libceph: convert ceph_snap_context.nref 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: Ilya Dryomov <idryomov@gmail.com>

include/linux/ceph/libceph.h

@@ -14,6 +14,7 @@
 #include <linux/wait.h>
 #include <linux/writeback.h>
 #include <linux/slab.h>
+#include <linux/refcount.h>
 
 #include <linux/ceph/types.h>
 #include <linux/ceph/messenger.h>
@@ -161,7 +162,7 @@ struct ceph_client {
  * dirtied.
  */
 struct ceph_snap_context {
-	atomic_t nref;
+	refcount_t nref;
 	u64 seq;
 	u32 num_snaps;
 	u64 snaps[];

net/ceph/snapshot.c

@@ -49,7 +49,7 @@ struct ceph_snap_context *ceph_create_snap_context(u32 snap_count,
 	if (!snapc)
 		return NULL;
 
-	atomic_set(&snapc->nref, 1);
+	refcount_set(&snapc->nref, 1);
 	snapc->num_snaps = snap_count;
 
 	return snapc;
@@ -59,7 +59,7 @@ EXPORT_SYMBOL(ceph_create_snap_context);
 struct ceph_snap_context *ceph_get_snap_context(struct ceph_snap_context *sc)
 {
 	if (sc)
-		atomic_inc(&sc->nref);
+		refcount_inc(&sc->nref);
 	return sc;
 }
 EXPORT_SYMBOL(ceph_get_snap_context);
@@ -68,7 +68,7 @@ void ceph_put_snap_context(struct ceph_snap_context *sc)
 {
 	if (!sc)
 		return;
-	if (atomic_dec_and_test(&sc->nref)) {
+	if (refcount_dec_and_test(&sc->nref)) {
 		/*printk(" deleting snap_context %p\n", sc);*/
 		kfree(sc);
 	}