Securing your cloud with Xen's advanced security features
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Xen is a mature enterprise-grade virtual machine with many advanced security features which are unique to Xen. For this reason it's the hypervisor of choice for the NSA, the DoD, and the new QubesOS Secure Desktop project. However, while much of the security of Xen is inherent in its design, many of the advanced security features, such as stub domains, driver domains, XSM, and so on are not enabled by default. This session will describe all of the advanced security features of Xen, and the best way to configure them for the Cloud environment.
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Securing your cloud with Xen's advanced security features
- 1. Intro Network path Bootloader Device model Xen Conclusion Securing your cloud with Xen’s advanced security features George Dunlap Brussels – February 2-3, 2013
- 2. Intro Network path Bootloader Device model Xen Conclusion Intro Xen is an enterprise-grade Type I hypervisor Built for the Cloud before it was called the Cloud A number of advanced security features Driver domains, stub domains, FLASK, &c Most of them aren’t or can’t be turned on by default Although simple to use, sometimes seem complicated Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 2 / 28
- 3. Intro Network path Bootloader Device model Xen Conclusion Goal Tools to think about security in Xen Know some key security features of Xen Equipped with the knowledge to get them working Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 3 / 28
- 4. Intro Network path Bootloader Device model Xen Conclusion Outline Overview of the Xen architecture Brief introduction to principles of security analysis Consider some attack surfaces and Xen features we can use to mitigate them Driver domains pvgrub stub domains PV vs HVM FLASK example policy Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 4 / 28
- 5. Intro Network path Bootloader Device model Xen Conclusion Xen Architecture dom 0 device model (qemu) toolstack Paravirtualized Fully (PV) Virtualized Domain (HVM) Hardware netback netfront Domain Drivers blkback blkfront Xen Hypervisor I/O Devices CPU Memory Hardware Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 5 / 28
- 6. Intro Network path Bootloader Device model Xen Conclusion Security Overview Threat Model Attacker can access network Attacker controls one guest OS Security considerations How much code is accessible? What is the interface like? (e.g., pointers vs scalars) Defense-in-depth Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 6 / 28
- 7. Intro Network path Bootloader Device model Xen Conclusion Example System Hardware setup Two networks: control network, guest network IOMMU with interrupt remapping (AMD or Intel VT-d v2) Default configuration Network drivers in dom0 PV guests with pygrub HVM guests with qemu running in domain 0 Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 7 / 28
- 8. Intro Network path Bootloader Device model Xen Conclusion Attack surface: Network path dom 0 toolstack Domain netfront iptables bridge Rogue Domain NIC netback netfront Driver Xen Hypervisor Control NIC Guest NIC Hardware How to break in? Bugs in hardware driver Bugs in bridging / filtering Bugs in netback via the ring protocol Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 8 / 28
- 9. Intro Network path Bootloader Device model Xen Conclusion Attack surface: Network path dom 0 toolstack Domain netfront iptables bridge Rogue Domain NIC netback netfront Driver Xen Hypervisor Control NIC Guest NIC Hardware What does it buy you? Control of domain 0 kernel Pretty much control of the whole system Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 9 / 28
- 10. Intro Network path Bootloader Device model Xen Conclusion Security feature: Driver Domains dom 0 Domain toolstack netfront Driver Domain iptables bridge Rogue Domain NIC netfront NIC netback Driver Driver Xen Hypervisor Control NIC Guest NIC Hardware What is it? Unprivileged VM which drives hardware, provides access to guests Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 10 / 28
- 11. Intro Network path Bootloader Device model Xen Conclusion Security feature: Driver Domains dom 0 Domain toolstack netfront Driver Domain iptables bridge Rogue Domain NIC netfront NIC netback Driver Driver Xen Hypervisor Control NIC Guest NIC Hardware Now an exploit buys you: Control of a PV VM (PV hypercall interface) Guest network traffic Control of NIC Opportunity to attack netfront of other guests Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 11 / 28
- 12. Intro Network path Bootloader Device model Xen Conclusion HowTo: Driver Domains Create a VM with appropriate drivers Any distro supporting dom0 should do Install the xen-related hotplug scripts Just installing the xen tools in the VM is usually good enough Give the VM access to the physical NIC with PCI pass-through Configure the network topology in the driver domain Just like you would for dom0 Configure the guest vif to use the new domain ID Add backend=domnet to vif declaration vif = [ ’type=pv, bridge=xenbr0, backend=domnet’ ] http://wiki.xen.org/wiki/Driver Domain Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 12 / 28
- 13. Intro Network path Bootloader Device model Xen Conclusion Attack surface: Pygrub dom 0 toolstack domain builder Paravirtualized pygrub (PV) Domain guest disk Xen Hypervisor What is it? grub implementation for PV guests Python program running in domain 0 Reads guest FS, parses grub.conf, presents menu Passes resulting kernel image to domain builder Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 13 / 28
