netdev - [PATCH 12/15] drivers, net, ppp: convert asyncppp.refcnt from atomic_t to refcount

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Message-Id: <1508484229-19706-13-git-send-email-elena.reshetova@intel.com>
Date:   Fri, 20 Oct 2017 10:23:46 +0300
From:   Elena Reshetova <elena.reshetova@...el.com>
To:     davem@...emloft.net
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-hams@...r.kernel.org, linux-ppp@...r.kernel.org,
        ganeshgr@...lsio.com, nbd@...nwrt.org, john@...ozen.org,
        sean.wang@...iatek.com, matthias.bgg@...il.com,
        yishaih@...lanox.com, saeedm@...lanox.com, matanb@...lanox.com,
        tariqt@...lanox.com, leonro@...lanox.com,
        ajk@...nets.uni-bremen.de, paulus@...ba.org, zbr@...emap.net,
        peterz@...radead.org, keescook@...omium.org,
        Elena Reshetova <elena.reshetova@...el.com>
Subject: [PATCH 12/15] 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@...omium.org>
Reviewed-by: David Windsor <dwindsor@...il.com>
Reviewed-by: Hans Liljestrand <ishkamiel@...il.com>
Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
---
 drivers/net/ppp/ppp_async.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/net/ppp/ppp_async.c b/drivers/net/ppp/ppp_async.c
index 814fd8f..1b28e6e 100644
--- 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);
 
-- 
2.7.4