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 <[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/pnfs.h

···
 #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>
···
 	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 */
 };

fs/nfs/pnfs_nfs.c

···
 		"        client %p\n"
 		"        cl_exchange_flags %x\n",
 		ds->ds_remotestr,
-		atomic_read(&ds->ds_count), ds->ds_clp,
+		refcount_read(&ds->ds_count), ds->ds_clp,
 		ds->ds_clp ? ds->ds_clp->cl_exchange_flags : 0);
 }
 
···
 
 void nfs4_pnfs_ds_put(struct nfs4_pnfs_ds *ds)
 {
-	if (atomic_dec_and_lock(&ds->ds_count,
+	if (refcount_dec_and_lock(&ds->ds_count,
 				&nfs4_ds_cache_lock)) {
 		list_del_init(&ds->ds_node);
 		spin_unlock(&nfs4_ds_cache_lock);
···
 		INIT_LIST_HEAD(&ds->ds_addrs);
 		list_splice_init(dsaddrs, &ds->ds_addrs);
 		ds->ds_remotestr = remotestr;
-		atomic_set(&ds->ds_count, 1);
+		refcount_set(&ds->ds_count, 1);
 		INIT_LIST_HEAD(&ds->ds_node);
 		ds->ds_clp = NULL;
 		list_add(&ds->ds_node, &nfs4_data_server_cache);
···
 	} else {
 		kfree(remotestr);
 		kfree(ds);
-		atomic_inc(&tmp_ds->ds_count);
+		refcount_inc(&tmp_ds->ds_count);
 		dprintk("%s data server %s found, inc'ed ds_count to %d\n",
 			__func__, tmp_ds->ds_remotestr,
-			atomic_read(&tmp_ds->ds_count));
+			refcount_read(&tmp_ds->ds_count));
 		ds = tmp_ds;
 	}
 	spin_unlock(&nfs4_ds_cache_lock);