Cloud computing environment supports
highly scalable hardware and software resource sharing platform through the
Internet. The cloud provider shares the hardware resources with the cloud
customers through the Virtual Machines (VM). Virtual Machines (VM) running on
the same physical server are denoted as Co-resident VMs. The Co-resident
Virtual Machines are logically isolated from each other. The logical isolation
is violated by the side channels of the malicious users. The sensitive
information from the Co-resident VMS are accessed by the malicious users is
defined as Co-resident attacks. The Cryptographic keys, workloads and web
traffic rates are the sensitive information accessed by the malicious
users.   The Co-resident attack is also
referred as co-location, co-residence or co-residency attacks.G1 G2 G3 

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The Virtual Machine allocation
policy is used to place the Virtual Machines on the physical server. The
malicious user co-locates their VM to the target VM. The security, workload
balance and power consumption parameters are considered in the Virtual Machine
placement process. Secure metrics are defined to measure the safety of the VM
allocation policy. The Balanced VM Allocation Policy is built to assign VMs to
the physical servers. The Previous Selected Server First (PSSF) policy is used
with security metrics. Least VM allocation policy, Most VM allocation policy,
and Random allocation policy are used with the workload balance parameter. The
data centers are connected to the Virtual Machines within the same environment.G4 G5 G6 G7 G8 G9 G10 

The attack resistant Virtual Machine
Management framework is built with centralized and distributed scheduling
schemes. The live VM migration is protected from the side channel attacks. The
system is enhanced with multiple data center management mechanisms. The
Distributed VM Placement (DVMP) policy is build to allocate the virtual
machines on the physical server.G11 G12 G13 


Index Terms: Cloud
Resources, Virtual Machine Allocation Policies, Side Channel Attacks,
Co-residential Attacks and Distributed Scheduling


1. Introduction

Public infrastructure-as-a-service
(IaaS) clouds enable the increasingly realistic threat of malicious customers
mounting side-channel attacks. An attacker obtains tenancy on the same physical
server as a target and then uses the careful timing of shared hardware
components to steal confidential data. Damaging attacks enable theft of
cryptographic secrets by way of shared per-core CPU state such as L1 data and
instruction caches, despite customers running within distinct virtual machines
(VMs).G14 G15 

general solution to prevent side-channel attacks is hard isolation: completely
prevent sharing of particular sensitive resources. Such isolation can be
obtained by avoiding multi-tenancy, new hardware that enforces cache isolation,
cache coloring, or software systems such as StealthMem. Hard isolation reduces
efficiency and raises costs because of stranded resources that are allocated to
a virtual machine yet left unused. Another approach has been to prevent attacks
by adding noise to the cache. For example, in the D¨uppel system, the guest
operating system protects against CPU cache side-channels by making spurious
memory requests to obfuscate cache usage. This incurs overheads, and also
requires users to identify the particular processes that should be protected.G16 

A final approach has been to
interfere with the ability to obtain accurate measurements of shared hardware
by removing or obscuring time sources. This can be done by removing hardware
timing sources, reducing the granularity of clocks exposed to guest VMs,
allowing only deterministic computations, or using replication of VMs to
normalize timing. These solutions either have significant overheads, as in the
last solution or severely limit functionality for workloads that need accurate

In addition to sharing resources and
having access to fine-grained clocks, shared-core side-channel attacks also
require the ability to measure the state of the cache frequently 10. For
example, Zhang et al.’s cross-VM attack on ElGamal preempted the victim every 16
?s on average. With less frequent interruptions, the attacker’s view of how
hardware state changes in response to a victim become obscured. Perhaps
surprisingly, then, is the lack of any investigation of the relationship
between CPU scheduling policies and side channel efficacy. In particular,
scheduling may enable what we call soft isolation: limiting the frequency of
potentially dangerous cross-VM interactions.G18 


2. Related work

variety of cross-VM side channels have been demonstrated in the academic literature.
Deficiencies in performance isolation, similar to those leveraged in this work,
have been exploited for a variety of purposes 7. Noting that cache and
network utilization are often contested between VMs, a resource freeing attack
(RFA) has been proposed that allows a greedy customer to manipulate the
performance of co-resident VMs by shifting their resource bottlenecks 4. This
work operates under a similar attack model as our own, targeting public network
cloud services and manipulating VMs from a helper host. However, where RFA is a
performance enhancement strategy for the cloud, co-resident watermarking is a
method of information extraction.

