fs, nfsd: convert nfs4_cntl_odstate.co_odcount 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 nfs4_cntl_odstate.co_odcount is used as pure reference counter.
Convert it to refcount_t and fix up the operations.
Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -568,7 +568,7 @@ alloc_clnt_odstate(struct nfs4_client *clp)
 	co = kmem_cache_zalloc(odstate_slab, GFP_KERNEL);
 	if (co) {
 		co->co_client = clp;
-		atomic_set(&co->co_odcount, 1);
+		refcount_set(&co->co_odcount, 1);
 	}
 	return co;
 }
@@ -586,7 +586,7 @@ static inline void
 get_clnt_odstate(struct nfs4_clnt_odstate *co)
 {
 	if (co)
-		atomic_inc(&co->co_odcount);
+		refcount_inc(&co->co_odcount);
 }
 
 static void
@@ -598,7 +598,7 @@ put_clnt_odstate(struct nfs4_clnt_odstate *co)
 		return;
 
 	fp = co->co_file;
-	if (atomic_dec_and_lock(&co->co_odcount, &fp->fi_lock)) {
+	if (refcount_dec_and_lock(&co->co_odcount, &fp->fi_lock)) {
 		list_del(&co->co_perfile);
 		spin_unlock(&fp->fi_lock);
 

fs/nfsd/state.h

@@ -468,7 +468,7 @@ struct nfs4_clnt_odstate {
 	struct nfs4_client	*co_client;
 	struct nfs4_file	*co_file;
 	struct list_head	co_perfile;
-	atomic_t		co_odcount;
+	refcount_t		co_odcount;
 };
 
 /*