Scotch: Combining Software Guard Extensions and System Management Mode to Monitor Cloud Resource Usage

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1 Scotch: Combining Software Guard Extensions and System Management Mode to Monitor Cloud Resource Usage Kevin Leach 1, Fengwei Zhang 2, and Westley Weimer 1 1 University of Michigan, 2 Wayne State University Leach, Zhang, & Weimer 1 / 19

2 Summary We use Intel Software Guard Extensions (SGX) and System Management Mode (SMM) to accurately monitor resource consumption of virtual machines (VMs) in the presence of a compromised VM or hypervisor Leach, Zhang, & Weimer 2 / 19

3 Motivation Multi-tenant cloud computing increases utilization Client agrees to pay Cloud provider for a particular service level e.g., $1 per hour of CPU time Leach, Zhang, & Weimer 3 / 19

4 Motivation Multi-tenant cloud computing increases utilization Client agrees to pay Cloud provider for a particular service level e.g., $1 per hour of CPU time Cloud provider depends on hypervisor/virtual machine monitor (VMM) platform to distribute resources Xen, QEMU, etc. Cloud VMM VM 1 VM 2 VM 3 Leach, Zhang, & Weimer 3 / 19

5 Motivation Multi-tenant cloud computing increases utilization Client agrees to pay Cloud provider for a particular service level e.g., $1 per hour of CPU time Cloud provider depends on hypervisor/virtual machine monitor (VMM) platform to distribute resources Xen, QEMU, etc. 1 Cloud VMM 1 3 CPU 3 CPU 3 CPU VM 1 VM 2 VM 3 If all 3 VMs peg the CPU, the VMM must decide how to allocate CPU time based on each client s service level. Leach, Zhang, & Weimer 3 / 19 1

6 Motivation Cloud provider depends on VMM platform to distribute resources Leach, Zhang, & Weimer 4 / 19

7 Motivation Cloud provider depends on VMM platform to distribute resources Two issues Leach, Zhang, & Weimer 4 / 19

8 Motivation Cloud provider depends on VMM platform to distribute resources Two issues 1. What if the VMM/cloud provider is malicious? Manipulate resource consumption to bill customers more Leach, Zhang, & Weimer 4 / 19

9 Motivation Cloud provider depends on VMM platform to distribute resources Two issues 1. What if the VMM/cloud provider is malicious? Manipulate resource consumption to bill customers more 2. What if the VMM is vulnerable to malicious VMs? Malicious VM manipulates resource consumption to steal resources from benign customers Leach, Zhang, & Weimer 4 / 19

10 Resource Accounting Attacks Benign Behavior VMM decides to bill a guest: ms 30ms 60ms 90ms 120ms 150ms 180ms CPU time The Xen hypervisor regularly checks which VM is active to determine how much CPU time each VM uses Leach, Zhang, & Weimer 5 / 19

11 Resource Accounting Attacks Malicious Behavior VMM decides to bill a guest: ms 30ms 60ms 90ms 120ms 150ms 180ms CPU time A malicious VM (2) with knowledge of the VMM can affect the appearance of resource consumption by itself and benign VMs. Leach, Zhang, & Weimer 6 / 19

12 Resource Interference Attacks Attacker can take advantage of known victim behavior Malicious VM Flood HTTP server Victim VM (webserver) Free CPU cycles Thrash from I/O Malicious VM can cause benign VM to free up resources for itself Leach, Zhang, & Weimer 7 / 19

13 VM Escape Attack Malicious VM can exploit buggy VMM implementation, allowing code execution with VMM privilege Could potentially alter resource consumption to hide itself Leach, Zhang, & Weimer 8 / 19

14 Scotch: Transparent Cloud Resource Accounting Two desired properties 1. Transparent The underlying VMM and VMs are not aware accounting occurs 2. Tamper-resistant A malicious VMM or VM guest cannot reliably alter accounting data Leach, Zhang, & Weimer 9 / 19

15 Insights for Scotch System Management Mode High priority System Management Interrupt causes CPU to atomically execute SMM handler code Leach, Zhang, & Weimer 10 / 19

16 Insights for Scotch System Management Mode High priority System Management Interrupt causes CPU to atomically execute SMM handler code Use SMM to collect raw resource consumption data Leach, Zhang, & Weimer 10 / 19

17 Insights for Scotch System Management Mode High priority System Management Interrupt causes CPU to atomically execute SMM handler code Use SMM to collect raw resource consumption data SMM logically collects data, then relays it to SGX enclave Leach, Zhang, & Weimer 10 / 19

