lockd: convert nlm_rqst.a_count from atomic_t to refcount_t

Web:        https://git.kernel.org/torvalds/c/d9226ec9ef01832b1edc1781241920614c6407db
Commit:     d9226ec9ef01832b1edc1781241920614c6407db
Parent:     8bb3ea77933e9796f8f15a5492481a96af8302d6
Refname:    refs/heads/master
Author:     Elena Reshetova <[email protected]>
AuthorDate: Wed Nov 29 13:15:46 2017 +0200
Committer:  J. Bruce Fields <[email protected]>
CommitDate: Thu Dec 21 15:24:19 2017 -0500

    lockd: convert nlm_rqst.a_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_rqst.a_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_rqst.a_count it might make a difference
    in following places:
     - nlmclnt_release_call() and nlmsvc_release_call(): decrement
       in refcount_dec_and_test() only
       provides RELEASE ordering and control dependency on success
       vs. fully ordered atomic counterpart
    
    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         | 8 ++++----
 fs/lockd/svcproc.c          | 2 +-
 include/linux/lockd/lockd.h | 2 +-
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/lockd/clntproc.c b/fs/lockd/clntproc.c
index 112173dbea76..a2c0dfc6fdc0 100644
--- a/fs/lockd/clntproc.c
+++ b/fs/lockd/clntproc.c
@@ -204,7 +204,7 @@ struct nlm_rqst *nlm_alloc_call(struct nlm_host *host)
 	for(;;) {
 		call = kzalloc(sizeof(*call), GFP_KERNEL);
 		if (call != NULL) {
-			atomic_set(&call->a_count, 1);
+			refcount_set(&call->a_count, 1);
 			locks_init_lock(&call->a_args.lock.fl);
 			locks_init_lock(&call->a_res.lock.fl);
 			call->a_host = nlm_get_host(host);
@@ -222,7 +222,7 @@ void nlmclnt_release_call(struct nlm_rqst *call)
 {
 	const struct nlmclnt_operations *nlmclnt_ops = call->a_host->h_nlmclnt_ops;
 
-	if (!atomic_dec_and_test(&call->a_count))
+	if (!refcount_dec_and_test(&call->a_count))
 		return;
 	if (nlmclnt_ops && nlmclnt_ops->nlmclnt_release_call)
 		nlmclnt_ops->nlmclnt_release_call(call->a_callback_data);
@@ -678,7 +678,7 @@ nlmclnt_unlock(struct nlm_rqst *req, struct file_lock *fl)
 		goto out;
 	}
 
-	atomic_inc(&req->a_count);
+	refcount_inc(&req->a_count);
 	status = nlmclnt_async_call(nfs_file_cred(fl->fl_file), req,
 			NLMPROC_UNLOCK, &nlmclnt_unlock_ops);
 	if (status < 0)
@@ -769,7 +769,7 @@ static int nlmclnt_cancel(struct nlm_host *host, int block, struct file_lock *fl
 	nlmclnt_setlockargs(req, fl);
 	req->a_args.block = block;
 
-	atomic_inc(&req->a_count);
+	refcount_inc(&req->a_count);
 	status = nlmclnt_async_call(nfs_file_cred(fl->fl_file), req,
 			NLMPROC_CANCEL, &nlmclnt_cancel_ops);
 	if (status == 0 && req->a_res.status == nlm_lck_denied)
diff --git a/fs/lockd/svcproc.c b/fs/lockd/svcproc.c
index 0d670c5c378f..ea77c66d3cc3 100644
--- a/fs/lockd/svcproc.c
+++ b/fs/lockd/svcproc.c
@@ -295,7 +295,7 @@ static void nlmsvc_callback_exit(struct rpc_task *task, void *data)
 
 void nlmsvc_release_call(struct nlm_rqst *call)
 {
-	if (!atomic_dec_and_test(&call->a_count))
+	if (!refcount_dec_and_test(&call->a_count))
 		return;
 	nlmsvc_release_host(call->a_host);
 	kfree(call);
diff --git a/include/linux/lockd/lockd.h b/include/linux/lockd/lockd.h
index 345fced8339e..94c9ff58fc91 100644
--- a/include/linux/lockd/lockd.h
+++ b/include/linux/lockd/lockd.h
@@ -136,7 +136,7 @@ struct nlm_wait;
  */
 #define NLMCLNT_OHSIZE		((__NEW_UTS_LEN) + 10u)
 struct nlm_rqst {
-	atomic_t		a_count;
+	refcount_t		a_count;
 	unsigned int		a_flags;	/* initial RPC task flags */
 	struct nlm_host *	a_host;		/* host handle */
 	struct nlm_args		a_args;		/* arguments */
--
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