Mitigating Multi-Tenancy Risks in IaaS Cloud Through Constraints-Driven Virtual Resource Scheduling

A major concern in the adoption of cloud infrastructure-as-a-service (IaaS) arises from multi-tenancy, where multiple tenants share the underlying physical infrastructure operated by a cloud service provider. A tenant could be an enterprise in the context of a public cloud or a department within an enterprise in the context of a private cloud. Enabled by virtualization technology, the service provider is able to minimize cost by providing virtualized hardware resources such as virtual machines, virtual storage and virtual networks, as a service to multiple tenants where, for instance, a tenant's virtual machine may be hosted in the same physical server as that of many other tenants. It is well-known that separation of execution environment provided by the hypervisors that enable virtualization technology has many limitations. In addition to inadvertent misconfigurations, a number of attacks have been demonstrated that allow unauthorized information flow between virtual machines hosted by a hypervisor on a given physical server. In this paper, we present attribute-based constraints specification and enforcement as a mechanism to mitigate such multi-tenancy risks that arise in cloud IaaS. We represent relevant properties of virtual resources (e.g., virtual machines, virtual networks, etc.) as their attributes. Conflicting attribute values are specified by the tenant or by the cloud IaaS system as appropriate. The goal is to schedule virtual resources on physical resources in a conflict-free manner. The general problem is shown to be NP-complete. We explore practical conflict specifications that can be efficiently enforced. We have implemented a prototype for virtual machine scheduling in OpenStack, a widely-used open-source cloud IaaS software, and evaluated its performance overhead, resource requirements to satisfy conflicts, and resource utilization.