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Terra: A Virtual Machine-Based Platform for Trusted Computing

Tal Garfinkel, Ben Pfaff, Jim Chow, Mendel Rosenblum, Dan Boneh

(Stanford)

SOSP’03

Presented by: Michael LeMay

Introduction

• VM-based architecture for supporting various security models on a single physical machine

• Context: Old paper, not fully implementable at the time and many of the techniques are outdated, but principles have still been very influential

Outline

• Background

• Security Requirements

• Terra Design

• Limitations of Terra

• Enhanced Technologies

• Conclusion

Standard Operating Systems

• Large codebases– Difficult to verify correctness and bug-

freeness

• Offer poor isolation between applications– Compromise in one can affect entire system

• Security reduced to level of most vulnerable application– Applications with varying security

requirements are unable to share a machine

• Don’t provide trusted paths

Closed-Box Platforms

• Advantages:– Greater security provided by custom

hardware and software, perhaps including tamper-resistance

– Embedded cryptographic keys can be used to authenticate platform to remote entity

• Disadvantages:– Expensive– Unable to utilize standard software for

commodity systems

Example Closed Boxes

• Automated Teller Machines

• Many cellphones

• Game consoles

• All three of the above have been cracked!

• Advanced meters– These can be easily attacked

• Are there any other closed-box platforms you can think of?

What is a closed-box, really?

• Something that doesn’t support standard development interfaces by default– Linux: GDB, /dev/kmem, cp, etc.

• Open-box

– Cellphone: Windows Mobile File Explorer– PS3: ?

• Closed boxes

Terra Objectives

• Combine best aspects of open- and closed-box platforms:– Strong support for privacy and integrity– Support for standard hardware and software

• Provide semantics of dedicated, tamper-evident hardware platform to each application

• Doesn’t actually use trusted hardware in implementation! Think about implications…

Realization

• Trusted Virtual Machine Monitor (TVMM)– Assurances:

• Isolates VMs• Will not falsely attest VM state• Will not disclose or allow tampering with contents

of closed-box VM• These assurances are root-secure

– Limitations:• Unable to guarantee availability

– Relies on Management VM to provide additional functionality

Terra Architecture

Trusted Platform Module

TPM Interconnection

14

TPM 1.0 Components

TCG 1.0 Architecture Overview

15

Credential Types

• TPM contains 5 types of credentials:– Important:

• Endorsement or EK credential: uniquely identifies TPM, privacy concern

• Identity or AIK credential: Issued by privacy CA to preserve privacy of EK credential

– Not-so-important:• Conformance credential: Certifies that TPM meets

specifications• Platform credential: Identifies TPM manufacturer

and capabilities• Validation credential: Associated with peripheral or

software to guarantee integrity

Threat Model

• Threat model dictated by hardware capabilities:– Tamper-evident, not tamper-resistant TPM

• Sets bit internally when compromised, but doesn’t zero information

– Hardware-protected CPU and memory against adversaries lacking electrical analysis equipment and expertise

• Joe pointed out that this assumption was violated on the Xbox

– Unprotected disk• Can be removed and inserted into new machine

Remote Attestation

• Three phases:– Measurement: machine to be attested must

measure its properties locally– Attestation: transfer measurements from

machine being attested to remote machine– Verification: remote machine examines

measurements transferred during attestation and decides whether they are valid and acceptable

Linux Integrity Measurement

Linux Attestation

Linux Verification

Terra Attestation Process

• Lower layers certify higher layers:– TPM → Firmware → Boot Loader → TVMM

→ VM → Application

• For each layer above TPM:– Upper layer generates public/private keypair– Upper layer requests that lower layer certify its public

key and perhaps other data– Lower layer signs certificate with hash over attestable

parts of requests as the common name (main identifier) and the hashed data as auxiliary information

TVMM Attestation (cont.)

• VM disk contents included in attestation– Simple hash tree used to optimize

performance• Permits VM to run for indefinite time using false

disk hash

– Encrypted, integrity-protected, and non-encrypted disks all supported

– Keys used to protect disks placed in sealed storage, to prevent attackers from removing disks and performing an offline compromise

Attestation Verification

• Verify certificate in each layer by ensuring that it is signed by lower layer– TPM certificate is signed by TPM

manufacturer, which is also responsible for issuing CRLs

• No TPM manufacturer currently does this

• Check software hashes and attested data contained within certificates, ensure they are all trusted.

