atm: fix a UAF in lec_arp_clear_vccs()

Gengming reported a UAF in lec_arp_clear_vccs(),
where we add a vcc socket to an entry in a per-device
list but free the socket without removing it from the
list when vcc->dev is NULL.

We need to call lec_vcc_close() to search and remove
those entries contain the vcc being destroyed. This can
be done by calling vcc->push(vcc, NULL) unconditionally
in vcc_destroy_socket().

Another issue discovered by Gengming's reproducer is
the vcc->dev may point to the static device lecatm_dev,
for which we don't need to register/unregister device,
so we can just check for vcc->dev->ops->owner.
Reported-by: Gengming Liu <l.dmxcsnsbh@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/atm/common.c

@@ -177,8 +177,7 @@ static void vcc_destroy_socket(struct sock *sk)
 
 	set_bit(ATM_VF_CLOSE, &vcc->flags);
 	clear_bit(ATM_VF_READY, &vcc->flags);
-	if (vcc->dev) {
-		if (vcc->dev->ops->close)
+	if (vcc->dev && vcc->dev->ops->close)
 		vcc->dev->ops->close(vcc);
 	if (vcc->push)
 		vcc->push(vcc, NULL); /* atmarpd has no push */
@@ -189,6 +188,7 @@ static void vcc_destroy_socket(struct sock *sk)
 		kfree_skb(skb);
 	}
 
+	if (vcc->dev && vcc->dev->ops->owner) {
 		module_put(vcc->dev->ops->owner);
 		atm_dev_put(vcc->dev);
 	}