Patent Publication Number: US-8972980-B2

Title: Automated provisioning of secure virtual execution environment using virtual machine templates based on requested activity

Description:
CLAIM OF PRIORITY 
     This application claims priority to U.S. provisional patent application No. 61/349,717, filed May 28, 2010, entitled “System and Method for Securing an Internet Endpoint using fine-grained Full OS Virtualization,” the contents of which are hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the invention relate to securing an Internet endpoint from being compromised by execution of malicious code or accessing protected resources thereon through the use of fine-grained operating system (OS) virtualization. 
     BACKGROUND 
     Ensuring the security of Internet users and Internet connected devices is one of the grand challenges facing us today. The current state of affairs is very problematic, as our cyber-security infrastructure is easily and routinely subverted by cyber criminals, resulting in great economic loss. Every year brings deeper and more complex dependence by society on our cyber-infrastructure, and yet at the same time the cyber-security problem only worsens as the capabilities of the cyber-criminal mature. In effect, we are building mission-critical dependence into virtually every aspect of human activities on a cyber-infrastructure that is very insecure at its core. 
     The current state of our cyber-security infrastructure is due to two fundamental limitations. The first limitation is a fundamental mismatch between the design assumptions made by computer security programmers with how the vast majority of users interact with the cyber-infrastructure (the “Security Model Complexity” problem. The second limitation is a lack of appropriate isolation of code and data from trusted and untrusted sources in modern computer systems (the “Lack of Isolation” problem). These two limitations of current systems are somewhat orthogonal, but are both very important for securing an endpoint. The “Lack of Isolation” problem, in particular is very important because modern computer systems that are used for everyday computing as endpoints are typically general purpose devices capable of running a vast variety of software from different sources. 
     The general purpose capability of modern endpoint systems is constructed using a layered stack of hardware and software. An example of the layered arrangement of hardware and software that is present in modern computer systems is shown in  FIG. 1 . At the lowest layer, there is hardware with a small number of basic general purpose programming capabilities. Upon this hardware layer sits the firmware and operating system (OS) layers. The firmware and OS layers provide higher-level but broad capabilities such as managing specific devices, files, and/or processes. On top of the OS layer run the various applications which provide user-visible rich functionality to the computer. The functionality provided by the application layer is typically the primary concern of the computer user. 
     One advantage and consequence of the layered nature of modern computer systems is that the various layers may come from different vendors, as long as the layers conform to the specifications governing the layer boundary (which may be based on open or proprietary industry standards). To illustrate an example, in a typical PC today the hardware may be constructed around processor and chipset technology provided by Intel or AMD. The firmware/BIOS may be provided by companies like Insyde, AMI or Phoenix and may be written to conform to several industry specifications such as UEFI and PI. The operating system (OS) may originate from a company like Microsoft or Apple or may be a flavor of the Linux open source OS. Finally, the applications themselves are usually written to the specification of one of the operating systems and may be provided by one of a large multitude of application vendors. 
     Note that some of the applications may themselves have a layered architecture. A web browser, for example, typically includes a browser core and may also download web applications in the form of HTML, Javascript and Flash programs from various Internet web sites. The web browser may run these downloaded web applications locally on top of the browser core. A typical web page contains HTML with embedded JavaScript that can change the HTML being rendered by the web browser dynamically based on user actions without having to re-download the web page from the web server. The HTML may also demarcate part of the web page to be rendered by a plugin, which is typically a separate program that is installed on the computer. Plugins are also often downloaded from different sources over the World Wide Web. Thus, a modern computer runs code that comes from a variety of different sources. In particular, application programs may originate from literally millions of different sources once we consider the collection of traditional local applications as well as web applications that are downloaded from websites. 
     The integrity of a computer system when it runs application code from different sources (or even the same program being run by different users of a shared computer) has traditionally been one of the responsibilities of the OS. The OS uses various hardware and software constructs like virtual memory, processes, and file permissions to prevent programs belonging to one user from affecting programs (and data) belonging to another user. This responsibility of the OS to “isolate” programs and data from one another often tends to compete with another responsibility of the OS, which is to allow for co-operation between programs especially between user application programs and system level services such as shared library modules, database services, and other higher-level common OS functionality. These two OS functions, to share and to isolate, require the OS designer to make some tradeoffs on how much to share and how much to isolate. 
     As a result of these tradeoffs, the resulting implementation of modern operating systems tends to be overly complex and typically exhibit numerous bugs. In mature operating systems, the security implementation is typically robust enough to work well for normal programs under normal usage with no adverse impact on the operation of the computer. However, most OS implementations are very large and complex bodies of computer code and have thousands of loopholes that cause the security system to break down under situations where programs are especially constructed to take advantage of less-tested or unvalidated corner cases in the operation of the security subsystem. Furthermore, the security implementation of modern operating systems does not perform well when all programs are initiated by the same user. 
