drivers, net, ppp: convert asyncppp.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 asyncppp.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/ppp/ppp_async.c b/drivers/net/ppp/ppp_async.c
index 814fd8fae67d095aad97e80a9648dea5587c2c4c..1b28e6e702f5249704238570a5c007d11266c973 100644 (file)
--- a/drivers/net/ppp/ppp_async.c
+++ b/drivers/net/ppp/ppp_async.c
@@ -69,7 +69,7 @@ struct asyncppp {
 
        struct tasklet_struct tsk;
 
-       atomic_t        refcnt;
+       refcount_t      refcnt;
        struct semaphore dead_sem;
        struct ppp_channel chan;        /* interface to generic ppp layer */
        unsigned char   obuf[OBUFSIZE];
@@ -140,14 +140,14 @@ static struct asyncppp *ap_get(struct tty_struct *tty)
        read_lock(&disc_data_lock);
        ap = tty->disc_data;
        if (ap != NULL)
-               atomic_inc(&ap->refcnt);
+               refcount_inc(&ap->refcnt);
        read_unlock(&disc_data_lock);
        return ap;
 }
 
 static void ap_put(struct asyncppp *ap)
 {
-       if (atomic_dec_and_test(&ap->refcnt))
+       if (refcount_dec_and_test(&ap->refcnt))
                up(&ap->dead_sem);
 }
 
@@ -185,7 +185,7 @@ ppp_asynctty_open(struct tty_struct *tty)
        skb_queue_head_init(&ap->rqueue);
        tasklet_init(&ap->tsk, ppp_async_process, (unsigned long) ap);
 
-       atomic_set(&ap->refcnt, 1);
+       refcount_set(&ap->refcnt, 1);
        sema_init(&ap->dead_sem, 0);
 
        ap->chan.private = ap;
@@ -234,7 +234,7 @@ ppp_asynctty_close(struct tty_struct *tty)
         * our channel ops (i.e. ppp_async_send/ioctl) are in progress
         * by the time it returns.
         */
-       if (!atomic_dec_and_test(&ap->refcnt))
+       if (!refcount_dec_and_test(&ap->refcnt))
                down(&ap->dead_sem);
        tasklet_kill(&ap->tsk);