1. Improving Xen Security through Disaggregation Derek MurrayGrzegorz MilosSteven Hand

2. Outline The myth of the secure hypervisor Trusted computing bases Disaggregating Xen Results Future work

3. Xen Small hypervisor –100k lines of code Provides isolation between VMs “Trusting the virtual machine monitor is akin to trusting a real processor” OS Xen Hardware VM

4. Domain Zero Full Linux distribution User-space tools for VM management Privileged hypervisor interface –Map foreign memory –Set foreign VCPU Therefore must be trusted VMDom0 Xen Hardware

5. Threat Model Malicious software running as Dom0 root –Root exploit on Dom0 –Untrusted administrator Want to protect security of other VMs –Confidentiality –Integrity Solution: disaggregation

6. Trusted Computing Base “The set of components on which a subsystem depends” “The totality of protection mechanisms... responsible for enforcing a computer security policy” Anything that can directly invoke a privileged operation –And hence undermine security

7. Call Graph PD z PD y PD x

8. Current Xen Control Stack Build VM Make hypercall Map memorySet VCPU Dom0 User Dom0 Kernel Hypervisor

9. Minimise the TCB? Build VM Make hypercall Map memorySet VCPU Dom0 User Dom0 Kernel Hypervisor

10. Smaller is not always better Build VM Make hypercall Map memorySet VCPU Dom0 User DomB Hypervisor

11. Implementation Xen Dom0 DomB DomU … Xend

12. Results Smaller, static TCB –No longer contains Dom0 userspace –Now only VMM, DomB and Dom0 kernel –With an I/O MMU, only VMM and DomB Other VMs protected from Dom0 root

13. Future Work Virtual TPM support Automated techniques for disaggregation Metrics for trustworthiness

14. Conclusions Virtualised TCB can be surprising Smaller TCB is not always better Choosing appropriate interfaces is crucial

15. Questions