lockd: convert nsm_handle.sm_count from atomic_t to refcount_t

Web:        https://git.kernel.org/torvalds/c/be819f7b66031c4a21fdc8edc47a3ecd4cac635d
Commit:     be819f7b66031c4a21fdc8edc47a3ecd4cac635d
Parent:     81833de1a46edce9ca20cfe079872ac1c20ef359
Refname:    refs/heads/master
Author:     Elena Reshetova <[email protected]>
AuthorDate: Wed Nov 29 13:15:44 2017 +0200
Committer:  J. Bruce Fields <[email protected]>
CommitDate: Thu Dec 21 15:24:18 2017 -0500

    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 <[email protected]>
    Reviewed-by: David Windsor <[email protected]>
    Reviewed-by: Hans Liljestrand <[email protected]>
    Signed-off-by: Elena Reshetova <[email protected]>
    Signed-off-by: J. Bruce Fields <[email protected]>
---
 fs/lockd/host.c             |  2 +-
 fs/lockd/mon.c              | 14 +++++++-------
 include/linux/lockd/lockd.h |  2 +-
 3 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/fs/lockd/host.c b/fs/lockd/host.c
index 826a89184f90..063095ee39ec 100644
--- a/fs/lockd/host.c
+++ b/fs/lockd/host.c
@@ -114,7 +114,7 @@ static struct nlm_host *nlm_alloc_host(struct nlm_lookup_host_info *ni,
 	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,
diff --git a/fs/lockd/mon.c b/fs/lockd/mon.c
index 96cfb2967ac7..654594ef4f94 100644
--- a/fs/lockd/mon.c
+++ b/fs/lockd/mon.c
@@ -191,7 +191,7 @@ void nsm_unmonitor(const struct nlm_host *host)
 	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);
 
@@ -279,7 +279,7 @@ static struct nsm_handle *nsm_create_handle(const struct sockaddr *sap,
 	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;
@@ -337,13 +337,13 @@ retry:
 		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;
 	}
 
@@ -388,12 +388,12 @@ struct nsm_handle *nsm_reboot_lookup(const struct net *net,
 		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;
 }
 
@@ -404,7 +404,7 @@ struct nsm_handle *nsm_reboot_lookup(const struct net *net,
  */
 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",
diff --git a/include/linux/lockd/lockd.h b/include/linux/lockd/lockd.h
index d7d313fb9cd4..809619553e9b 100644
--- a/include/linux/lockd/lockd.h
+++ b/include/linux/lockd/lockd.h
@@ -83,7 +83,7 @@ struct nlm_host {
 
 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;
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