lockd: convert nlm_lockowner.count from atomic_t to refcount_t

Web:        https://git.kernel.org/torvalds/c/8bb3ea77933e9796f8f15a5492481a96af8302d6
Commit:     8bb3ea77933e9796f8f15a5492481a96af8302d6
Parent:     be819f7b66031c4a21fdc8edc47a3ecd4cac635d
Refname:    refs/heads/master
Author:     Elena Reshetova <[email protected]>
AuthorDate: Wed Nov 29 13:15:45 2017 +0200
Committer:  J. Bruce Fields <[email protected]>
CommitDate: Thu Dec 21 15:24:18 2017 -0500

    lockd: convert nlm_lockowner.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 nlm_lockowner.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 nlm_lockowner.count it might make a difference
    in following places:
     - nlm_put_lockowner(): 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 changes in 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/clntproc.c         | 6 +++---
 include/linux/lockd/lockd.h | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/lockd/clntproc.c b/fs/lockd/clntproc.c
index 066ac313ae5c..112173dbea76 100644
--- a/fs/lockd/clntproc.c
+++ b/fs/lockd/clntproc.c
@@ -48,13 +48,13 @@ void nlmclnt_next_cookie(struct nlm_cookie *c)
 
 static struct nlm_lockowner *nlm_get_lockowner(struct nlm_lockowner *lockowner)
 {
-	atomic_inc(&lockowner->count);
+	refcount_inc(&lockowner->count);
 	return lockowner;
 }
 
 static void nlm_put_lockowner(struct nlm_lockowner *lockowner)
 {
-	if (!atomic_dec_and_lock(&lockowner->count, &lockowner->host->h_lock))
+	if (!refcount_dec_and_lock(&lockowner->count, &lockowner->host->h_lock))
 		return;
 	list_del(&lockowner->list);
 	spin_unlock(&lockowner->host->h_lock);
@@ -105,7 +105,7 @@ static struct nlm_lockowner *nlm_find_lockowner(struct nlm_host *host, fl_owner_
 		res = __nlm_find_lockowner(host, owner);
 		if (res == NULL && new != NULL) {
 			res = new;
-			atomic_set(&new->count, 1);
+			refcount_set(&new->count, 1);
 			new->owner = owner;
 			new->pid = __nlm_alloc_pid(host);
 			new->host = nlm_get_host(host);
diff --git a/include/linux/lockd/lockd.h b/include/linux/lockd/lockd.h
index 809619553e9b..345fced8339e 100644
--- a/include/linux/lockd/lockd.h
+++ b/include/linux/lockd/lockd.h
@@ -122,7 +122,7 @@ static inline struct sockaddr *nlm_srcaddr(const struct nlm_host *host)
  */
 struct nlm_lockowner {
 	struct list_head list;
-	atomic_t count;
+	refcount_t count;
 
 	struct nlm_host *host;
 	fl_owner_t owner;
--
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