Eliminating Fine Grained Timers in Xen

Bhanu Vattikondawith Sambit Das and Hovav Shacham

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• Motivation

• Project goals

• Goals of the paper

• Discussion

• Future work

Motivation

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Recent research efforts have shown that covert channel attacks are possible in the cloud using fine grained timers [Hey, You, Get Off of My Cloud: Exploring Information Leakage in Third-Party Compute Clouds, Ristenpart et al.]

Presence of covert channels indicates the likelihood of side channels

Side channels could be exploited to obtain confidential information from the victim VM

Motivation

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Some attacks do not require fine grained timers:Determining whether two VMs are co-resident This is done by using network addresses and verifying that the dom0 IP address is the same for both the VMs

Server

VM1

VM2

dom0ProberTraceroute probes

Motivation

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Whereas some attacks require access to fine grained timers:

Sanboxed VMs can communicate using cache as a covert channelThis attack assumes that the VMs are co-located

Load cache and de-schedule

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Disturb cache2

High read time for loaded data

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Load cache and de-schedule

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Do nothing2

Low read time for loaded data

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SenderReceiver

Send “1” Send “0”

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• Motivation

• Project goals

• Goals of the paper

• Discussion

• Future work

Project goals

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Prevent covert channel and side channel attacks

Eliminating access to fine grained timers could prevent such attacks [Reducing timing channels with fuzzy time, Wei-Ming Hu]

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• Motivation

• Project goals

• Goals of the paper

• Discussion

• Future work

Goals of the paper

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Can fine grained timers be eliminated from the system?

What impact does it have on the utility of the system?

How coarse can the timers be?

Not addressed in the paper

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Thorough evaluation of security benefits

Preventing alternative attack strategies to obtain fine grained timers

Can fine grained timers be eliminated from the system?

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Various sources of fine grained timersRDTSC instruction (cycle counter on x86 processors)gettimeofday clock_gettime

System time read by gettimeofday and clock_gettime gets updated using the value of RDTSC register Modifying value

returned by RDTSC instruction should affect all timers

Can fine grained timers be eliminated from the system?

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Yes!

Xen has a mode in which it traps and emulates the RDTSC instruction

Can fine grained timers be eliminated from the system?

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In our experiments, we returned the actual RDTSC value rounded off to a certain number of cycles

To measure the impact, we measure the difference between return values of consecutive RDTSC instructions

What impact does it have on the utility of the system?

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We evaluate the impact of clock fuzziness on a small testbed of two machines running Xen 4.0.1 on Centos 5.5

Server1

VM1

Server2

VM2

What impact does it have on the utility of the system?

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Compute intensive jobs are not affected

We run a fast fourier transform and measure the completion time

What impact does it have on the utility of the system?

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Network performance is not affected either, impact on throughput and RTT is insignificant

RTT is measured using a UDP based ping between the VMs

Throughput is measured using a long lasting TCP flow between the VMs

What impact does it have on the utility of the system?

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Negligible impact on the finish times of all to all transfer

The two VMs send 1GB of data to each other and we measure the completion time

What impact does it have on the utility of the system?

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Performance of Apache web server is also unaffected

Requests per second measured by requesting a ~200KB file

Throughput measured by requesting a ~15MB file from a web server running on the VM

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• Motivation

• Project goals

• Goals of the paper

• Discussion

• Future work

How coarse can the timers be?

Para-virtualized system becomes unusable at a fuzziness of 10000 cycles (10s of microseconds)

In the case of fully virtualized systems the system becomes unusable at a fuzziness of 100 million cycles (100s of milliseconds)

How coarse can the timers be?

Performance of TCP applications begins to degrade at high fuzziness (1ms)

Performance of other applications is not affected

It should be possible to modify granularity of

clock to 100s of microseconds

Fine grained timing using userspace counter

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On a multi-processor system, a thread can be dedicated to maintain a counter and used in place of RDTSC counter

The amount of time taken for a constant number of nops has variance

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• Motivation

• Project goals

• Goals of the paper

• Discussion

• Future work

Future work

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Push the limit on coarseness of the timer in the case of para-virtualized system

We only explored a step function for eliminating fine grained timers, other functions need to be explored

Evaluate the feasibility of existing attack strategies on the modified system

Summary

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Fine grained timers in Xen can be eliminated

Performance of typical cloud based applications is not affected by the elimination of fine grained timers

Security benefits achieved due to the elimination of timers must be evaluated

Impact of using other methods to obtain fine grained timers, on security must be evaluated

Thank you!

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