net, sctp: convert sctp_datamsg.refcnt from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

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

include/net/sctp/structs.h

@@ -496,7 +496,7 @@ struct sctp_datamsg {
 	/* Chunks waiting to be submitted to lower layer. */
 	struct list_head chunks;
 	/* Reference counting. */
-	atomic_t refcnt;
+	refcount_t refcnt;
 	/* When is this message no longer interesting to the peer? */
 	unsigned long expires_at;
 	/* Did the messenge fail to send? */

net/sctp/chunk.c

@@ -49,7 +49,7 @@
 /* Initialize datamsg from memory. */
 static void sctp_datamsg_init(struct sctp_datamsg *msg)
 {
-	atomic_set(&msg->refcnt, 1);
+	refcount_set(&msg->refcnt, 1);
 	msg->send_failed = 0;
 	msg->send_error = 0;
 	msg->can_delay = 1;
@@ -136,13 +136,13 @@ static void sctp_datamsg_destroy(struct sctp_datamsg *msg)
 /* Hold a reference. */
 static void sctp_datamsg_hold(struct sctp_datamsg *msg)
 {
-	atomic_inc(&msg->refcnt);
+	refcount_inc(&msg->refcnt);
 }
 
 /* Release a reference. */
 void sctp_datamsg_put(struct sctp_datamsg *msg)
 {
-	if (atomic_dec_and_test(&msg->refcnt))
+	if (refcount_dec_and_test(&msg->refcnt))
 		sctp_datamsg_destroy(msg);
 }