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

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Message-ID: <aSnd-8wK-wNx8cV6@t14s.localdomain>
Date: Fri, 28 Nov 2025 14:38:03 -0300
From: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Hithashree Bojanala <bojanalahithashri@...il.com>,
	linux-kernel@...r.kernel.org, davem@...emloft.net,
	syzbot+e94b93511bda261f4c43@...kaller.appspotmail.com
Subject: Re: [PATCH] Fix refcount bug in time scheduled paths

On Fri, Nov 28, 2025 at 09:05:05AM -0800, Eric Dumazet wrote:
> On Fri, Nov 28, 2025 at 8:40 AM Hithashree Bojanala
> <bojanalahithashri@...il.com> 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@...kaller.appspotmail.com
> > Fixes: https://syzkaller.appspot.com/bug?extid=e94b93511bda261f4c43
> > Signed-off-by: Hithashree Bojanala <bojanalahithashri@...il.com>
> > ---
> >  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.

I was just scratching my head here thinking "waaaat"... Thanks Eric :)

Btw, Hithashree, next time please don't forget to post networking
patches to netdev@ instead, otherwise they won't be picked up.

> 
> 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.