ax25: Fix ax25 session cleanup problems

There are session cleanup problems in ax25_release() and
ax25_disconnect(). If we setup a session and then disconnect,
the disconnected session is still in "LISTENING" state that
is shown below.

Active AX.25 sockets
Dest Source Device State Vr/Vs Send-Q Recv-Q
DL9SAU-4 DL9SAU-3 ??? LISTENING 000/000 0 0
DL9SAU-3 DL9SAU-4 ??? LISTENING 000/000 0 0

The first reason is caused by del_timer_sync() in ax25_release().
The timers of ax25 are used for correct session cleanup. If we use
ax25_release() to close ax25 sessions and ax25_dev is not null,
the del_timer_sync() functions in ax25_release() will execute.
As a result, the sessions could not be cleaned up correctly,
because the timers have stopped.

In order to solve this problem, this patch adds a device_up flag
in ax25_dev in order to judge whether the device is up. If there
are sessions to be cleaned up, the del_timer_sync() in
ax25_release() will not execute. What's more, we add ax25_cb_del()
in ax25_kill_by_device(), because the timers have been stopped
and there are no functions that could delete ax25_cb if we do not
call ax25_release(). Finally, we reorder the position of
ax25_list_lock in ax25_cb_del() in order to synchronize among
different functions that call ax25_cb_del().

The second reason is caused by improper check in ax25_disconnect().
The incoming ax25 sessions which ax25->sk is null will close
heartbeat timer, because the check "if(!ax25->sk || ..)" is
satisfied. As a result, the session could not be cleaned up properly.

In order to solve this problem, this patch changes the improper
check to "if(ax25->sk && ..)" in ax25_disconnect().

What`s more, the ax25_disconnect() may be called twice, which is
not necessary. For example, ax25_kill_by_device() calls
ax25_disconnect() and sets ax25->state to AX25_STATE_0, but
ax25_release() calls ax25_disconnect() again.

In order to solve this problem, this patch add a check in
ax25_release(). If the flag of ax25->sk equals to SOCK_DEAD,
the ax25_disconnect() in ax25_release() should not be executed.

Fixes: 82e31755e55f ("ax25: Fix UAF bugs in ax25 timers")
Fixes: 8a367e74c012 ("ax25: Fix segfault after sock connection timeout")
Reported-and-tested-by: Thomas Osterried <[e-mail]>
Signed-off-by: Duoming Zhou <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

include/net/ax25.h

···
 	ax25_dama_info		dama;
 #endif
 	refcount_t		refcount;
+	bool device_up;
 } ax25_dev;
 
 typedef struct ax25_cb {

net/ax25/af_ax25.c

···
  */
 static void ax25_cb_del(ax25_cb *ax25)
 {
+	spin_lock_bh(&ax25_list_lock);
 	if (!hlist_unhashed(&ax25->ax25_node)) {
-		spin_lock_bh(&ax25_list_lock);
 		hlist_del_init(&ax25->ax25_node);
-		spin_unlock_bh(&ax25_list_lock);
 		ax25_cb_put(ax25);
 	}
+	spin_unlock_bh(&ax25_list_lock);
 }
 
 /*
···
 
 	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
 		return;
+	ax25_dev->device_up = false;
 
 	spin_lock_bh(&ax25_list_lock);
 again:
···
 				spin_unlock_bh(&ax25_list_lock);
 				ax25_disconnect(s, ENETUNREACH);
 				s->ax25_dev = NULL;
+				ax25_cb_del(s);
 				spin_lock_bh(&ax25_list_lock);
 				goto again;
 			}
···
 				dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
 				ax25_dev_put(ax25_dev);
 			}
+			ax25_cb_del(s);
 			release_sock(sk);
 			spin_lock_bh(&ax25_list_lock);
 			sock_put(sk);
···
 	if (sk->sk_type == SOCK_SEQPACKET) {
 		switch (ax25->state) {
 		case AX25_STATE_0:
-			release_sock(sk);
-			ax25_disconnect(ax25, 0);
-			lock_sock(sk);
+			if (!sock_flag(ax25->sk, SOCK_DEAD)) {
+				release_sock(sk);
+				ax25_disconnect(ax25, 0);
+				lock_sock(sk);
+			}
 			ax25_destroy_socket(ax25);
 			break;
 
···
 		ax25_destroy_socket(ax25);
 	}
 	if (ax25_dev) {
-		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);
+		if (!ax25_dev->device_up) {
+			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);
+		}
 		dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
 		ax25_dev_put(ax25_dev);
 	}

net/ax25/ax25_dev.c

···
 	ax25_dev->dev     = dev;
 	dev_hold_track(dev, &ax25_dev->dev_tracker, GFP_ATOMIC);
 	ax25_dev->forward = NULL;
+	ax25_dev->device_up = true;
 
 	ax25_dev->values[AX25_VALUES_IPDEFMODE] = AX25_DEF_IPDEFMODE;
 	ax25_dev->values[AX25_VALUES_AXDEFMODE] = AX25_DEF_AXDEFMODE;

net/ax25/ax25_subr.c

···
 		del_timer_sync(&ax25->t3timer);
 		del_timer_sync(&ax25->idletimer);
 	} else {
-		if (!ax25->sk || !sock_flag(ax25->sk, SOCK_DESTROY))
+		if (ax25->sk && !sock_flag(ax25->sk, SOCK_DESTROY))
 			ax25_stop_heartbeat(ax25);
 		ax25_stop_t1timer(ax25);
 		ax25_stop_t2timer(ax25);