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

1. Alibi: Attested Instruction Tracing as a Secure Thin Intermediate Layer (STIL) Primitive AmitVasudevan, Chen Chen, Adrian Perrig CyLab, Carnegie Mellon University Vyas Sekar, Petros Maniatis ISTC for Secure Computing

2. 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

3. State of cloud computing today .. 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. 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. Need stronger, verifiable resource accounting!

4. What capabilities do we want? Task (T) Provider (P) Verifier T,R,W,A Report (R) Witness (W) Trusted Layer Attribution Model (A) • Did I use the resources billed? • e.g., T did physically consume X cycles • Is 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?

5. Did-I/Should-I Clean Slate Solution Customer Verifier Provider Task1 Task2 Resource 1 “Trusted” Resource 2 Interrupts Requests Log of Requests, interrupts Hidden Allocator Template Private Policy Decisions Allocator Template Decisions Log of Decisions Task1 Task2 No spurious reports Visibility into low-level Resource 1 Resource 2 “Trusted” Hardware-root-of-trust “Witness”

6. Challenges • 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? Attested Instruction Tracing

7. Attested Instruction Trace Program void alibi(int p, intv, inta){ if( (p+v) >= a) printf("\nGreater/Equal"); else printf("\nLesser"); } • 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 Instruction Trace Instruction Trace … leal (%edx,%eax), %eax cmpl 16(%ebp), %eax jle .L2 movl $.LC0, (%esp) call printf jmp .L4 .L2: movl $.LC1, (%esp) call printf .L4: leave ret … leal (%edx,%eax), %eax cmpl 16(%ebp), %eax jle .L2 movl $.LC0, (%esp) call printf jmp .L4 .L2: movl $.LC1, (%esp) call printf .L4: leave ret

8. 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!

9. Alibi: Architectural Overview Operating System Verifier 2. Register Process 4. Instruction Trace Collection Callback Report 3. Entry /Exit Alibi Hypervisor Alibi Hypervisor 5. Reporting 1. Trusted Launch Process Customer Callback Provider

10. 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 • …

11. Alibi: Attested Instruction Tracing as a Secure Thin Intermediate Layer (STIL) Primitive • Introduction and Motivation • Attested Instruction Tracing • Machinery • Summary and Discussion

12. Prior work from CMU: XTRec Shaded = Trusted Leverage Branch Trace Message (BTM) feature

13. Branch Trace Messages • 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 13

14. 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

15. 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

16. Alibi: Attested Instruction Tracing as a Secure Thin Intermediate Layer (STIL) Primitive • Introduction and Motivation • Attested Instruction Tracing • Machinery • Summary and Discussion

17. 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