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DESCRIPTION
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Remote Virtual Machine Monitor Detection
Jason Franklin, Mark Luk, Jonathan McCune, Arvind Seshadri, Adrian
Perrig, Leendert van Doorn
Remote Virtual Machine Monitor Detection
Problem Statement• Determine if a remote machine is virtual or real
Challenges• VMM provides an accurate abstraction of the underlying hardware• VMM controls execution of code and may return arbitrary values
ExternalVerifier
Remote Machine
Are you virtual?
`
VMM Detection and Botnets (1/2)
Scenario 1• Bots may install a stealthy virtual machine
based rootkit (VMBR) to avoid detection by traditional malware scanners
• Stealthy rootkits prevent administered machines from removing bots• You run an AV, update, patch, yet never
locate/remove the bot
• Detecting VMMs allows us to detect bots
VMM Detection and Botnets (2/2)
Scenario 2• Bots may check for the existence of a VMM in
order to prevent dynamic analysis • “Detecting the sandbox”• Real threat & mentioned several times
yesterday• Agobot uses a heuristic to check for VMWare
• Studying VMM detection helps us understand how to enable VMM-based dynamic analysis
State of the Art in VMM Detection
Check for software-implementation artifacts• Redpill checks the location of the IDT (different
location under VMWare)• VMWare’s Back checks for VMWare I/O port
Other approaches• Make restrictive assumptions• Easy to thwart• Require benchmarking
Our Goals
Develop a VMM detection algorithm:• VMM implementation independent• Accurate• Practical/relies on few assumptions
Leverage fundamental differences between virtual and real machines
VMM Model
Popek and Goldberg ’74 formally defined the properties a control program must satisfy to be deemed a VMM• Efficiency Property• Resource Control Property• Equivalence Property
• Program execution in a virtual environment must be indistinguishable from execution in a real environment
Indistinguishable? Oh no! If a program executes
indistinguishably, we can’t detect a virtual execution environment
Don’t worry! There are exceptions to the equivalence property• Timing dependency exception
• Certain sequences of instructions may take longer to execute
• Resource availability exception
Does the timing dependency exception necessarily exist?
Empirically, yes.• Programs executing in a VMM experience
VMM overhead In theory, yes.
• Intuition is that VMM must maintain control of executing code by interposing on the operations or rewrite the binary
Exploiting the timing dependency exception to
detect a VMM Algorithm:
Given: • Real machine R with configuration C e.g.,
C={Pentium IV, 2.0GHz}• Remote machine M with configuration C• Program P with control-modifying instructions
1: Time the execution of P on R and store the value in r2: Time the execution of P on M and store the value in m3: IF m > r + k THEN M is virtual [note: k is the detection
constant]
4: ELSE M is real
Tasks Remaining
Achieve accurate high-integrity execution timing
Construct program P with externally noticeable VMM overhead
Determine configuration of remote machine
Determine detection constant k
Accurate High-Integrity Execution Timing
Can’t trust the integrity of the timing measurements returned by the VMM
Use an external source of time (e.g., remote machine, watch, etc…)
Constructing P with VMM Overhead
P is a sequence of sensitive (potentially control modifying) instructions that requires VMM interposition
P is designed to invoke VMM overhead Design decisions in developing P
include:• Sensitive instruction selection• Number of instructions
Selecting Sensitive Instructions
R/W cr3 R/W cr2
R/W cr0 cli
Number of Instructions in P
Assume we have complete configuration information for remote machine M
Easy to determine the number of instructions required to overcome experimental noise• Variance in execution time• Variance in network latency
Complete Configuration Information
Given an estimate of the noise N in the environment (i.e., 10 ms variation in network latency)
Select x s.t. FV(x) – RM(x) >> N
Fastest VMM = FV(x)
Real Machine = RM(x)
Incomplete Configuration Information
Unreasonable to assume complete configuration information is available for a remote machine
Use “hardware discovery” heuristic• Intuition: certain properties of the underlying
hardware are difficult to mask through the VMM and are unique to a particular architecture
• Discovering these hardware artifacts gives us partial configuration information about a remote machine
Incomplete Configuration Information
Given a subset C’ of the complete configuration information C• C = {Pentium IV, 2.0 GHz} and C’ = {Pentium IV}
Bound the execution time of P on the fastest and slowest machines that satisfy C’• Works because P is CPU bound• We can time the execution of P on a x GHz machine and then
use the ratio of the fastest and slowest machines to bound the execution times
Hardware Discovery on the Pentium IV
P4 has a unique trace cache which “shines” through the VMM
With sequences of register-to-register arithmetic instructions without data hazards populate the trace cache of the Intel Pentium IV, a CPI of 1/3 is attainable
Once an instruction sequence exceeds the trace cache’s size of 12KB, the CPI becomes 1
Remote Trace Cache Discovery
11264 instructions fit in the trace cache 11328 instructions exceeds the size of the trace cache A considerable jump in overhead occurs when the trace cache
overflows
Putting it All Together
Remotely timed overhead from reading and writing x86 Control Register 3 multiple times consecutively
Despite not being included in our analysis, remote detection works against a machine running Xen with hardware virtualization support (HVM Xen)• We conclude that hardware virtualization support is not
sufficient to prevent VMM detection
Detection Algorithm Limitations
VMM could tamper with execution of detection code• Countermeasure: Leverage software-based attestation
(Pioneer) VMM could prevent communication to external
timer• Countermeasure: Containment policy-based detection
Receive incorrect response from hardware discovery heuristic
VMM may be incorporated with OS• Malware can still own the lowest layer• Virtual-machine-based rootkits are a threat today
Conclusion Developed a remote VMM detection algorithm
• Attempts to be independent of VMM software implementation details
• Practical/relies on fewer assumptions than previous schemes
• Accurate, configurable, and effective over the Internet
Hardware virtualization support is not sufficient to mask differences between real and virtual environments