- 14. Intro Network path Bootloader Device model Xen Conclusion Attack surface: Pygrub dom 0 toolstack domain builder Paravirtualized pygrub (PV) Domain guest disk Xen Hypervisor How to break in? Bugs in file system parser Bugs in menu parser Bugs in domain builder Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 14 / 28
- 15. Intro Network path Bootloader Device model Xen Conclusion Attack surface: Pygrub dom 0 toolstack domain builder Paravirtualized pygrub (PV) Domain kernel guest disk Xen Hypervisor What does it buy you? Control of domain 0 user space Pretty much control of the whole system Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 15 / 28
- 16. Intro Network path Bootloader Device model Xen Conclusion Security practice: Fixed kernels dom 0 toolstack kernel domain image builder Paravirtualized (PV) Domain guest disk Xen Hypervisor What is it? Passing a known-good kernel from domain 0 Removes attacker avenue to domain builder Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 16 / 28
- 17. Intro Network path Bootloader Device model Xen Conclusion Security practice: Fixed kernels dom 0 toolstack kernel domain image builder Paravirtualized (PV) Domain guest disk Xen Hypervisor Disadvantages Host admin must keep up with kernel updates Guest admin can’t pass kernel parameters, custom kernels, Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 17 / 28
- 18. Intro Network path Bootloader Device model Xen Conclusion Security feature: pvgrub dom 0 toolstack domain builder pvgrub MiniOS guest disk Xen Hypervisor What is it? MiniOS + pv port of grub running in a guest context PV equivalent of HVM “BIOS + grub” Now an exploit buys you: Control of your own VM Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 18 / 28
- 19. Intro Network path Bootloader Device model Xen Conclusion HowTo: pvgrub Make sure that you have the pvgrub image pvgrub-$ARCH.gz Normally lives in /usr/lib/xen/boot Included in Fedora Xen packages Debian-based: need to build yourself Use appropriate pvgrub as bootloader in guest config kernel="/usr/lib/xen/boot/pvgrub-x86_32.gz" http://wiki.xen.org/wiki/Pvgrub Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 19 / 28
- 20. Intro Network path Bootloader Device model Xen Conclusion Attack surface: Device model (qemu) How to break in? Bugs in NIC emulator parsing packets Bugs in emulation of virtual devices What does it buy you? Domain 0 privileged userspace Pretty much control of the whole system Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 20 / 28
- 21. Intro Network path Bootloader Device model Xen Conclusion Security feature: qemu stub domains What is it? Stub domain: a small “service” domain running just one application qemu stub domain: run each qemu in its own domain Now an exploit buys you: Control of the stubom VM ...which we assume you control Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 21 / 28
- 22. Intro Network path Bootloader Device model Xen Conclusion HowTo: qemu stub domains Make sure that you have the pvgrub image: ioemu-$ARCH.gz Normally lives in /usr/lib/xen/boot Included in Fedora Xen packages Debian-based: need to build yourself Specify stub domains in your guest config device_model_stubdomain_override = 1 http://wiki.xen.org/wiki/Device Model Stub Domains Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 22 / 28
- 23. Intro Network path Bootloader Device model Xen Conclusion Attack Surface: Xen PV guests PV Hypercalls HVM guests HVM hypercalls (Subset of PV hypercalls) Instruction emulation (MMIO, shadow pagetables) Emulated platform devices: APIC, HPET, PIT Nested virtualization Security practice: Use PV VMs Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 23 / 28
- 24. Intro Network path Bootloader Device model Xen Conclusion Security feature: FLASK example policy What is FLASK? Xen Security Module (XSM): Xen equivalent of LSM FLASK: Framework for XSM developed by NSA Xen Equivalent of SELinux Uses same concepts, tools as SELinux Allows a policy to restrict hypercalls What can FLASK do? Basic: Restricts hypercalls to those needed by a particular guest Advanced: Allows more fine-grained granting of privileges FLASK example policy This contains example roles for dom0, domU, stub domains, driver domains, &c Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 24 / 28
- 25. Intro Network path Bootloader Device model Xen Conclusion HowTo: Use the example FLASK policy Build Xen with XSM enabled Build the example policy Add the appropriate label to guest config files seclabel=[foo] stubdom label=[foo] WARNING: In 4.2, the example policy not extensively tested. Use with care! http://wiki.xen.org/wiki/Xen Security Modules : XSM- FLASK Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 25 / 28
- 26. Intro Network path Bootloader Device model Xen Conclusion Outline Overview of the Xen architecture Brief introduction to principles of security analysis Consider some attack surfaces and Xen features we can use to mitigate them Driver domains pvgrub stub domains PV vs HVM Experimental: Example policy for FLASK Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 26 / 28
- 27. Intro Network path Bootloader Device model Xen Conclusion Goal Tools to think about security in Xen Know some key security features of Xen Equipped with the knowledge to get them working Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 27 / 28
- 28. Intro Network path Bootloader Device model Xen Conclusion Questions Questions? More info at http://wiki.xen.org/wiki/Securing Xen Check out our blog: http://blog.xen.org/ Brussels – February 2-3, 2013 Securing your cloud with Xen’s advanced security features 28 / 28