[PATCH 14/15] drivers, net, ppp: convert syncppp.refcnt from atomic_t to refcount_t

[Elena Reshetova]

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 syncppp.refcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keesc...@chromium.org>
Reviewed-by: David Windsor <dwind...@gmail.com>
Reviewed-by: Hans Liljestrand <ishkam...@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshet...@intel.com>
---
 drivers/net/ppp/ppp_synctty.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
index 7868c29..7196f00 100644
--- a/drivers/net/ppp/ppp_synctty.c
+++ b/drivers/net/ppp/ppp_synctty.c
@@ -46,6 +46,7 @@
 #include <linux/init.h>
 #include <linux/interrupt.h>
 #include <linux/slab.h>
+#include <linux/refcount.h>
 #include <asm/unaligned.h>
 #include <linux/uaccess.h>
 
@@ -72,7 +73,7 @@ struct syncppp {
 
        struct tasklet_struct tsk;
 
-       atomic_t        refcnt;
+       refcount_t      refcnt;
        struct completion dead_cmp;
        struct ppp_channel chan;        /* interface to generic ppp layer */
 };
@@ -141,14 +142,14 @@ static struct syncppp *sp_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 sp_put(struct syncppp *ap)
 {
-       if (atomic_dec_and_test(&ap->refcnt))
+       if (refcount_dec_and_test(&ap->refcnt))
                complete(&ap->dead_cmp);
 }
 
@@ -182,7 +183,7 @@ ppp_sync_open(struct tty_struct *tty)
        skb_queue_head_init(&ap->rqueue);
        tasklet_init(&ap->tsk, ppp_sync_process, (unsigned long) ap);
 
-       atomic_set(&ap->refcnt, 1);
+       refcount_set(&ap->refcnt, 1);
        init_completion(&ap->dead_cmp);
 
        ap->chan.private = ap;
@@ -232,7 +233,7 @@ ppp_sync_close(struct tty_struct *tty)
         * our channel ops (i.e. ppp_sync_send/ioctl) are in progress
         * by the time it returns.
         */
-       if (!atomic_dec_and_test(&ap->refcnt))
+       if (!refcount_dec_and_test(&ap->refcnt))
                wait_for_completion(&ap->dead_cmp);
        tasklet_kill(&ap->tsk);
 
-- 
2.7.4