dlm: fix user space lkb refcounting

This patch fixes to check on the right return value if it was the last
callback. The rv variable got overwritten by the return of
copy_result_to_user(). Fixing it by introducing a second variable for
the return value and don't let rv being overwritten.

Cc: stable@vger.kernel.org
Fixes: 61bed0ba ("fs: dlm: use a non-static queue for callbacks")
Reported-by: Valentin Vidić <vvidic@valentin-vidic.from.hr>
Closes: https://lore.kernel.org/gfs2/Ze4qSvzGJDt5yxC3@valentin-vidic.from.hrSigned-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>

fs/dlm/user.c

@@ -806,7 +806,7 @@ static ssize_t device_read(struct file *file, char __user *buf, size_t count,
 	struct dlm_lkb *lkb;
 	DECLARE_WAITQUEUE(wait, current);
 	struct dlm_callback *cb;
-	int rv, copy_lvb = 0;
+	int rv, ret, copy_lvb = 0;
 	int old_mode, new_mode;
 
 	if (count == sizeof(struct dlm_device_version)) {
@@ -906,9 +906,9 @@ static ssize_t device_read(struct file *file, char __user *buf, size_t count,
 		trace_dlm_ast(lkb->lkb_resource->res_ls, lkb);
 	}
 
-	rv = copy_result_to_user(lkb->lkb_ua,
-				 test_bit(DLM_PROC_FLAGS_COMPAT, &proc->flags),
-				 cb->flags, cb->mode, copy_lvb, buf, count);
+	ret = copy_result_to_user(lkb->lkb_ua,
+				  test_bit(DLM_PROC_FLAGS_COMPAT, &proc->flags),
+				  cb->flags, cb->mode, copy_lvb, buf, count);
 
 	kref_put(&cb->ref, dlm_release_callback);
 
@@ -916,7 +916,7 @@ static ssize_t device_read(struct file *file, char __user *buf, size_t count,
 	if (rv == DLM_DEQUEUE_CALLBACK_LAST)
 		dlm_put_lkb(lkb);
 
-	return rv;
+	return ret;
 }
 
 static __poll_t device_poll(struct file *file, poll_table *wait)