[PATCH] ncpfs: convert ncp_request_reply.refs 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 ncp_request_reply.refs is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
---
 fs/ncpfs/sock.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/ncpfs/sock.c b/fs/ncpfs/sock.c
index 98b6db0..523d864 100644
--- a/fs/ncpfs/sock.c
+++ b/fs/ncpfs/sock.c
@@ -29,6 +29,7 @@
 #include <linux/ipx.h>
 #include <linux/poll.h>
 #include <linux/file.h>
+#include <linux/refcount.h>
 
 #include "ncp_fs.h"
 
@@ -52,7 +53,7 @@ static int _send(struct socket *sock, const void *buff, int len)
 struct ncp_request_reply {
 	struct list_head req;
 	wait_queue_head_t wq;
-	atomic_t refs;
+	refcount_t refs;
 	unsigned char* reply_buf;
 	size_t datalen;
 	int result;
@@ -72,7 +73,7 @@ static inline struct ncp_request_reply* ncp_alloc_req(void)
 		return NULL;
 
 	init_waitqueue_head(&req->wq);
-	atomic_set(&req->refs, (1));
+	refcount_set(&req->refs, (1));
 	req->status = RQ_IDLE;
 
 	return req;
@@ -80,12 +81,12 @@ static inline struct ncp_request_reply* ncp_alloc_req(void)
 
 static void ncp_req_get(struct ncp_request_reply *req)
 {
-	atomic_inc(&req->refs);
+	refcount_inc(&req->refs);
 }
 
 static void ncp_req_put(struct ncp_request_reply *req)
 {
-	if (atomic_dec_and_test(&req->refs))
+	if (refcount_dec_and_test(&req->refs))
 		kfree(req);
 }
 
-- 
2.7.4