lockd: convert nsm_handle.sm_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 nsm_handle.sm_count is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

**Important note for maintainers:

Some functions from refcount_t API defined in lib/refcount.c
have different memory ordering guarantees than their atomic
counterparts.
The full comparison can be seen in
https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
in state to be merged to the documentation tree.
Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in
some rare cases it might matter.
Please double check that you don't have some undocumented
memory guarantees for this variable usage.

For the nsm_handle.sm_count it might make a difference
in following places:
- nsm_release(): decrement in refcount_dec_and_lock() only
provides RELEASE ordering, control dependency on success
and holds a spin lock on success vs. fully ordered atomic
counterpart. No change for the spin lock guarantees.

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: Trond Myklebust <[e-mail]>

fs/lockd/host.c

···
 	unsigned long now = jiffies;
 
 	if (nsm != NULL)
-		atomic_inc(&nsm->sm_count);
+		refcount_inc(&nsm->sm_count);
 	else {
 		host = NULL;
 		nsm = nsm_get_handle(ni->net, ni->sap, ni->salen,

fs/lockd/mon.c

···
 	struct nsm_res	res;
 	int status;
 
-	if (atomic_read(&nsm->sm_count) == 1
+	if (refcount_read(&nsm->sm_count) == 1
 	 && nsm->sm_monitored && !nsm->sm_sticky) {
 		dprintk("lockd: nsm_unmonitor(%s)\n", nsm->sm_name);
 
···
 	if (unlikely(new == NULL))
 		return NULL;
 
-	atomic_set(&new->sm_count, 1);
+	refcount_set(&new->sm_count, 1);
 	new->sm_name = (char *)(new + 1);
 	memcpy(nsm_addr(new), sap, salen);
 	new->sm_addrlen = salen;
···
 		cached = nsm_lookup_addr(&ln->nsm_handles, sap);
 
 	if (cached != NULL) {
-		atomic_inc(&cached->sm_count);
+		refcount_inc(&cached->sm_count);
 		spin_unlock(&nsm_lock);
 		kfree(new);
 		dprintk("lockd: found nsm_handle for %s (%s), "
 				"cnt %d\n", cached->sm_name,
 				cached->sm_addrbuf,
-				atomic_read(&cached->sm_count));
+				refcount_read(&cached->sm_count));
 		return cached;
 	}
 
···
 		return cached;
 	}
 
-	atomic_inc(&cached->sm_count);
+	refcount_inc(&cached->sm_count);
 	spin_unlock(&nsm_lock);
 
 	dprintk("lockd: host %s (%s) rebooted, cnt %d\n",
 			cached->sm_name, cached->sm_addrbuf,
-			atomic_read(&cached->sm_count));
+			refcount_read(&cached->sm_count));
 	return cached;
 }
 
···
  */
 void nsm_release(struct nsm_handle *nsm)
 {
-	if (atomic_dec_and_lock(&nsm->sm_count, &nsm_lock)) {
+	if (refcount_dec_and_lock(&nsm->sm_count, &nsm_lock)) {
 		list_del(&nsm->sm_link);
 		spin_unlock(&nsm_lock);
 		dprintk("lockd: destroyed nsm_handle for %s (%s)\n",

include/linux/lockd/lockd.h

···
 
 struct nsm_handle {
 	struct list_head	sm_link;
-	atomic_t		sm_count;
+	refcount_t		sm_count;
 	char			*sm_mon_name;
 	char			*sm_name;
 	struct sockaddr_storage	sm_addr;