Attestation Binding

• Verification must be bound to attested process in some way

• Exchange certificate chains during SSL handshake

• If software is good, it will not persist session key

• Prevents system from rebooting and continuing execution in unattested state– This problem was pointed out by John

Attestation Limitations

• In Terra, PCRs are not necessarily utilized, so no boot history is maintained

• Verifying entity must have comprehensive and up-to-date list of trusted software configurations– A simple patch or software upgrade generates

entirely new hash

• Difficult to obtain meaningful security assurances from simple software hashes

Policy-Reduced Integrity Measurement Architecture

JaegerSS 2006

Attestation Limitations (cont.)

• Compromising a TPM key undermines entire process– TPM vendors do not maintain CAs or CRLs

• Platform privacy compromised by attestation– Endorsement Key uniquely identifies platform– Privacy CAs issue alternative keys (AIKs), but

have an unsustainable business model– Terra proposes group signatures


• Attestation can increase vendor lock-in and platform discrimination by permitting verifying entities to check the exact, complete software stack of the system.

• Attestation can only guarantee past and present system properties, it can say very little about the future.– Owner can always turn machine off– Operator can often load new software that

violates trust (can be prevented)


• John pointed out that TPM revocation is an issue. Fortunately, TPMs are unlikely to be compromised while the system is running.

• Must decide how much state to attest. Should updating the high scores in Trusted Quake break the attestation?– Use separate images for configuration data


• Can’t directly detect MITM exploits– Attestation simply tells you what software is

running on system, not the identity of the system

– Could corrupt DNS or routing tables and run legitimate server, client wouldn’t be able to detect.

– Is this a bad thing?– No, leads to privacy problems.


• Useful for clients attempting to verify security of servers?– No, clients couldn’t possibly be provided with

source code for major websites, so attestation would be meaningless.

• Is isolation important from the standpoint of attestation?– Somewhat orthogonal, but often an important

property being attested.

Management VM

• Analogous to Xen Dom0

• Performs administrative functions:– Create virtual device (NIC, block dev, etc.)– Creates new VM– Attach/detach virtual device to/from VM– Powers VM up or down– Suspends and resumes VM

• Hosts drivers

Driver Security

• Terra designers do not wish to include drivers in TCB, since they are often buggy– They suggest nooks, but the VM-based driver

isolation technique we discussed would also be applicable

• Just one of the problems with the suggested Terra architecture

Security-Enhanced Xen

• Would provide significant benefits to Terra

• Mandatory Access Control (MAC) for VM objects and commands– Would permit controlled data sharing between

Terra VMs, using shared memory buffers– Currently implemented by IBM as sHype– Xen Security Modules (XSM) provides

extended hooks, backwards compatibility with sHype, and support for SELinux-style Type Enforcement policies

Security-Enhanced Xen (cont.)

• Better TPM support:– Trusted/secure boot– TPM virtualization

• Dom0 decomposition– Break management interface into pieces,

allow different domains to use various parts– Run drivers in separate domains

• Secure I/O– IO-MMU support

http://www.xensource.com/files/xs0106_intel_xen_security.pdf, http://www.xensource.com/files/XenSecurity_SHand.pdf

TPM Virtualization

http://www.xensource.com/files/XenSecurity_Intel_CRozas.pdf

Intel LaGrande

Intel Trusted Execution Technology (TET)

http://www.intel.com/technology/security/downloads/arch-overview.pdf

TET System Architecture

TET System Implementation

• Enter VMM mode using GETSEC[SENTER] instruction, measures VMM before transferring control

• CPU provides internal RAM that can execute code after hashing code and verifying against embedded digital signature. Enter Authenticated Code (AC) mode using GETSEC[ENTERACCS] instruction.– Will only run software signed by Intel– Is this a good thing? What if Intel decides they

don’t like ATI, so you have to choose between an ATI graphics card and an Intel processor?

http://download.intel.com/technology/security/downloads/31516803.pdf

ARM TrustZone

http://www.arm.com/products/esd/trustzone_home.html

42

Microsoft NGSCB

• Microsoft, AMD, HP, IBM, Infineon, Intel, Sun, … all members of TCG

• Uses TPM to partitionsystem into two parts:Nexus and L.H.S.

