Securing Your Cloud with Xen Project s Advanced Security Features

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Securing Your Cloud with Xen Project’s Advanced Security Features

Russell Pavlicek, Xen Project Evangelist

CloudOpen North America 2013

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Who is the Old, Fat Geek Up Front?

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Xen Project Evangelist

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Employed by Citrix, focused entirely on the Xen Project

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History with Open Source begins in 1997

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Former columnist with Infoworld, Processor magazines

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Former panelist on The Linux Show webcast, repeat guest on The Linux Link Tech Show

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Over 150 pieces published, plus one book on Open Source

development and several blogs

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Introduction: Xen Project and Security

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Xen Project is an enterprise-grade Type I hypervisor

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Built for the Cloud before it was called the Cloud

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A number of advanced security features

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Driver Domains, Stub Domains, FLASK, and more

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Most of them are not (or cannot) be turned on by default

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Although they are simple to use, sometimes they can appear

to be complicated

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Presentation Goals

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Introduce you to key Xen Project Security Tools

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Discuss some key Xen security features

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Get you started in the right direction toward securing your

Xen installation

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Presentation Outline

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A few thoughts on the problem of securing the Cloud

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Overview of the Xen architecture

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Brief introduction to principles of security analysis

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Consider some attack surfaces and Xen features we can use to mitigate them:

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Driver Domains

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PVgrub

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Stub Domains

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Paravirtualization (PV) mode vs Hardware Virtualization (HVM) mode

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FLASK example policy

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The Cloud Security Conundrum

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Security: The 800 pound gorilla of the Cloud world

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Nothing generates more fear in specific, and FUD in general

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Probably the single greatest barrier to Cloud adoption

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Immediately behind it is the inability to get out of a 20th century IT mindset (i.e., ”Change is Bad”)

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The good news: we don’t need to fear it – we just need to

solve it

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Cloud Security: New Visibility to Old Problem

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Security has always been an issue

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Putting a truly secure system in the open does not reduce its security, just increases the frequency of attack

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Unfortunately, system security behind the firewall has not always been comprehensive

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Having solutions in Clouds forces us to solve the security issues we should have already solved

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Security through obscurity is no longer sufficient

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Security by Design, not by Wishful Thinking

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Security by Wishful Thinking is Officially Dead

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Merely hoping that your firewall holds off the marauding hordes is NOT good enough

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Addressing security in one area while ignoring others is NOT good enough

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Saying, ”We’ve never had a problem before” is NOT good enough

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Comprehensive security starts with design

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It needs to planned and carefully thought through

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It needs to be implemented at multiple levels

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It needs components which are themselves securable

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Xen Project: Security by Design

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Xen Project was designed for Clouds before the term ”Cloud”

was coined in the industry

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Designers foresaw the day of an ”infrastructure for wide-area distributed computing” which we now call ”the Cloud”

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http://www.cl.cam.ac.uk/research /srg/netos/xeno/publications.html

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Xen is designed to thwart attacks from many attack vectors,

using different defensive techniques

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Xen Architecture: A Basic Picture

Xen Hypervisor Hardware

device model (qemu) toolstack dom 0

Hardware Drivers

I/O Devices CPU Memory

Paravirtualized (PV) Domain

Fully Virtualized

(HVM) Domain netback

blkback

netfront blkfront

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Security Overview

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Threat Models:

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Attacker can access network

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Attacker controls one Guest VM

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Security considerations to evaluate:

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How much code is accessible?

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What is the interface like? (e.g., pointers vs scalars)

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Defense-in-depth

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Then combine security tactics to secure the installation

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There is no single ”magic bullet”

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Individual tactics reduce danger; combined tactics go even

farther

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Example System, for our Discussion

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Hardware setup

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Two networks: one Control network, one Guest network

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IOMMU with interrupt remapping (AMD or Intel VT-d v2) to allow for full Hardware Virtualization (HVM)

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Default configuration

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Network drivers in the Control Domain (aka ”Domain 0” or just ”Dom0”)

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Paravirtualized (PV) guests using PyGrub (grub-like boot utility within context of Guest Domain)

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Hardware Virtualized (HVM) guests using Qemu (as the device

model) running in the Control Domain

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Attack surface: Network path

Xen Hypervisor Hardware

toolstack dom 0

NIC Driver

Control NIC

Rogue Domain netback netfront

Guest NIC bridge iptables

Domain

netfront

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Where might an exploit focus?

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Bugs in hardware driver

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Bugs in bridging / filtering

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Bugs in netback (via the ring protocol)

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Attack surface: Network path

Xen Hypervisor Hardware

toolstack dom 0

NIC Driver

Control NIC

Guest NIC bridge iptables

Domain

netfront

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What could a successful exploit yield?

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Control of Domain 0 kernel

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Pretty much control of the whole system

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Security feature: Driver Domains

Xen Hypervisor Hardware

toolstack dom 0

NIC Driver

Control NIC

Guest NIC

bridge iptables

Domain

netfront

NIC Driver

Driver Domain

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What is a Driver Domain?

