fs, nfs: convert nfs_lock_context.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 nfs_lock_context.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/include/linux/nfs_fs.h b/include/linux/nfs_fs.h
index 453f491..e670691 100644
--- a/include/linux/nfs_fs.h
+++ b/include/linux/nfs_fs.h
@@ -22,6 +22,7 @@
 #include <linux/mm.h>
 #include <linux/pagemap.h>
 #include <linux/rbtree.h>
+#include <linux/refcount.h>
 #include <linux/rwsem.h>
 #include <linux/wait.h>
 
@@ -55,7 +56,7 @@ struct nfs_access_entry {
 };
 
 struct nfs_lock_context {
-	atomic_t count;
+	refcount_t count;
 	struct list_head list;
 	struct nfs_open_context *open_context;
 	fl_owner_t lockowner;