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, 3 insertions, 3 deletions

diff --git a/fs/nfs/nfs4state.c b/fs/nfs/nfs4state.c
index 1887134d5231..3bd79b8c016b 100644
--- a/fs/nfs/nfs4state.c
+++ b/fs/nfs/nfs4state.c
@@ -1177,7 +1177,7 @@ void nfs4_schedule_state_manager(struct nfs_client *clp)
         if (test_and_set_bit(NFS4CLNT_MANAGER_RUNNING, &clp->cl_state) != 0)
                 return;
         __module_get(THIS_MODULE);
-        atomic_inc(&clp->cl_count);
+        refcount_inc(&clp->cl_count);
 
         /* The rcu_read_lock() is not strictly necessary, as the state
          * manager is the only thread that ever changes the rpc_xprt
@@ -1269,7 +1269,7 @@ int nfs4_wait_clnt_recover(struct nfs_client *clp)
 
         might_sleep();
 
-        atomic_inc(&clp->cl_count);
+        refcount_inc(&clp->cl_count);
         res = wait_on_bit_action(&clp->cl_state, NFS4CLNT_MANAGER_RUNNING,
                                  nfs_wait_bit_killable, TASK_KILLABLE);
         if (res)
@@ -2510,7 +2510,7 @@ static void nfs4_state_manager(struct nfs_client *clp)
                         break;
                 if (test_and_set_bit(NFS4CLNT_MANAGER_RUNNING, &clp->cl_state) != 0)
                         break;
-        } while (atomic_read(&clp->cl_count) > 1);
+        } while (refcount_read(&clp->cl_count) > 1);
         return;
 out_error:
         if (strlen(section))