netrom: Decrease sock refcount when sock timers expire

Commit 63346650 ("netrom: switch to sock timer API") switched to use
sock timer API. It replaces mod_timer() by sk_reset_timer(), and
del_timer() by sk_stop_timer().

Function sk_reset_timer() will increase the refcount of sock if it is
called on an inactive timer, hence, in case the timer expires, we need to
decrease the refcount ourselves in the handler, otherwise, the sock
refcount will be unbalanced and the sock will never be freed.
Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
Reported-by: syzbot+10f1194569953b72f1ae@syzkaller.appspotmail.com
Fixes: 63346650 ("netrom: switch to sock timer API")
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netrom/nr_timer.c

@@ -121,11 +121,9 @@ static void nr_heartbeat_expiry(struct timer_list *t)
 		   is accepted() it isn't 'dead' so doesn't get removed. */
 		if (sock_flag(sk, SOCK_DESTROY) ||
 		    (sk->sk_state == TCP_LISTEN && sock_flag(sk, SOCK_DEAD))) {
-			sock_hold(sk);
 			bh_unlock_sock(sk);
 			nr_destroy_socket(sk);
-			sock_put(sk);
-			return;
+			goto out;
 		}
 		break;
 
@@ -146,6 +144,8 @@ static void nr_heartbeat_expiry(struct timer_list *t)
 
 	nr_start_heartbeat(sk);
 	bh_unlock_sock(sk);
+out:
+	sock_put(sk);
 }
 
 static void nr_t2timer_expiry(struct timer_list *t)
@@ -159,6 +159,7 @@ static void nr_t2timer_expiry(struct timer_list *t)
 		nr_enquiry_response(sk);
 	}
 	bh_unlock_sock(sk);
+	sock_put(sk);
 }
 
 static void nr_t4timer_expiry(struct timer_list *t)
@@ -169,6 +170,7 @@ static void nr_t4timer_expiry(struct timer_list *t)
 	bh_lock_sock(sk);
 	nr_sk(sk)->condition &= ~NR_COND_PEER_RX_BUSY;
 	bh_unlock_sock(sk);
+	sock_put(sk);
 }
 
 static void nr_idletimer_expiry(struct timer_list *t)
@@ -197,6 +199,7 @@ static void nr_idletimer_expiry(struct timer_list *t)
 		sock_set_flag(sk, SOCK_DEAD);
 	}
 	bh_unlock_sock(sk);
+	sock_put(sk);
 }
 
 static void nr_t1timer_expiry(struct timer_list *t)
@@ -209,8 +212,7 @@ static void nr_t1timer_expiry(struct timer_list *t)
 	case NR_STATE_1:
 		if (nr->n2count == nr->n2) {
 			nr_disconnect(sk, ETIMEDOUT);
-			bh_unlock_sock(sk);
-			return;
+			goto out;
 		} else {
 			nr->n2count++;
 			nr_write_internal(sk, NR_CONNREQ);
@@ -220,8 +222,7 @@ static void nr_t1timer_expiry(struct timer_list *t)
 	case NR_STATE_2:
 		if (nr->n2count == nr->n2) {
 			nr_disconnect(sk, ETIMEDOUT);
-			bh_unlock_sock(sk);
-			return;
+			goto out;
 		} else {
 			nr->n2count++;
 			nr_write_internal(sk, NR_DISCREQ);
@@ -231,8 +232,7 @@ static void nr_t1timer_expiry(struct timer_list *t)
 	case NR_STATE_3:
 		if (nr->n2count == nr->n2) {
 			nr_disconnect(sk, ETIMEDOUT);
-			bh_unlock_sock(sk);
-			return;
+			goto out;
 		} else {
 			nr->n2count++;
 			nr_requeue_frames(sk);
@@ -241,5 +241,7 @@ static void nr_t1timer_expiry(struct timer_list *t)
 	}
 
 	nr_start_t1timer(sk);
+out:
 	bh_unlock_sock(sk);
+	sock_put(sk);
 }