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

diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
index bbc7b7808a31c2c1e21b34e89552ee4146db52a6..32f49c4ce45783aa507b8473f4a5ab60e495fa8f 100644 (file)
--- a/drivers/net/hamradio/6pack.c
+++ b/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