[PATCH] Fix refcount bug in time scheduled paths

[Hithashree Bojanala]

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;
 	}
 
-- 
2.47.0