer was processed
> multiple times, which can happen as multiple paths can trigger it.  This of
> course leads to the BUG halt in add_timer getting called.
> 
> Fix seems pretty straightforward, just check before the timer is added if its
> already been started.  If it has mod the timer instead to min(current
> expiration, new expiration)
> 
> Its been tested but not confirmed to fix the problem, as the issue has only
> occured in production environments where test kernels are enjoined from being
> installed.  It appears to be a sane fix to me though.  Also, recentely,
> Jere found a reproducer posted on list to confirm that this resolves the
> issues
> 
> Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
> CC: Vlad Yasevich <vyasevich@gmail.com>
> CC: "David S. Miller" <davem@davemloft.net>
> CC: Jere Leppänen <jere.leppanen@nokia.com>
> CC: marcelo.leitner@gmail.com
> ---
>  net/sctp/sm_sideeffect.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/net/sctp/sm_sideeffect.c b/net/sctp/sm_sideeffect.c
> index 2bc29463e1dc..43a256b5a5f0 100644
> --- a/net/sctp/sm_sideeffect.c
> +++ b/net/sctp/sm_sideeffect.c
> @@ -1523,9 +1523,24 @@ static int sctp_cmd_interpreter(enum sctp_event_type event_type,
>  			timeout = asoc->timeouts[cmd->obj.to];
>  			BUG_ON(!timeout);
>  
> -			timer->expires = jiffies + timeout;
> -			sctp_association_hold(asoc);
> -			add_timer(timer);
> +			/*
> +			 * SCTP has a hard time with timer starts.  Because we process
> +			 * timer starts as side effects, it can be hard to tell if we
> +			 * have already started a timer or not, which leads to BUG
> +			 * halts when we call add_timer. So here, instead of just starting
> +			 * a timer, if the timer is already started, and just mod
> +			 * the timer with the shorter of the two expiration times
> +			 */
> +			if (timer_pending(timer)) {
> +				if (time_after(timer->expires, (jiffies + timeout))) {
> +					timer->expires = jiffies+timeout;
> +					mod_timer(timer, timer->expires);
> +				}
> +			} else {
> +				timer->expires = jiffies + timeout;
> +				sctp_association_hold(asoc);
> +				mod_timer(timer, timer->expires);
> +			}

Jere, can you then update it to use timer_reduce(), as you had
suggested?

>  			break;
>  
>  		case SCTP_CMD_TIMER_RESTART:
> -- 
> 2.25.3
>