fs, nfsd: convert nfs4_file.fi_ref 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_file.fi_ref 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

@@ -359,7 +359,7 @@ put_nfs4_file(struct nfs4_file *fi)
 {
 	might_lock(&state_lock);
 
-	if (atomic_dec_and_lock(&fi->fi_ref, &state_lock)) {
+	if (refcount_dec_and_lock(&fi->fi_ref, &state_lock)) {
 		hlist_del_rcu(&fi->fi_hash);
 		spin_unlock(&state_lock);
 		WARN_ON_ONCE(!list_empty(&fi->fi_clnt_odstate));
@@ -3401,7 +3401,7 @@ static void nfsd4_init_file(struct knfsd_fh *fh, unsigned int hashval,
 {
 	lockdep_assert_held(&state_lock);
 
-	atomic_set(&fp->fi_ref, 1);
+	refcount_set(&fp->fi_ref, 1);
 	spin_lock_init(&fp->fi_lock);
 	INIT_LIST_HEAD(&fp->fi_stateids);
 	INIT_LIST_HEAD(&fp->fi_delegations);
@@ -3697,7 +3697,7 @@ find_file_locked(struct knfsd_fh *fh, unsigned int hashval)
 
 	hlist_for_each_entry_rcu(fp, &file_hashtbl[hashval], fi_hash) {
 		if (fh_match(&fp->fi_fhandle, fh)) {
-			if (atomic_inc_not_zero(&fp->fi_ref))
+			if (refcount_inc_not_zero(&fp->fi_ref))
 				return fp;
 		}
 	}

fs/nfsd/state.h

@@ -484,7 +484,7 @@ struct nfs4_clnt_odstate {
  * the global state_lock spinlock.
  */
 struct nfs4_file {
-	atomic_t		fi_ref;
+	refcount_t		fi_ref;
 	spinlock_t		fi_lock;
 	struct hlist_node       fi_hash;	/* hash on fi_fhandle */
 	struct list_head        fi_stateids;
@@ -637,7 +637,7 @@ struct nfs4_file *find_file(struct knfsd_fh *fh);
 void put_nfs4_file(struct nfs4_file *fi);
 static inline void get_nfs4_file(struct nfs4_file *fi)
 {
-	atomic_inc(&fi->fi_ref);
+	refcount_inc(&fi->fi_ref);
 }
 struct file *find_any_file(struct nfs4_file *f);