fs, nfsd: convert nfs4_stid.sc_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 nfs4_stid.sc_count is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/fs/nfsd/nfs4layouts.c b/fs/nfsd/nfs4layouts.c
index e122da696f1b..fed076069dd2 100644
--- a/fs/nfsd/nfs4layouts.c
+++ b/fs/nfsd/nfs4layouts.c
@@ -335,7 +335,7 @@ nfsd4_recall_file_layout(struct nfs4_layout_stateid *ls)
 
 	trace_layout_recall(&ls->ls_stid.sc_stateid);
 
-	atomic_inc(&ls->ls_stid.sc_count);
+	refcount_inc(&ls->ls_stid.sc_count);
 	nfsd4_run_cb(&ls->ls_recall);
 
 out_unlock:
@@ -440,7 +440,7 @@ nfsd4_insert_layout(struct nfsd4_layoutget *lgp, struct nfs4_layout_stateid *ls)
 			goto done;
 	}
 
-	atomic_inc(&ls->ls_stid.sc_count);
+	refcount_inc(&ls->ls_stid.sc_count);
 	list_add_tail(&new->lo_perstate, &ls->ls_layouts);
 	new = NULL;
 done: