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 <[e-mail]>
Reviewed-by: David Windsor <[e-mail]>
Reviewed-by: Hans Liljestrand <[e-mail]>
Signed-off-by: Elena Reshetova <[e-mail]>
Signed-off-by: Anna Schumaker <[e-mail]>

fs/nfs/inode.c

···
 
 static void nfs_init_lock_context(struct nfs_lock_context *l_ctx)
 {
-	atomic_set(&l_ctx->count, 1);
+	refcount_set(&l_ctx->count, 1);
 	l_ctx->lockowner = current->files;
 	INIT_LIST_HEAD(&l_ctx->list);
 	atomic_set(&l_ctx->io_count, 0);
···
 	do {
 		if (pos->lockowner != current->files)
 			continue;
-		atomic_inc(&pos->count);
+		refcount_inc(&pos->count);
 		return pos;
 	} while ((pos = list_entry(pos->list.next, typeof(*pos), list)) != head);
 	return NULL;
···
 	struct nfs_open_context *ctx = l_ctx->open_context;
 	struct inode *inode = d_inode(ctx->dentry);
 
-	if (!atomic_dec_and_lock(&l_ctx->count, &inode->i_lock))
+	if (!refcount_dec_and_lock(&l_ctx->count, &inode->i_lock))
 		return;
 	list_del(&l_ctx->list);
 	spin_unlock(&inode->i_lock);
···
 struct nfs_open_context *get_nfs_open_context(struct nfs_open_context *ctx)
 {
 	if (ctx != NULL)
-		atomic_inc(&ctx->lock_context.count);
+		refcount_inc(&ctx->lock_context.count);
 	return ctx;
 }
 EXPORT_SYMBOL_GPL(get_nfs_open_context);
···
 	struct super_block *sb = ctx->dentry->d_sb;
 
 	if (!list_empty(&ctx->list)) {
-		if (!atomic_dec_and_lock(&ctx->lock_context.count, &inode->i_lock))
+		if (!refcount_dec_and_lock(&ctx->lock_context.count, &inode->i_lock))
 			return;
 		list_del(&ctx->list);
 		spin_unlock(&inode->i_lock);
-	} else if (!atomic_dec_and_test(&ctx->lock_context.count))
+	} else if (!refcount_dec_and_test(&ctx->lock_context.count))
 		return;
 	if (inode != NULL)
 		NFS_PROTO(inode)->close_context(ctx, is_sync);

include/linux/nfs_fs.h

···
 #include <linux/mm.h>
 #include <linux/pagemap.h>
 #include <linux/rbtree.h>
+#include <linux/refcount.h>
 #include <linux/rwsem.h>
 #include <linux/wait.h>
 
···
 };
 
 struct nfs_lock_context {
-	atomic_t count;
+	refcount_t count;
 	struct list_head list;
 	struct nfs_open_context *open_context;
 	fl_owner_t lockowner;