ax25: Fix NULL pointer dereferences in ax25 timers

The previous commit 7ec02f5a ("ax25: fix NPD bug in ax25_disconnect")
move ax25_disconnect into lock_sock() in order to prevent NPD bugs. But
there are race conditions that may lead to null pointer dereferences in
ax25_heartbeat_expiry(), ax25_t1timer_expiry(), ax25_t2timer_expiry(),
ax25_t3timer_expiry() and ax25_idletimer_expiry(), when we use
ax25_kill_by_device() to detach the ax25 device.

One of the race conditions that cause null pointer dereferences can be
shown as below:

      (Thread 1)                    |      (Thread 2)
ax25_connect()                      |
 ax25_std_establish_data_link()     |
  ax25_start_t1timer()              |
   mod_timer(&ax25->t1timer,..)     |
                                    | ax25_kill_by_device()
   (wait a time)                    |  ...
                                    |  s->ax25_dev = NULL; //(1)
   ax25_t1timer_expiry()            |
    ax25->ax25_dev->values[..] //(2)|  ...
     ...                            |

We set null to ax25_cb->ax25_dev in position (1) and dereference
the null pointer in position (2).

The corresponding fail log is shown below:
===============================================================
BUG: kernel NULL pointer dereference, address: 0000000000000050
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.17.0-rc6-00794-g45690b7d0
RIP: 0010:ax25_t1timer_expiry+0x12/0x40
...
Call Trace:
 call_timer_fn+0x21/0x120
 __run_timers.part.0+0x1ca/0x250
 run_timer_softirq+0x2c/0x60
 __do_softirq+0xef/0x2f3
 irq_exit_rcu+0xb6/0x100
 sysvec_apic_timer_interrupt+0xa2/0xd0
...

This patch moves ax25_disconnect() before s->ax25_dev = NULL
and uses del_timer_sync() to delete timers in ax25_disconnect().
If ax25_disconnect() is called by ax25_kill_by_device() or
ax25->ax25_dev is NULL, the reason in ax25_disconnect() will be
equal to ENETUNREACH, it will wait all timers to stop before we
set null to s->ax25_dev in ax25_kill_by_device().

Fixes: 7ec02f5a ("ax25: fix NPD bug in ax25_disconnect")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ax25/af_ax25.c

@@ -89,20 +89,20 @@ static void ax25_kill_by_device(struct net_device *dev)
 			sk = s->sk;
 			if (!sk) {
 				spin_unlock_bh(&ax25_list_lock);
-				s->ax25_dev = NULL;
 				ax25_disconnect(s, ENETUNREACH);
+				s->ax25_dev = NULL;
 				spin_lock_bh(&ax25_list_lock);
 				goto again;
 			}
 			sock_hold(sk);
 			spin_unlock_bh(&ax25_list_lock);
 			lock_sock(sk);
+			ax25_disconnect(s, ENETUNREACH);
 			s->ax25_dev = NULL;
 			if (sk->sk_socket) {
 				dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
 				ax25_dev_put(ax25_dev);
 			}
-			ax25_disconnect(s, ENETUNREACH);
 			release_sock(sk);
 			spin_lock_bh(&ax25_list_lock);
 			sock_put(sk);

net/ax25/ax25_subr.c

@@ -261,12 +261,20 @@ void ax25_disconnect(ax25_cb *ax25, int reason)
 {
 	ax25_clear_queues(ax25);
 
+	if (reason == ENETUNREACH) {
+		del_timer_sync(&ax25->timer);
+		del_timer_sync(&ax25->t1timer);
+		del_timer_sync(&ax25->t2timer);
+		del_timer_sync(&ax25->t3timer);
+		del_timer_sync(&ax25->idletimer);
+	} else {
 		if (!ax25->sk || !sock_flag(ax25->sk, SOCK_DESTROY))
 			ax25_stop_heartbeat(ax25);
 		ax25_stop_t1timer(ax25);
 		ax25_stop_t2timer(ax25);
 		ax25_stop_t3timer(ax25);
 		ax25_stop_idletimer(ax25);
+	}
 
 	ax25->state = AX25_STATE_0;