     These “security vulnerabilities” are not important for well behaved programs during typical operation, but are used extensively by cyber criminals to subvert the computer&#39;s security subsystems. Once the system&#39;s security is subverted, it is generally possible for cyber criminals to run any software under their control on the subverted computer system. 
     The Lack of Isolation problem stems from the fact that a large amount of code executed by computers today comes from sources outside the computer, some of which have explicit intentions of committing criminal activities. This includes any program downloaded from the Internet or any web site visited by the computer. All downloaded programs (good and bad) have the same OS and library services available to them to use during their operation. Consequently, any program (even malware), can exploit any security vulnerability in the complex OS or web browser environment and subvert the security subsystem that isolates applications from one other. For example, when a user visits a web site, he or she is really running web application code developed by the publisher of the web site. If this web site is malicious, then malware may be executed on the computer. Malware may be designed to exploit a security vulnerability in the web browser to take control of the computer system during subsequent web site visits, e.g., if you visit your bank&#39;s web site, your key strokes may be captured and your login/password information for the bank may be transmitted to the malware publisher. 
     Most computer security professionals understand the existence of the Lack of Isolation problem, but consider it hard to solve in any practical way because better achieving the goal of isolation between applications fundamentally tends to conflict with achieving the goal of increasing seamless communication between different local and web applications. There has been some work towards the isolation of web code from different sources being run by a web browser. Modern browsers have attempted to create a level of sandboxing around downloaded web application code in order to isolate downloaded code from the rest of the computer and from each other. However, these models are fairly primitive in their ability to deal with the full gamut of security issues that arise during the course of a typical user&#39;s web experience. For example, certain versions of Google&#39;s Chrome web browser&#39;s sandboxing does not address safety issues arising from downloaded browser plugins and various types of native executables; thus, every computer system running certain versions of Chrome is vulnerable to a zero day exploit attack against Adobe Flash or Microsoft Word as much as if the system was running a less secure or older browser with the same Adobe Flash Plugin or Microsoft Word plugin. 
     Web browsers have been burdened with the need to ensure full compatibility to older and non-standard web pages in their efforts to provide superior safety and privacy. For example, web browser programmers have had to make some relaxations around the same-origin policy in order to correctly render popular web sites that rely on the sharing of information between web sites. 
     Last but not least, most web browsers vendors suffer from a huge conflict of interest because their business relies upon monetizing the web browsing habits of their users within their own business processes and with their industry partners. This monetization relies on data about users&#39; browsing habits which is contained in the web cookies that are set and later provided to web servers during the course of web sessions. Companies such as Google and Microsoft have a great interest in learning as much as possible about a person&#39;s browsing habits and typically arrange the default privacy settings of web browsers to be advantageous to them (but less than optimal from a security and privacy standpoint). This choice of default privacy and core functionality settings causes web browsers to transfer large amounts of sensitive information from end users&#39; machines to Internet related businesses, such as Google, Microsoft, Apple, etc., thereby allowing such businesses to better monetize their customer base by offering appropriate products and services and serving targeted ads. These same settings, however, can be leveraged by malicious parties to exploit security vulnerabilities. While all web browsers provide some level of control to the sophisticated user to tune his or her web browser functionality and/or privacy/safety settings to browse more securely, the vast majority of users never change these default settings. 
     Some security researchers have also proposed the use of “client virtualization” (also called “Virtualization using a Hypervisor” in the desktop) to solve the Lack of Isolation Problem. In one form of client virtualization, the user runs multiple independent operating systems on their laptop or desktop on multiple virtual machines (VMs) within the client system which have been created using a hypervisor, such as from VMWare of Palo Alto, Calif. or Virtual PC, available from Microsoft Corporation of Redmond, Wash. When client virtualization is used to achieve improved security, different VMs are used to run applications from different sources or of different types. For example, an OS in one VM may be dedicated for accessing the corporate network that the user may be part of and running corporate applications (local and web). Another OS in a second VM might be used by the user to run his or her personal programs and store personal documents. Finally, a different OS in a third VM may be used for general web browsing on the wider Internet and running native executables that may have been downloaded from the Internet. An example of such a solution is XenClient, which is made by Citrix Systems of Ft Lauderdale, Fla. 
     The use of classical client virtualization, as discussed above, to solve the general code isolation problem in the context of Internet endpoints suffers from several drawbacks. A first drawback is that there is too much management overhead for the end-user. The end-user has the onus of making the decision as to what VM to use for each activity. Any mistake, intentional or accidental, may subvert the integrity of the system. While many safeguards can be added as a layer on top of the core virtualization technology to prevent the user from making mistakes, this has not yet been demonstrated to work in a practical and robust fashion. 