• NCAs: Nexus Comput-ing Agents

• Only two compartments

43

NGSCB Architecture – WinHEC 2004

ScenariosAdministration &

ManagementServices and User

I/OWindows

Nexus

WindowsApps

Main OS

USB DriverDriver NexusMgr.sys

HAL

Secure Input

Secure Video

TPM 1.2

CPU RAM

WindowsAppsWindows

Apps

AgentTrusted User Engine (TUE)

TSP

TSP

Runtime

Most hardware

TSP

Runtime Runtime

TSP

Agent

Windows Owns most HW Only real-time OS Security benefits via scenarios

Compartments are Windows-based Significantly reduced footprint Strongly Isolated, hardened and armored

Secure device ownership Nexus or service compartments

•Great device diversity •Thousands of drivers•MLOC

•Little device diversity•Only a few drivers•KLOC

Biddle, 2004

Additional Questions

• What are some closed-box platforms that could not be easily adapted to run within Terra?

• What are some closed-box platforms that could benefit from Terra?

• What changes to Terra or its infrastructure are necessary to shrink the first list and/or expand the second?

Additional Questions (cont.)

• Are Terra-provided closed-box platforms more secure than standard closed-box platforms?– Use tamper-evident hardware for key storage– Would your answer change if the TPM were

integrated into the CPU so it can’t be snooped?

Conclusion

• Very influential general architecture

• Proposed before much needed functionality was available, but could be easily updated to use new functionality

• More general than NGSCB, but should benefit from NGSCB-inspired hardware

APPENDICES

48

Attested Meter

• Distributed Energy Resource management

• Demand Reducation/Load Management

• Automated Meter Reading/Real Time Pricing

49

Problem

• For real-time pricing to work, power company has to know exactly how much power was used by each customer at each point in time– Could be privacy problem

• User should be able to access consumer portal software on meter from local network– We’re taking a closed-box platform, a meter,

and adding an isolated open-box application– Same thing suggested by Ravinder for Xbox

Attested Meter Architecture

Motivating Applications

• Trusted Access Point

• High-Assurance Terminals

• Isolated Monitors

• Virtual Secure Coprocessors

• Trusted Quake

• Attested Meter

Trusted Access Point

High-Assurance Terminals

• E-voting machines that attest their proper operation to the central tallying authority

• Stock feed viewers

Isolated Monitors

• Key stores– Confidentiality must be very carefully

protected

• Intrusion detection systems and virus scanners– The VMI paper covered these in detail

• Secure loggers– Should not be possible for compromised

system being logged to remove entries

Virtual Secure Coprocessors

• Privacy-preserving databases

• Secure auctions

• Online commerce applications

Trusted Quake

• Game clients and servers can be modified to provide additional functionality to players– Aiming proxies: modify network commands to

stabilize or otherwise assist in aiming weapons

– Eavesdropping: determine information about other players’ activities

• Puts other players at disadvantage

SECURITY REQUIREMENTS

Root Security

• System administrator should not be able to compromise fundamental security assurances of system– Non-traditional security model: protect system

from its owner/user

Remote Attestation

• Allow remote entity to determine properties about particular system– What hardware is present?– What software is running?– What security properties does the software

provide?

Trusted Path

• Guarantees that specific input was received from legitimate user

• Guarantees that output is only provided to legitimate user

• Addresses social engineering attacks, such as phishing

PROPERTIES OF COMMODITY SYSTEMS

Implications of Characteristics

• Remote systems must be assumed to be malicious– E.g. Game server must assume that game

clients have been modified to cheat– May prevent cooperation between components,

or necessitate additional external monitoring of behavior

• Example: P2P client– If trusted, not necessary to audit operation

– If untrusted, might ask surrounding clients to monitor behavior of client.

