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>

1 files changed, 2 insertions, 2 deletions

diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
index 2ed368a91a10..f3772ea8ba0d 100644
--- a/fs/nfsd/state.h
+++ b/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);