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Unprivileged VM which drives hardware, provides access to

guests

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Security feature: Driver Domains

Xen Hypervisor Hardware

toolstack dom 0

NIC Driver

Control NIC

Guest NIC

bridge iptables

Domain

netfront

NIC Driver

Driver Domain

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Now a successful exploit could yield:

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Control of the Driver Domain (PV hypercall interface)

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Control of that guest’s network traffic

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HowTo: Driver Domains

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Create a VM with appropriate drivers

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Use any distribution suitable as a Control Domain

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Install the Xen-related hotplug scripts

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Just installing the Xen tools in the VM is usually good enough

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Give the VM access to the physical NIC with PCI pass-through

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Configure the network topology in the Driver domain

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Just like you would for the Control Domain

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Configure the guest Virtual Network Interface (vif) to use the new domain ID

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Add backend=domnet to vif declaration

vif = [ ’type=pv, bridge=xenbr0, backend=domnet’ ]

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http://wiki.xenproject.org/wiki/Driver Domain

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Attack surface: PyGrub

Xen Hypervisor

toolstack dom 0

Paravirtualized (PV) Domain domain

builder

pygrub

guest disk

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What is PyGrub?

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grub implementation for PV guests

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Python program running in Control Domain

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Attack surface: PyGrub

Xen Hypervisor

toolstack dom 0

builder

pygrub

guest disk

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Where might an exploit focus?

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Bugs in file system parser

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Bugs in menu parser

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Bugs in domain builder

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Attack surface: PyGrub

Xen Hypervisor

toolstack dom 0

builder

pygrub

guest kernel disk

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What could a successful exploit yield?

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Control of Domain 0 user space

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Pretty much control of the whole system

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Security practice: Fixed kernels

Xen Hypervisor

toolstack dom 0

builder kernel

image

guest disk

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What is a fixed kernel?

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Passing a known-good kernel from Control Domain

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Removes attacker avenue to domain builder

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Security practice: Fixed kernels

Xen Hypervisor

toolstack dom 0

builder kernel

image

guest disk

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Disadvantages

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Host admin must keep up with kernel updates

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Guest admin can’t pass kernel parameters, custom kernels,

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Security feature: PVgrub

Xen Hypervisor

toolstack dom 0

domain builder

guest disk

MiniOS pvgrub

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What is PVgrub?

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MiniOS + PV port of grub running in a guest context

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PV equivalent of HVM “BIOS + grub”

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Now a successful exploit could yield:

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Control of the attacked guest domain alone

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HowTo: PVgrub

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Make sure that you have the PVgrub image

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pvgrub-$ARCH.gz

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Normally lives in /usr/lib/xen/boot

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Included in Fedora Xen packages

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Debian-based: need to build yourself

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Use appropriate PVgrub as bootloader in guest configuration kernel="/usr/lib/xen/boot/pvgrub-x86_32.gz"

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http://wiki.xenproject.org/wiki/Pvgrub

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Attack surface: Device model (Qemu)

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Where might an exploit focus?

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Bugs in NIC emulator parsing packets

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Bugs in emulation of virtual devices

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What could a successful exploit yield?

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Control Domain privileged userspace

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Pretty much control of the whole system

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Security feature: Qemu stub domains

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What is a stub domain?

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Stub domain: a small “service” domain running just one application

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Qemu stub domain: run each Qemu in its own domain

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What could a successful exploit yield?

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Control only of the stub domain VM (which, if FLASK is employed, is a relatively small universe)

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You need to devise another attack entirely to do anything

more significant

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HowTo: Qemu stub domains

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Make sure that you have the PVgrub image:

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ioemu-$ARCH.gz

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Normally lives in /usr/lib/xen/boot

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Included in Fedora Xen packages

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On Debian (and offshoots), you will need to build it yourself

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Specify stub domains in your guest config device_model_stubdomain_override = 1

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http://wiki.xenproject.org/wiki/Device Model Stub Domains

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Attack Surface: Xen Hypervisor itself

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Where might an exploit focus?

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On Paravirtualized (PV) Guests:

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PV Hypercalls

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On full Hardware Virtualized (HVM) Guests:

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HVM hypercalls (Subset of PV hypercalls)

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Instruction emulation (MMIO, shadow pagetables)

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Emulated platform devices: APIC, HPET, PIT

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Nested virtualization

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Security practice: Use PV VMs whenever possible

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Security feature: FLASK example policy

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What is FLASK?

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Xen Security Module (XSM): Xen equivalent of LSM

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FLASK: Framework for XSM developed by NSA

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Xen equivalent of SELinux

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Uses same concepts and tools as SELinux

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Allows a policy to restrict hypercalls

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What can FLASK do?

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Basic: Restricts hypercalls to those needed by a particular guest

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Advanced: Allows more fine-grained granting of privileges

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FLASK example policy

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This contains example roles for the Control Domain (dom0),

User/Guest Domain(domU), stub domains, driver domains,

etc.

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HowTo: Use the example FLASK policy

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Build Xen with XSM enabled

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Build the example policy

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Add the appropriate label to guest config files

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seclabel=[foo]

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stubdom label=[foo]

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Make sure you TEST the example policy in your environment BEFORE putting it into production!

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NOTE: As an example policy, it is not as rigorously tested as other parts of Xen during release, and it may not be suitable as-is if you are doing unusual things

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http://wiki.xenproject.org/wiki

/Xen Security Modules : XSM-FLASK

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ARM: Right solution for security

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Stays in ARM Hypervisor Mode

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The ARM architecture has separate Hypervisor and Kernel modes

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Because Xen’s architecture maps so well to the ARM architecture, Xen never has to use Kernel mode

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Other hypervisors have to flip back and forth between modes

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If a hypervisor has to enter Kernel mode, it loses the security of running in a privileged mode, isolated from the rest of the system

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This is a non-issue with the Xen Hypervisor on ARM

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Does not need to use device emulation

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No emulation means a smaller attack surface for bad guys

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