18 Insights for Scotch Software Guard Extensions Enclave-based trusted execution environment (TEE); userspace code runs in isolation Leach, Zhang, & Weimer 11 / 19

19 Insights for Scotch Software Guard Extensions Enclave-based trusted execution environment (TEE); userspace code runs in isolation Use SGX enclave so that benign user can monitor and verify their resource consumption Leach, Zhang, & Weimer 11 / 19

20 Insights for Scotch Software Guard Extensions Enclave-based trusted execution environment (TEE); userspace code runs in isolation Use SGX enclave so that benign user can monitor and verify their resource consumption Raw data collected by SMM is relayed to SGX enclave Leach, Zhang, & Weimer 11 / 19

21 Scotch: Transparent Cloud Resource Accounting Protected System 2 1 VMM (e.g., Xen) VM1 VM2 VM3 4 SGX Enclave SGX Enclave VM1 data VM2 data VM3 data True timer 3 SMI Handler Data 5 1. VMM decides to switch between VM guests Leach, Zhang, & Weimer 12 / 19

22 Scotch: Transparent Cloud Resource Accounting Protected System 2 1 VMM (e.g., Xen) VM1 VM2 VM3 4 SGX Enclave SGX Enclave VM1 data VM2 data VM3 data True timer 3 SMI Handler Data 5 2. Scotch measures resource consumption by invoking SMM every context switch Leach, Zhang, & Weimer 12 / 19

23 Scotch: Transparent Cloud Resource Accounting Protected System 2 1 VMM (e.g., Xen) VM1 VM2 VM3 4 SGX Enclave SGX Enclave VM1 data VM2 data VM3 data True timer 3 SMI Handler Data 5 3. SMM handler executes resource accounting in isolation Leach, Zhang, & Weimer 12 / 19

24 Scotch: Transparent Cloud Resource Accounting Protected System 2 1 VMM (e.g., Xen) VM1 VM2 VM3 4 SGX Enclave SGX Enclave VM1 data VM2 data VM3 data True timer 3 SMI Handler Data 5 4. Data is marshalled to SGX enclave within VM Leach, Zhang, & Weimer 12 / 19

25 Scotch: Transparent Cloud Resource Accounting Protected System 2 1 VMM (e.g., Xen) VM1 VM2 VM3 4 SGX Enclave SGX Enclave VM1 data VM2 data VM3 data True timer 3 SMI Handler Data 5 5. Benign VM can monitor resource accounting data with high integrity Leach, Zhang, & Weimer 12 / 19

26 Evaluation Research Questions RQ1: Can we maintain accurate accounting during scheduler attacks? RQ2: What is our overhead on benign workloads? RQ3: Can we maintain accurate accounting during resource interference attacks? RQ4: Can we maintain accurate accounting during VM escape attacks? Leach, Zhang, & Weimer 13 / 19

27 RQ1: Scheduler Attacks Implement controllable scheduler Simulate attacker by altering the CPU time allocation by a varying degree Leach, Zhang, & Weimer 14 / 19

28 RQ1: Scheduler Attacks Implement controllable scheduler Simulate attacker by altering the CPU time allocation by a varying degree Run two VMs, one simulated attacker and one benign Both are computing indicative workloads: pi, gzip, and the PARSEC benchmarks Leach, Zhang, & Weimer 14 / 19

29 RQ1: Scheduler Attacks Implement controllable scheduler Simulate attacker by altering the CPU time allocation by a varying degree Run two VMs, one simulated attacker and one benign Both are computing indicative workloads: pi, gzip, and the PARSEC benchmarks Compare observed CPU time consumption presented by Xen vs. Scotch Leach, Zhang, & Weimer 14 / 19

30 RQ1: Scheduler Attacks Implement controllable scheduler Simulate attacker by altering the CPU time allocation by a varying degree Run two VMs, one simulated attacker and one benign Both are computing indicative workloads: pi, gzip, and the PARSEC benchmarks Compare observed CPU time consumption presented by Xen vs. Scotch TL;DR Scotch shows significant difference in allocated CPU time Leach, Zhang, & Weimer 14 / 19

31 RQ1: Scheduler Attacks Table : Ratio of attacker VM CPU time to guest VM CPU time. Scheduler attack severity level Benign Scotch ground truth Leach, Zhang, & Weimer 15 / 19