side-channel attacks, in which timing differences in access latencies between
the cache and main memory are exploited, have attracted the most attention for
cloud computing. Most notably, Zhang et al. 6 demonstrated that the machine
instructions of a co-resident VM can be recovered from shared L1 caches,
permitting the reconstruction of secret keys in the circumstance that they
influence the code path of a decryption routine. Ristenpart et al. showed that
cache usage can be examined as a means to measure the activity of other
instances co-resident with the attacker. Furthermore, they demonstrated that
they can detect co-residency with a victim’s instance if they have information
about the instance’s computational load. In contrast, Zhang et al. 5 utilized
cache-based side channels as a defensive mechanism. Their scheme works by
measuring cache footprints for evidence of other VMs. Leveraging this scheme,
they can challenge correct functionality on the part of the cloud provider and
discover other unanticipated instances sharing the same host.G19 

et al. 8 have proposed the use of a different network timing side channel in
order to challenge fault tolerance guarantees in storage clouds. This work
measures the response time of random data reads in order to confirm that a
given file’s storage redundancy meets expectations. This scheme can be used to
detect drive-failure vulnerabilities and expose cloud provider negligence. We
intend to investigate the applicability of storage cloud co-resident
watermarking in future work.G20 

et al. proposed two other mechanisms for preventing cache-based side channels,
cache hierarchy aware core assignment, and page-coloring-based cache
partitioning. The former groups CPU cores based on last level cache (LLC)
organization and checks whether such organization has any conflict with the SLA
of the clients. The latter is a software method that monitors how the physical
memory used by applications maps to cache hardware, grouping applications
accordingly to isolate clients. Another effective defense against cache-based
side channels is changing how caches assign memory to applications, such as
non-deterministic caches. Non-deterministic caches control the lifetime of
cache items. By assigning a random decay interval to cache items, the cache
behavior becomes nondeterministic, and hence, side channels cannot exploit it.
Work in performance isolation in Xen can also lead to added security benefits.G21 vm22 

work aims to combat virtualization vulnerabilities by reducing the role and
size of the hypervisor. Most drastically, Keller et al. 2 eliminate a large
attack surface by proposing the near elimination of the hypervisor. This is
achieved through pre-allocation of resources, limited virtualized I/O devices,
and modified guest operating systems. While this approach inarguably reduces
the likelihood of exploitable implementation flaws in the virtualization code
base, it necessarily places VMs closer to the underlying hardware. Intuitively,
this can only increase the bandwidth of the isolation-compromising side channel
that we explore in this work. Other proposals reduce the hypervisor attack
surface by considering only specific virtualization applications such as
rootkit detection or integrity assurance for critical portions of
security-sensitive code 3 or by distributing administrative responsibilities
across multiple VMs 10. We do not consider these systems in our work because
they are not intended for the third-party compute cloud model.G23 G24 


3. Virtual Machine Allocation Policies

Security is one of the major concerns against cloud computing. From
the customer’s perspective, migrating to the cloud means they are exposed to
the additional risks brought about by the other tenants with whom they share
the resources?are these neighbors trustworthy, or they may compromise the
integrity of others? This paper concentrates on one form of this security
problem: the co-resident attack.

Virtual machines (VM) area commonly used the resource in cloud computing
environments. For cloud providers, VMs help increases the utilization rate of
the underlying hardware platforms. For cloud customers, it enables on-demand
resource scaling and outsources the maintenance of computing resources.
However, apart from all these benefits, it also brings a new security threat.
In theory, VMs running on the same physical server are logically isolated from
each other. In practice, nevertheless, malicious users can build various side
channels to circumvent the logical isolation, and obtain sensitive information
from co-resident VMs, ranging from the coarse-grained, e.g., workloads and web
traffic rates to the fine-grained, e.g., cryptographic keys For clever
attackers, even seemingly innocuous information like workload statistics can be
useful 9. For example, such data can be used to identify when the system is
most vulnerable, i.e., the time to launch further attacks, such as
Denial-of-Service attacks. G25 G26 G27 

