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Alibi: Attested Instruction Tracing as a Secure Thin Intermediate Layer (STIL)

Primitive

Vyas Sekar, Petros ManiatisISTC for Secure

Computing

Amit Vasudevan, Chen Chen, Adrian PerrigCyLab, Carnegie Mellon

University

Ubiquity of Outsourcing Computation

• Fundamental advantage of cloud paradigm is computation outsourcing

• Trend shows we are increasingly placing trust on remote parties for computation

• To be sustainable we need stronger assurances

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State of cloud computing today ..

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It's that dreaded time of the month again, the time of the month that we, the 400,000+ Amazon Web Service consumers await with great anticipation / horror. What I'm talking about is the Amazon Web Services Billing Statement sent at beginning of each month.

As it turns out, Microsoft's doesn't disclose revenues related to its cloud services. And on that matter, it's not alone. Neither do Amazon, Google, or IBM.

Need stronger, verifiable resource accounting!

What capabilities do we want?

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Provider (P)Task (T)

Attribution Model (A)

Report (R)

Witness (W)

Verifier T,R,W,A

Trusted Layer

1. Did I use the resources billed?e.g., T did physically consume X cyclesIs P double counting or overcharging?

2. Should I have used these resources?e.g., Was it because of poor scheduling by P?Did T consume more due to “contention” with T’ on same CPU?

Epoch Resource1 Resource2

1 T1=5, T2=0 T1=1,T2=2

2 T1=1, T2=10

T1=0,T2=10

….

Did-I/Should-I Clean Slate Solution

DecisionsAllocatorTemplate

Requests

Interrupts

AllocatorTemplate

Decisions

PrivatePolicy

Hidden Log of

Requests, interrupts

Log of Decisions

VerifierCustomerProvider

Resource 1

Resource 2

“Trusted”

Task1 Task2

• Lack of suitable h/w accounting primitives– Most resources have “allocators” within the OS (or

VMM). E.g., memory, scheduler, disk etc. – Use a s/w monitoring layer that can deliver

required measurements• How does the provider justify what it did?

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Challenges

Attested Instruction Tracing

Attested Instruction Trace

• Atomic component of any program is an instruction

• Enables precise reasoning of what the program did during runtime

• What You See Is What Executed (WYSIWE) if the collection mechanism is robust to tampering 7

void alibi(int p, int v, int a){ if( (p+v) >= a) printf("\nGreater/Equal"); else printf("\nLesser");}

Program

…leal (%edx,%eax), %eaxcmpl 16(%ebp), %eaxjle .L2movl $.LC0, (%esp)call printfjmp .L4

.L2:movl $.LC1, (%esp)call printf

.L4:leaveret

Instruction Trace

…leal (%edx,%eax), %eaxcmpl 16(%ebp), %eaxjle .L2movl $.LC0, (%esp)call printfjmp .L4

.L2:movl $.LC1, (%esp)call printf

.L4:leaveret

Instruction Trace

Why the resource accounting problem is not solved by * ?

• Client-side sanity checks– Useful, but can’t really use it to justify anything

• Launch-time Attestation– E.g., TPM++, Flickr++– Need run time, not just load time

• Deterministic replay– E.g., AVM– Too much to trust, might give away too much info

• Control Flow Integrity++– Want actual run time accounting, not plausible flows!

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Operating System

Process

Callback

Alibi: Architectural Overview

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2. Register

3. Entry/Exit

5. Reporting1. Trusted Launch

Alibi Hypervisor

Customer

Provider

Verifier

Repo

rt

Alibi Hypervisor

Process

Callback

4. Instruction TraceCollection

Attested Instruction Tracing Benefits and Applications

• A strong general primitive– Attesting actual execution vs. launch-time attestation

• Benefits– fine-grained and accurate– minimal-TCB approach -> robust to tampering– can be used on commodity x86 platforms today!

• Applications– Attested Read from an input port– Attested accesses to disk – Attested CPU cycle counter read– Malware detection– …

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Alibi: Attested Instruction Tracing as a Secure Thin Intermediate Layer (STIL)

Primitive

• Introduction and Motivation• Attested Instruction Tracing• Machinery• Summary and Discussion

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Prior work from CMU: XTRec

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Shaded = Trusted

Leverage Branch Trace Message (BTM) feature

Branch Trace Messages

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• Emitted by the CPU for all branch instructions decoded at the Instruction Pointer

• Available on commodity x86 class CPUs; primarily used for debugging purposes

• Generated irrespective of the code privilege level (Ring 0-3, SMM and even the hypervisor!)

• Usually sent out on the system bus, BUT can be stored to system memory

Dynamic Code Capture

• BTMs record only control-flow instructions– What about other instructions?

• Hardware managed physical memory page tables– W XOR X policy on physical memory pages – Record contents of page prior to converting it to

executable status– Superimpose BTMs over corresponding code page

contents

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Alibi: Current status

• Port XTRec primitives to IA-32• Addressing performance issues– Selective logging, BTM cache, LBA-style offload

• Using Alibi for Did-I verifiability– e.g., challenges with dynamic libraries, sys-calls

• Using Alibi for Should-I verifiability– Guarantee privacy (code + data)– API for selective logging

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Alibi: Attested Instruction Tracing as a Secure Thin Intermediate Layer (STIL)

Primitive

• Introduction and Motivation• Attested Instruction Tracing• Machinery• Summary and Discussion

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Summary and Discussion• Attested Instruction tracing– A strong general primitive• Attesting actual execution vs. launch-time attestation

– fine-grained, accurate, minimal-TCB approach, robust to tampering and applicable to commodity x86 platforms!

• Verifiable Resource Accounting– Did-I and Should-I properties– Provider incentives• Adoption to avoid underutilization, less conservative in

accounting, prevent customers from gaming the system

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