fs, nfs: convert nfs4_ff_layout_mirror.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_ff_layout_mirror.ref 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/flexfilelayout/flexfilelayout.c

···
 			continue;
 		if (!ff_mirror_match_fh(mirror, pos))
 			continue;
-		if (atomic_inc_not_zero(&pos->ref)) {
+		if (refcount_inc_not_zero(&pos->ref)) {
 			spin_unlock(&inode->i_lock);
 			return pos;
 		}
···
 	mirror = kzalloc(sizeof(*mirror), gfp_flags);
 	if (mirror != NULL) {
 		spin_lock_init(&mirror->lock);
-		atomic_set(&mirror->ref, 1);
+		refcount_set(&mirror->ref, 1);
 		INIT_LIST_HEAD(&mirror->mirrors);
 	}
 	return mirror;
···
 
 static void ff_layout_put_mirror(struct nfs4_ff_layout_mirror *mirror)
 {
-	if (mirror != NULL && atomic_dec_and_test(&mirror->ref))
+	if (mirror != NULL && refcount_dec_and_test(&mirror->ref))
 		ff_layout_free_mirror(mirror);
 }
 
···
 		if (!test_and_clear_bit(NFS4_FF_MIRROR_STAT_AVAIL, &mirror->flags))
 			continue;
 		/* mirror refcount put in cleanup_layoutstats */
-		if (!atomic_inc_not_zero(&mirror->ref))
+		if (!refcount_inc_not_zero(&mirror->ref))
 			continue;
 		dev = &mirror->mirror_ds->id_node; 
 		memcpy(&devinfo->dev_id, &dev->deviceid, NFS4_DEVICEID4_SIZE);

fs/nfs/flexfilelayout/flexfilelayout.h

···
 #define FF_FLAGS_NO_IO_THRU_MDS  2
 #define FF_FLAGS_NO_READ_IO      4
 
+#include <linux/refcount.h>
 #include "../pnfs.h"
 
 /* XXX: Let's filter out insanely large mirror count for now to avoid oom
···
 	nfs4_stateid			stateid;
 	struct rpc_cred	__rcu		*ro_cred;
 	struct rpc_cred	__rcu		*rw_cred;
-	atomic_t			ref;
+	refcount_t			ref;
 	spinlock_t			lock;
 	unsigned long			flags;
 	struct nfs4_ff_layoutstat	read_stat;