A straightforward solution to this novel attack
is to eliminate the side channels, which has been the focus of most previous
works. However, most of these methods are not suitable for immediate deployment
due to the required modifications to current cloud platforms. In our work, we
approach this problem from a completely different perspective. Before the
attacker is able to extract any private information from the victim, they first
need to co-locate their VMs with the target VMs. It has been shown that the
attacker can achieve an efficiency rate of as high as 40%, which means 4 out of
10 attacker’s VMs can co-locate with the target. This motivates us to study how
to effectively minimise this value. From a cloud provider’s point of view, the
VM allocation policy (also known as VM placement?we use these two terms
interchangeably in this paper)is the most important and direct control that can
be used to influence the probability of co-location. Consequently, we aim to
design a security policy that can substantially increase the difficulty for
attackers to achieve co-residence.G28 G29 G30 G31 

In our earlier work, we have
proposed a prototype of such a security policy, called the previous-selected-server-first
policy (PSSF). However, this prototype policy only focuses on the problem of
security, and hence has obvious limitations in terms of:G32 G33 G34 

1. Workload balance?Workload here refers to the VM requests. From the
cloud provider’s point of view, spreading VMs among the servers that have

already been switched on can help
reduce the probability of servers being over-utilized, which may cause SLA
(service level agreement) breaches. From the customer’s perspective, it is also
preferable if their VMs are distributed across the system, rather than being
allocated together on the same server. Otherwise, the failure of one server
will impact all the VMs of a user.

2. Power consumption?It has been estimated that the power consumption
of an average datacentre is as much as 25,000 households and it is expected to
double every 5 years. Therefore, managing the servers in an energy efficient
way is crucial for cloud providers in order to reduce the power consumption and
hence the overall cost. This has also been the focus of many previous works.

In this paper, we take all three
aspects of security, workload balance, and power consumption into consideration
to make PSSF more applicable to existing commercial cloud platforms. Since
these three objectives are conflicting to some extent, we improve our earlier
policy by applying multi-objective optimisation techniques. In addition, we
have implemented PSSF on the simulation environment CloudSim as well as on the
real cloud platform OpenStack and performed large-scale experiments that
involve hundreds of servers and thousands of VMs, to demonstrate that it meets
the requirements of all three criteria.G35 G36 G37 

Specifically, our contributions include: (1) we define
secure metrics that measure the safety of a VM allocation policy, in terms of
its ability to defend against co-resident attacks; (2) we model these metrics
under three basic but commonly used VM allocation policies, and conduct
extensive experiments on the widely used simulation plat-form CloudSim to
validate the models; (3) we propose a new security policy, which not only
significantly decreases the probability of attackers co-locating with their
targets but also satisfies the constraints in workload balance and power
consumption; and (4) we implement and verify the effectiveness of our new
policy using the popular open-source cloud software OpenStack as well as on
CloudSim.G38 G39 G40 


4. Issues on VM Allocation Policies

The Virtual Machine allocation policy is used to place the Virtual
Machines on the physical server. The malicious user co-locates their VM to the
target VM. The security, workload balance and power consumption parameters are
considered in the Virtual Machine placement process. Secure metrics are defined
to measure the safety of the VM allocation policy. The Balanced VM Allocation
Policy is built to assign VMs to the physical servers. The Previous Selected
Server First (PSSF) policy is used with security metrics. Least VM allocation
policy, Most VM allocation policy, and Random allocation policy are used with
the workload balance parameter. The data centers are connected to the Virtual
Machines within the same environment. The following issues are discovered from
the current virtual machine allocation policies against co-residential attacks.
G41 G42 G43 G44 G45 G46 G47 G48 

The system supports centralized
allocation policy only

Live VM migration is not protected

Multiple data center management is
not supported

The system state information is
required for the scheduling process


5. Distributed Scheduling and Virtual Machine Management

The virtual machine placement operations are performed with
centralized and distributed manner. The virtual machines are placed with
workload and energy levels. The data center selection process is used to detect
the suitable data center for the workloads. The system is divided into six
major modules. They are Physical server deployment, Data center management,
Workload controller, Security Analyzer, Centralized VM placement and
Distributed VM placement.G49 G50 G51 G52 

The physical servers and virtual machines are configured I
the deployment process. The data center management is build to organize the
data centers and shared data items. The workload controller is used to collect
workloads from the users. The security analyzer is built to estimate the
security metrics for the VMs. The centralized VM placement is employed to
control co-resident attacks. Live VM migration and multiple data center based
allocation is performed under the distributed VM placement model.G53 G54 G55 G56 

5.1. Physical Server Deployment

physical server deployment is used to set up the cloud with shared resources.
The physical servers and configuration levels are collected from the cloud
provider. The virtual machine configurations are assigned with provider choice.
The physical server and virtual machine association levels are updated under
the deployment process.

