fs, nfs: convert nfs4_pnfs_ds.ds_count 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_pnfs_ds.ds_count 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: Anna Schumaker <Anna.Schumaker@Netapp.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/nfs/pnfs.h b/fs/nfs/pnfs.h
index 87f144f14d1e..cefa7daf6e85 100644
--- a/fs/nfs/pnfs.h
+++ b/fs/nfs/pnfs.h
@@ -30,6 +30,7 @@
 #ifndef FS_NFS_PNFS_H
 #define FS_NFS_PNFS_H
 
+#include <linux/refcount.h>
 #include <linux/nfs_fs.h>
 #include <linux/nfs_page.h>
 #include <linux/workqueue.h>
@@ -54,7 +55,7 @@ struct nfs4_pnfs_ds {
 	char			*ds_remotestr;	/* comma sep list of addrs */
 	struct list_head	ds_addrs;
 	struct nfs_client	*ds_clp;
-	atomic_t		ds_count;
+	refcount_t		ds_count;
 	unsigned long		ds_state;
 #define NFS4DS_CONNECTING	0	/* ds is establishing connection */
 };