fs, nfs: convert nfs_client.cl_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 nfs_client.cl_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: Anna Schumaker <Anna.Schumaker@Netapp.com>

1 files changed, 5 insertions, 5 deletions

diff --git a/fs/nfs/nfs4client.c b/fs/nfs/nfs4client.c
index e9bea90dc017..31b5bc0f10a4 100644
--- a/fs/nfs/nfs4client.c
+++ b/fs/nfs/nfs4client.c
@@ -483,7 +483,7 @@ static int nfs4_match_client(struct nfs_client  *pos,  struct nfs_client *new,
          * ID and serverowner fields.  Wait for CREATE_SESSION
          * to finish. */
         if (pos->cl_cons_state > NFS_CS_READY) {
-                atomic_inc(&pos->cl_count);
+                refcount_inc(&pos->cl_count);
                 spin_unlock(&nn->nfs_client_lock);
 
                 nfs_put_client(*prev);
@@ -559,7 +559,7 @@ int nfs40_walk_client_list(struct nfs_client *new,
                  * way that a SETCLIENTID_CONFIRM to pos can succeed is
                  * if new and pos point to the same server:
                  */
-                atomic_inc(&pos->cl_count);
+                refcount_inc(&pos->cl_count);
                 spin_unlock(&nn->nfs_client_lock);
 
                 nfs_put_client(prev);
@@ -715,7 +715,7 @@ int nfs41_walk_client_list(struct nfs_client *new,
                         continue;
 
 found:
-                atomic_inc(&pos->cl_count);
+                refcount_inc(&pos->cl_count);
                 *result = pos;
                 status = 0;
                 break;
@@ -749,7 +749,7 @@ nfs4_find_client_ident(struct net *net, int cb_ident)
         spin_lock(&nn->nfs_client_lock);
         clp = idr_find(&nn->cb_ident_idr, cb_ident);
         if (clp)
-                atomic_inc(&clp->cl_count);
+                refcount_inc(&clp->cl_count);
         spin_unlock(&nn->nfs_client_lock);
         return clp;
 }
@@ -804,7 +804,7 @@ nfs4_find_client_sessionid(struct net *net, const struct sockaddr *addr,
                     sid->data, NFS4_MAX_SESSIONID_LEN) != 0)
                         continue;
 
-                atomic_inc(&clp->cl_count);
+                refcount_inc(&clp->cl_count);
                 spin_unlock(&nn->nfs_client_lock);
                 return clp;
         }