Re: [PATCH] Fix refcount bug in time scheduled paths

[Eric Dumazet]

On Fri, Nov 28, 2025 at 8:40 AM Hithashree Bojanala
<bojanalahithashri@xxxxxxxxx> wrote:
>
> The SCTP heartbeat timer callback can cause a refcount underflow when
> rescheduling the timer. The issue occurs when mod_timer() is called
> inside sctp_generate_heartbeat_event() to reschedule an already-pending
> timer.
>
> The current approach only takes a reference if mod_timer() returns 0
> (timer was not pending). However, when rescheduling inside a timer
> callback, we're consuming the reference that was held for the current
> timer firing. If we reschedule without taking a new reference, the
> subsequent timer callback will do sctp_transport_put() without a
> corresponding hold, leading to refcount underflow.
>
> The fix is to always take a reference when rescheduling inside a timer
> callback, since the callback will always drop a reference at the end.
>
> Reported-by: syzbot+e94b93511bda261f4c43@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: https://syzkaller.appspot.com/bug?extid=e94b93511bda261f4c43
> Signed-off-by: Hithashree Bojanala <bojanalahithashri@xxxxxxxxx>
> ---
>  net/sctp/sm_sideeffect.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/net/sctp/sm_sideeffect.c b/net/sctp/sm_sideeffect.c
> index 424f10a6fdba..733617781ed9 100644
> --- a/net/sctp/sm_sideeffect.c
> +++ b/net/sctp/sm_sideeffect.c
> @@ -377,9 +377,10 @@ void sctp_generate_heartbeat_event(struct timer_list *t)
>         if (sock_owned_by_user(sk)) {
>                 pr_debug("%s: sock is busy\n", __func__);
>
> -               /* Try again later.  */
> -               if (!mod_timer(&transport->hb_timer, jiffies + (HZ/20)))
> -                       sctp_transport_hold(transport);
> +               /* Always hold a reference when rescheduling inside timer callback
> +               * because this callback will put the reference at the end */
> +               sctp_transport_hold(transport);
> +               mod_timer(&transport->hb_timer, jiffies + (HZ/20));
>                 goto out_unlock;
>         }
>
> @@ -388,8 +389,10 @@ void sctp_generate_heartbeat_event(struct timer_list *t)
>         timeout = sctp_transport_timeout(transport);
>         if (elapsed < timeout) {
>                 elapsed = timeout - elapsed;
> -               if (!mod_timer(&transport->hb_timer, jiffies + elapsed))
> -                       sctp_transport_hold(transport);
> +               /* Always hold a reference when rescheduling inside timer callback
> +               * because this callback will put the reference at the end*/
> +               sctp_transport_hold(transport);
> +               mod_timer(&transport->hb_timer, jiffies + elapsed);
>                 goto out_unlock;
>         }

sk_reset_timer() has been using this construct for years, it can be
called from timer handlers just fine.

Can you explain how you have tested this patch ?

Beware that syzbot reports can sometimes point to some fine piece of
code, that can misbehave
if another layer did a random memory mangling.