net, lapb: convert lapb_cb.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 <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/lapb.h

@@ -1,6 +1,7 @@
 #ifndef _LAPB_H
 #define _LAPB_H 
 #include <linux/lapb.h>
+#include <linux/refcount.h>
 
 #define	LAPB_HEADER_LEN	20		/* LAPB over Ethernet + a bit more */
 
@@ -101,7 +102,7 @@ struct lapb_cb {
 	struct lapb_frame	frmr_data;
 	unsigned char		frmr_type;
 
-	atomic_t		refcnt;
+	refcount_t		refcnt;
 };
 
 /* lapb_iface.c */

net/lapb/lapb_iface.c

@@ -54,12 +54,12 @@ static void lapb_free_cb(struct lapb_cb *lapb)
 
 static __inline__ void lapb_hold(struct lapb_cb *lapb)
 {
-	atomic_inc(&lapb->refcnt);
+	refcount_inc(&lapb->refcnt);
 }
 
 static __inline__ void lapb_put(struct lapb_cb *lapb)
 {
-	if (atomic_dec_and_test(&lapb->refcnt))
+	if (refcount_dec_and_test(&lapb->refcnt))
 		lapb_free_cb(lapb);
 }
 
@@ -136,7 +136,7 @@ static struct lapb_cb *lapb_create_cb(void)
 	lapb->mode    = LAPB_DEFAULT_MODE;
 	lapb->window  = LAPB_DEFAULT_WINDOW;
 	lapb->state   = LAPB_STATE_0;
-	atomic_set(&lapb->refcnt, 1);
+	refcount_set(&lapb->refcnt, 1);
 out:
 	return lapb;
 }