sctp: change to hold/put transport for proto_unreach_timer

A call trace was found in Hangbin's Codenomicon testing with debug kernel:

  [ 2615.981988] ODEBUG: free active (active state 0) object type: timer_list hint: sctp_generate_proto_unreach_event+0x0/0x3a0 [sctp]
  [ 2615.995050] WARNING: CPU: 17 PID: 0 at lib/debugobjects.c:328 debug_print_object+0x199/0x2b0
  [ 2616.095934] RIP: 0010:debug_print_object+0x199/0x2b0
  [ 2616.191533] Call Trace:
  [ 2616.194265]  <IRQ>
  [ 2616.202068]  debug_check_no_obj_freed+0x25e/0x3f0
  [ 2616.207336]  slab_free_freelist_hook+0xeb/0x140
  [ 2616.220971]  kfree+0xd6/0x2c0
  [ 2616.224293]  rcu_do_batch+0x3bd/0xc70
  [ 2616.243096]  rcu_core+0x8b9/0xd00
  [ 2616.256065]  __do_softirq+0x23d/0xacd
  [ 2616.260166]  irq_exit+0x236/0x2a0
  [ 2616.263879]  smp_apic_timer_interrupt+0x18d/0x620
  [ 2616.269138]  apic_timer_interrupt+0xf/0x20
  [ 2616.273711]  </IRQ>

This is because it holds asoc when transport->proto_unreach_timer starts
and puts asoc when the timer stops, and without holding transport the
transport could be freed when the timer is still running.

So fix it by holding/putting transport instead for proto_unreach_timer
in transport, just like other timers in transport.

v1->v2:
  - Also use sctp_transport_put() for the "out_unlock:" path in
    sctp_generate_proto_unreach_event(), as Marcelo noticed.

Fixes: 50b5d6ad63 ("sctp: Fix a race between ICMP protocol unreachable and connect()")
Reported-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Link: https://lore.kernel.org/r/102788809b554958b13b95d33440f5448113b8d6.1605331373.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/sctp/input.c

@@ -449,7 +449,7 @@ void sctp_icmp_proto_unreachable(struct sock *sk,
 		else {
 			if (!mod_timer(&t->proto_unreach_timer,
 						jiffies + (HZ/20)))
-				sctp_association_hold(asoc);
+				sctp_transport_hold(t);
 		}
 	} else {
 		struct net *net = sock_net(sk);
@@ -458,7 +458,7 @@ void sctp_icmp_proto_unreachable(struct sock *sk,
 			 "encountered!\n", __func__);
 
 		if (del_timer(&t->proto_unreach_timer))
-			sctp_association_put(asoc);
+			sctp_transport_put(t);
 
 		sctp_do_sm(net, SCTP_EVENT_T_OTHER,
 			   SCTP_ST_OTHER(SCTP_EVENT_ICMP_PROTO_UNREACH),

net/sctp/sm_sideeffect.c

@@ -419,7 +419,7 @@ void sctp_generate_proto_unreach_event(struct timer_list *t)
 		/* Try again later.  */
 		if (!mod_timer(&transport->proto_unreach_timer,
 				jiffies + (HZ/20)))
-			sctp_association_hold(asoc);
+			sctp_transport_hold(transport);
 		goto out_unlock;
 	}
 
@@ -435,7 +435,7 @@ void sctp_generate_proto_unreach_event(struct timer_list *t)
 
 out_unlock:
 	bh_unlock_sock(sk);
-	sctp_association_put(asoc);
+	sctp_transport_put(transport);
 }
 
  /* Handle the timeout of the RE-CONFIG timer. */

net/sctp/transport.c

@@ -133,7 +133,7 @@ void sctp_transport_free(struct sctp_transport *transport)
 
 	/* Delete the ICMP proto unreachable timer if it's active. */
 	if (del_timer(&transport->proto_unreach_timer))
-		sctp_association_put(transport->asoc);
+		sctp_transport_put(transport);
 
 	sctp_transport_put(transport);
 }