32 RQ1: Scheduler Attacks Table : Ratio of attacker VM CPU time to guest VM CPU time. Scheduler attack severity level Benign Scotch ground truth The attacker receives disproportionate CPU time. Ground truth obtained with Xentrace. Leach, Zhang, & Weimer 15 / 19

33 RQ2: Overhead Invoking SMIs to run accounting code can be costly Leach, Zhang, & Weimer 16 / 19

34 RQ2: Overhead Invoking SMIs to run accounting code can be costly Accounting code takes 2248 ± 69 cycles to execute Roughly 1µs incurred every context switch Leach, Zhang, & Weimer 16 / 19

35 RQ2: Overhead Invoking SMIs to run accounting code can be costly Accounting code takes 2248 ± 69 cycles to execute Roughly 1µs incurred every context switch Xen takes roughly 20,000 cycles (7.1µs) per context switch Leach, Zhang, & Weimer 16 / 19

36 RQ2: Overhead Invoking SMIs to run accounting code can be costly Accounting code takes 2248 ± 69 cycles to execute Roughly 1µs incurred every context switch Xen takes roughly 20,000 cycles (7.1µs) per context switch Scotch adds 14% overhead per context switch Leach, Zhang, & Weimer 16 / 19

37 RQ2: Overhead Invoking SMIs to run accounting code can be costly Accounting code takes 2248 ± 69 cycles to execute Roughly 1µs incurred every context switch Xen takes roughly 20,000 cycles (7.1µs) per context switch Scotch adds 14% overhead per context switch.0033% system overhead on CPU-bound workloads Leach, Zhang, & Weimer 16 / 19

38 RQ3: Resource Interference Attacks By construction, Scotch provides accurate accounting information Leach, Zhang, & Weimer 17 / 19

39 RQ3: Resource Interference Attacks By construction, Scotch provides accurate accounting information Scotch does not automatically detect Resource Interference Attacks Leach, Zhang, & Weimer 17 / 19

40 RQ3: Resource Interference Attacks By construction, Scotch provides accurate accounting information Scotch does not automatically detect Resource Interference Attacks However, SGX allows userspace access to reliable accounting information Client can monitor their resource usage and perform their own analysis for their case Leach, Zhang, & Weimer 17 / 19

41 RQ4: VM Escape Attacks Accounting code stored in isolated SMRAM and SGX enclave Leach, Zhang, & Weimer 18 / 19

42 RQ4: VM Escape Attacks Accounting code stored in isolated SMRAM and SGX enclave Even if attacker roots hypervisor, they cannot change the accounting code Leach, Zhang, & Weimer 18 / 19

43 RQ4: VM Escape Attacks Accounting code stored in isolated SMRAM and SGX enclave Even if attacker roots hypervisor, they cannot change the accounting code BIOS locks SMRAM, so no opportunity for attacker to infiltrate SMM if BIOS is trusted Leach, Zhang, & Weimer 18 / 19

44 Conclusion Scotch is an SMM- and SGX-based framework for accurately accounting resource consumption in cloud infrastructure Leach, Zhang, & Weimer 19 / 19

45 Conclusion Scotch is an SMM- and SGX-based framework for accurately accounting resource consumption in cloud infrastructure Scotch accounts for resource usage every context switch, introducing minimal overhead on indicative workloads Leach, Zhang, & Weimer 19 / 19

46 Conclusion Scotch is an SMM- and SGX-based framework for accurately accounting resource consumption in cloud infrastructure Scotch accounts for resource usage every context switch, introducing minimal overhead on indicative workloads Scotch accurately accounts for CPU time consumption in the presence of scheduler attack Porting drivers to SMM would readily admit incorporating accounting for additional types of resources, such as network usage Leach, Zhang, & Weimer 19 / 19

47 Conclusion Scotch is an SMM- and SGX-based framework for accurately accounting resource consumption in cloud infrastructure Scotch accounts for resource usage every context switch, introducing minimal overhead on indicative workloads Scotch accurately accounts for CPU time consumption in the presence of scheduler attack Porting drivers to SMM would readily admit incorporating accounting for additional types of resources, such as network usage By construction, Scotch protects the hypervisor from VM escape and other control hijacking attacks Leach, Zhang, & Weimer 19 / 19

Scotch: Combining Software Guard Extensions and System Management Mode to Monitor Cloud Resource Usage

Scotch: Combining Software Guard Extensions and System Management Mode to Monitor Cloud Resource Usage Kevin Leach 1, Fengwei Zhang 2, and Westley Weimer 3 1 University of Virginia, Charlottesville, VA