In an Infrastructure-as-a-Service cloud, tenants rely on the cloud provider to provide "value-added" services such as data security and reliability. However, this provider-controlled service model is less flexible and cannot be customized to meet individual tenants' needs. In this paper, we present StorM, a novel middle-box service platform that allows each tenant to deploy tenant-specific security and reliability services -- in virtualized middle-boxes -- for their cloud data. With such middle-boxes, StorM divides the responsibilities of service creation between tenants and the provider by allowing tenants to customize their own cloud data polices and the provider to offer corresponding infrastructural support. In developing StorM, we address key challenges including network splicing, platform efficiency, and semantic gap. We implement a StorM prototype on top of OpenStack and demonstrate three tenant-defined security/reliability middle-box services, with low performance overhead (