Bluetooth: convert rfcomm_dlc.refcnt from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <[e-mail]>
Signed-off-by: Hans Liljestrand <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: David Windsor <[e-mail]>
Signed-off-by: Marcel Holtmann <[e-mail]>

include/net/bluetooth/rfcomm.h

···
    SOFTWARE IS DISCLAIMED.
 */
 
+#include <linux/refcount.h>
+
 #ifndef __RFCOMM_H
 #define __RFCOMM_H
 
···
 	struct mutex  lock;
 	unsigned long state;
 	unsigned long flags;
-	atomic_t      refcnt;
+	refcount_t    refcnt;
 	u8            dlci;
 	u8            addr;
 	u8            priority;
···
 
 static inline void rfcomm_dlc_hold(struct rfcomm_dlc *d)
 {
-	atomic_inc(&d->refcnt);
+	refcount_inc(&d->refcnt);
 }
 
 static inline void rfcomm_dlc_put(struct rfcomm_dlc *d)
 {
-	if (atomic_dec_and_test(&d->refcnt))
+	if (refcount_dec_and_test(&d->refcnt))
 		rfcomm_dlc_free(d);
 }
 

net/bluetooth/rfcomm/core.c

···
 
 	skb_queue_head_init(&d->tx_queue);
 	mutex_init(&d->lock);
-	atomic_set(&d->refcnt, 1);
+	refcount_set(&d->refcnt, 1);
 
 	rfcomm_dlc_clear_state(d);
 
···
 {
 	struct rfcomm_session *s = d->session;
 
-	BT_DBG("dlc %p refcnt %d session %p", d, atomic_read(&d->refcnt), s);
+	BT_DBG("dlc %p refcnt %d session %p", d, refcount_read(&d->refcnt), s);
 
 	list_del(&d->list);
 	d->session = NULL;