5.2. Data Center Management

The data centers and its contents
are maintained under the data center management. Data center storage and usage
levels are monitored at intervals. Shared data and its request frequency are
maintained in the data centerG57 . Virtual machines and data center
communication is controlled with security levels.G58 G59 G60 



5.3. Workload Controller

workload controller monitors the workload execution process. The workloads are
collected from the cloud users. The workloads and data association are verified
by the controller. The workload status is monitored and updated by the

5.4. Security Analyzer

The security analyzer is used to
estimate the security levels for the virtual machines. The secure metrics are
used to estimate the security levels. The workload balance parameter is
considered in the security metrics. The power consumption levels are estimated
in the secure metric estimation process.G61 G62 

5.5. Centralized VM Placement

The centralized VM placement is
carried out with the balanced VM allocation policy. The Previous Selected
Server First (PSSF) algorithm is used for the VM placement process. The single
data center is used to provide the data values. The secure metrics are used in
the VM placement process.G63 G64 

5.6. Distributed VM Placement

secure metrics estimation and initial VM placement operation are tuned for the
distributed scheduling model. The Distributed VM placement algorithm is used to
allocate the virtual machines in a distributed manner. The Datacenter
allocation algorithm is used to select data centers for the virtual machines.
The live VM migration operations are secured with the Attack resistant VM
migration algorithm.G65 G66 



Cloud computing environment provides
IT resources to the users based on their demand. The Co-resident attacks are
raised by the co-located malicious users with side channels. The Previous
Selected Server First (PSSF) policy is applied for the VM placement with
security. The attack resistant VM placement is built with centralized and
distributed scheduling, Live VM migration and Multiple data center management.
The Virtual Machine placement operations are carried out with side channel
attack control mechanism. The VM placement policies are improved to manage
centralized and distributed placement models. The co-location control model is
tuned to handle the VM migration tasks. Data center communications are also
protected in the allocation scheme.G67 G68 


1 Hao Wu, Shangping Ren, Gabriele
Garzoglio, Keith Chadwick and Seo-Young Noh, “A Reference Model for Virtual
Machine Launching Overhead”, IEEE Transactions On Cloud Computing,
July-September 2016.

2 Keller, E., Szefer, J., Rexford,
J., Lee, R.B.: Eliminating the hypervisor attack surface for a more secure
cloud. In: Proceedings of ACM Conference on Computer and Communications,
Security (CCS’11) (2011)

3 McCune, J.M., Li, Y., Qu, N.,
Zhou, Z., Datta, A., Gligor, V., Perrig, A.: TrustVisor: efficient TCB
reduction and attestation. In: Proceedings of 2010 IEEE Symposium on Security
and Privacy, Oakland (2010)

4 Varadarajan, T., Farley, B.,
Ristenpart, T., Swift, M.M.: Resource-freeing attacks: improve your cloud
performance. In: Proceedings of ACM Conference on Computer and Communications
Security, Raleigh (2012)

5 Zhang, Y., Juels, A., Oprea, A.,
Reiter, M.K.: Home Alone: Co- Residency Detection in the Cloud via Side-Channel
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Berkeley (2011)

5 Zhang, Y., Juels, A., Reiter,
M.K., Reiter, M., Ristenpart, T.: Cross- VM side channels and their use to
extract private keys. In: Proceedings of 2012 ACM Conference on Computer and
Communications Security, Raleigh (2012)

7 Adam Bates,
Benjamin Mood,  Joe Pletcher, Hannah
Pruse, Masoud Valafar and Kevin Butler, “On detecting co-resident cloud
instances using network flow watermarking techniques”, Springer-Verlag Berlin
Heidelberg 2013

8 Bowers, K.D., van Dijk, M.,
Juels, A., Oprea, A., Rivest R.L.: How to tell if your cloud files are
vulnerable to drive crashes. In: CCS ’11: Proceedings of 18th ACM Conference on
Computer and Communications Security, Chicago, (2011)

9 Eun Kyung Lee, Hariharasudhan
Viswanathan and Dario Pompili, “Proactive Thermal-aware Resource Management in
Virtualized HPC Cloud Datacenters”, IEEE Transactions on Cloud Computing, June

10 Butt, S., Lagar-Cavilla, H.A.,
Srivastava, A., Ganapathy V.: Self-service cloud computing. In: Proceedings of
2012 ACM Conference on Computer and Communications Security, Raleigh (2012)G69