drivers, net: convert masces_rx_sc.refcnt from atomic_t to refcount_t

atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable masces_rx_sc.refcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.
Suggested-by: NKees Cook <keescook@chromium.org>
Reviewed-by: NDavid Windsor <dwindsor@gmail.com>
Reviewed-by: NHans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: NElena Reshetova <elena.reshetova@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/macsec.c

@@ -172,7 +172,7 @@ struct macsec_rx_sc {
 	bool active;
 	struct macsec_rx_sa __rcu *sa[MACSEC_NUM_AN];
 	struct pcpu_rx_sc_stats __percpu *stats;
-	atomic_t refcnt;
+	refcount_t refcnt;
 	struct rcu_head rcu_head;
 };
 
@@ -331,12 +331,12 @@ static void free_rx_sc_rcu(struct rcu_head *head)
 
 static struct macsec_rx_sc *macsec_rxsc_get(struct macsec_rx_sc *sc)
 {
-	return atomic_inc_not_zero(&sc->refcnt) ? sc : NULL;
+	return refcount_inc_not_zero(&sc->refcnt) ? sc : NULL;
 }
 
 static void macsec_rxsc_put(struct macsec_rx_sc *sc)
 {
-	if (atomic_dec_and_test(&sc->refcnt))
+	if (refcount_dec_and_test(&sc->refcnt))
 		call_rcu(&sc->rcu_head, free_rx_sc_rcu);
 }
 
@@ -1411,7 +1411,7 @@ static struct macsec_rx_sc *create_rx_sc(struct net_device *dev, sci_t sci)
 
 	rx_sc->sci = sci;
 	rx_sc->active = true;
-	atomic_set(&rx_sc->refcnt, 1);
+	refcount_set(&rx_sc->refcnt, 1);
 
 	secy = &macsec_priv(dev)->secy;
 	rcu_assign_pointer(rx_sc->next, secy->rx_sc);