drivers, net, hamradio: convert sixpack.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 sixpack.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/hamradio/6pack.c

@@ -35,7 +35,7 @@
 #include <linux/tcp.h>
 #include <linux/semaphore.h>
 #include <linux/compat.h>
-#include <linux/atomic.h>
+#include <linux/refcount.h>
 
 #define SIXPACK_VERSION    "Revision: 0.3.0"
 
@@ -120,7 +120,7 @@ struct sixpack {
 
 	struct timer_list	tx_t;
 	struct timer_list	resync_t;
-	atomic_t		refcnt;
+	refcount_t		refcnt;
 	struct semaphore	dead_sem;
 	spinlock_t		lock;
 };
@@ -381,7 +381,7 @@ static struct sixpack *sp_get(struct tty_struct *tty)
 	read_lock(&disc_data_lock);
 	sp = tty->disc_data;
 	if (sp)
-		atomic_inc(&sp->refcnt);
+		refcount_inc(&sp->refcnt);
 	read_unlock(&disc_data_lock);
 
 	return sp;
@@ -389,7 +389,7 @@ static struct sixpack *sp_get(struct tty_struct *tty)
 
 static void sp_put(struct sixpack *sp)
 {
-	if (atomic_dec_and_test(&sp->refcnt))
+	if (refcount_dec_and_test(&sp->refcnt))
 		up(&sp->dead_sem);
 }
 
@@ -576,7 +576,7 @@ static int sixpack_open(struct tty_struct *tty)
 	sp->dev = dev;
 
 	spin_lock_init(&sp->lock);
-	atomic_set(&sp->refcnt, 1);
+	refcount_set(&sp->refcnt, 1);
 	sema_init(&sp->dead_sem, 0);
 
 	/* !!! length of the buffers. MTU is IP MTU, not PACLEN!  */
@@ -670,7 +670,7 @@ static void sixpack_close(struct tty_struct *tty)
 	 * We have now ensured that nobody can start using ap from now on, but
 	 * we have to wait for all existing users to finish.
 	 */
-	if (!atomic_dec_and_test(&sp->refcnt))
+	if (!refcount_dec_and_test(&sp->refcnt))
 		down(&sp->dead_sem);
 
 	/* We must stop the queue to avoid potentially scribbling