• Increases system complexity

TERRA DESIGN

64

VM

TVMM Attestation

• Each layer of software has a keypair

• Lower layers certify higher layers

• Enables attestation ofentire stack

Hardware (TPM)

Firmware

Operating System

Application

Bootloader

TVMM (Terra)Hash of Attestable Data

Higher Public Key

Other Application Data

Signed by Lower Level

Certificate Layers

HARDWARE SUPPORT

Required Hardware

• Hardware attestation– Securely measure system

configuration and issue digitally-signed certificates for that state

• Sealed storage– Store persistent data such that

only system with specific configuration can access it

TPM

Required Hardware (cont.)

• Hardware virtualization acceleration– False requirement, just helps to minimize

VMM (TCB) size and increase performance

• Secure I/O– Secure connection between TVMM and

mouse, keyboard, video card, etc.

• Secure counter– Prevents replay/rollback attacks

Required Hardware (cont.)

• Device isolation– Prevent devices from using DMA to overwrite

TVMM structures, etc.

• Real-time support– Many closed-box platforms have real-time

requirements that are not supported by current commodity architectures

69

TCG Layers

http://trousers.sourceforge.net

TPM 1.2

• Additions:– Direct Anonymous Attestation– Symmetric crypto: 3-DES, AES192,256– Removable Endorsement Key

Next Try TCG 1.2: Trustworthy or Treacherous? (warning: conspiratorial)

71

Opposition

• Trusted Computing has many opponents, because it considers the computer operator to be a potential attacker:– EFF: Trust Computing: Promise and Risk– Against-TCPA – LAFKON - A movie about Trusted Computing

• And, a rebuttal:– TCPA Misinformation Rebuttal and Linux

drivers

Credential Relationships

DevID Relationship to TPM

TERRA IMPLEMENTATION

Basic Implementation

• VMware Server GSX 2.0.1 on Debian

• Python management VM interface

• Serial port interface to TVMM attestation mechanism– Doesn’t use TPM

Implementation Performance

• Trusted Quake:– Direct boot (no attestation): 26.6 seconds– Optimistic attestation: 27.1 seconds– Encrypted optimistic attestation: 29.1 seconds– Ahead-of-time attestation: 57.1 seconds– Interactive performance apparently equal

across the board (but much slower than native I’m sure!)

SAMPLE APPLICATIONS - REVISITED

78

Trusted Access Points

• VPN client can be implemented as closed-box VM and distributed to visitors when they first connect to a regulated network

• VM can attest to VPN gateway that it is operating properly, and will enforce intended traffic regulations

79

TAP Benefits

• Prevents source forgery: TAP can reliably check all outgoing packets

• Prevents DoS attacks: TAP can block DoS attacks at their source, before they even reach the network

• Scalability: Clients enforce regulations on their own traffic

• Network Scalability: TAP can perform local vulnerability scan on host before permitting it to connect

80

Example #1

• Online gaming: Quake• Players often modify Quake to provide additional

capabilities to their characters, or otherwise cheat

• Quake can be transformed into a closed-box VM and distributed to players

• Remote attestation shows that it is unmodified• Very little performance degradation• Covert channels remain, such as frame rate

statistics

81

Trusted Quake Assurances

• Secure Communication: VM can’t be inspected, so shared key can be embedded in VM image to protect network communication– Any software can be reverse engineered, so

is this a good idea?

• Client Integrity: maps and media files are protected from modification on client

• Server Integrity: Bad clients can’t connect

82

Trusted Quake Weaknesses

• Bugs and Undesirable Features: Rendered polygon OSD permits prediction of impending character appearances

• Network DoS Attacks: Terra does nothing in this regard

• Out-of-Band Collusion: Players can still communicate if they’re sitting together in a basement or using IM

ANALYSIS

Advantages of Terra

• Applications can customize their software stacks

• Supports attestation primitives directly

• Same hardware interface as physical machine

• Hardware memory and CPU protection, and software protection of disks prevents tampering by platform owner and other attackers

Limitations of Terra

• Obsolete attestation system (easily enhanced)

• Lack of trusted path support (coming soon)

• Lack of any inter-VM sharing (porting to Xen would provide this)

• Monolithic management VM (resolved by XenSE)

RELATED WORK

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