libceph: Fix NULL pointer dereference in auth client code

A malicious monitor can craft an auth reply message that could cause a
NULL function pointer dereference in the client's kernel.

To prevent this, the auth_none protocol handler needs an empty
ceph_auth_client_ops->build_request() function.

CVE-2013-1059

Signed-off-by: Tyler Hicks <[e-mail]>
Reported-by: Chanam Park <[e-mail]>
Reviewed-by: Seth Arnold <[e-mail]>
Reviewed-by: Sage Weil <[e-mail]>
Cc: [e-mail]

net/ceph/auth_none.c

···
 	return xi->starting;
 }
 
+static int build_request(struct ceph_auth_client *ac, void *buf, void *end)
+{
+	return 0;
+}
+
 /*
  * the generic auth code decode the global_id, and we carry no actual
  * authenticate state, so nothing happens here.
···
 	.destroy = destroy,
 	.is_authenticated = is_authenticated,
 	.should_authenticate = should_authenticate,
+	.build_request = build_request,
 	.handle_reply = handle_reply,
 	.create_authorizer = ceph_auth_none_create_authorizer,
 	.destroy_authorizer = ceph_auth_none_destroy_authorizer,