     Another drawback is that this arrangement of VMs is very static and does not lend itself to the dynamic and varied nature of the typical user&#39;s activities. For example, depending on the time of the day, many users may need to isolate programs from 2 sources to 10 s of sources. In the arrangement described above, there is no VM based isolation between all the web sessions of the user on the general Internet. Running 10 s of VMs on a single client system all the time leads to too much performance and management overhead using prior approaches. Starting VMs on demand for individual activities suffers from huge latencies to start activities and a limitation on the number of concurrent activities supported, which adversely affects the user experience. 
     An additional drawback is that client virtualization, as described above, suffers from the problem that any VM that is used for general web browsing is just as vulnerable to a security problem as any monolithic system running a single VM while accessing web sites on the general Internet. Therefore, it is quite likely that the VM dedicated to web browsing described in the arrangement above will be subverted by malware eventually. Any subsequent activities in that VM, then, will be compromised. 
     Due to these reasons client virtualization has not been used widely to improve the security of Internet endpoints. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  is an illustration of the layered arrangement of hardware and software present in modern computer systems; 
         FIG. 2  is an block diagram of the functional components of one embodiment of the invention; 
         FIG. 3  is block diagram of the functional components involved in exposing a restricted copy of the file system to different UCVMs (and VVMs) according to an embodiment of the invention; 
         FIG. 4  is a flowchart illustrating the steps involved in a UCVM obtaining a copy of a new user file maintained in the file system stored elsewhere according to an embodiment of the invention; 
         FIG. 5  is an illustration of instantiating a plurality of different virtual machines using different templates according to an embodiment of the invention; 
         FIG. 6  is an illustration of a virtual disk based on VSS shadow copies according to an embodiment of the invention; 
         FIG. 7  is an illustration of exemplary desktop of a client according to an embodiment of the invention; 
         FIG. 8  is an illustration of safely installing an untrusted application according to an embodiment of the invention of the invention; and 
         FIG. 9  is a block diagram that illustrates a computer system upon which an embodiment of the invention may be implemented. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Approaches for securing an Internet endpoint from being compromised by malicious code executing thereon through the use of fine-grained operating system (OS) virtualization are presented herein. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the invention described herein. It will be apparent, however, that the embodiments of the invention described herein may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form or discussed at a high level in order to avoid unnecessarily obscuring teachings of embodiments of the invention. 
     Functional Overview 
     Embodiments of the invention prevent malicious code, introduced into a computer system, from compromising the resources of the computer system through the use of dynamic operating system (OS) micro-virtualization. A computer system of an embodiment includes a number of independent virtual machines (VMs) that each executes a full operating system (OS). A block diagram of client  200  according to one embodiment of the invention is shown in  FIG. 2 . The term “client,” as broadly used herein, represents any type of Internet endpoint or computer system capable of connecting to a network and executing a virtual machine. Non-limiting, illustrative examples of client  200  include a PC, a laptop computer, a tablet computer, a cell phone, a personal digital assistant (PDA), and the like. 
     In an embodiment, client  200  may correspond to a server. Thus, while use of the term “client” in other contexts might exclude an interpretation that includes a server, as broadly used herein, client  200  may be embodied on a wide variety of machines, one example of such being a server. Thus, as the Applicant may be his or her own lexicographer, as used herein, the term client  200  expressly includes a server. For example, non-limiting, illustrative examples of client  200  include a web server, an application server, a file server, and a cloud server. Indeed, implementing embodiments of the invention upon a server may yield many benefits. The micro-virtualization techniques employed by embodiments provide an efficient mechanism for eliminating the risk of executing untrusted code and/or interpreting untrusted data in accordance with different policies to manage such risk. As such, a device, such as a server, which interacts with (a) numerous sources of untrusted code and/or data and/or (b) two or more corporate entities having different policies towards managing the risk of untrusted code and/or data, may benefit from embodiments of the invention. 
     Client  200  includes a number of virtual machines (such as  230 ,  240 ,  250 , and  260 , for example) that execute on hardware  210  of client  200 . The various VMs within client  200  may be used for separately executing processes associated with different activities. One such VM, namely “VM 0 ” (i.e., VM 0   230  of  FIG. 2 ), is secured so that VM 0  may serve as the root of trust with a guaranteed integrity. VM 0  may contain core operating system  232  and one or more applications  234 . In the embodiment shown in  FIG. 2 , VM 0  is not accessible over any network, such as the Internet. As shall be explained below, VM 0  provides a secure environment in which operating system  232  and one or more applications  234  may execute without risk of exposure to malicious code. 
     Other VMs, such as VMs  260 ,  262 ,  264 , and  266  in  FIG. 2 , may be created, maintained, and destroyed on-demand using a very efficient micro-virtualizing hypervisor  220 . Using efficient micro-virtualization techniques, the latency of starting and stopping activities or applications which run in their own VM in embodiments is very low, thereby providing a practical user experience while employing full OS virtualization. 
     Embodiments address and overcome many disadvantages, such as the Lack of Isolation Problem, experienced by modern general purpose computer systems that execute code from different sources and of differing trust levels; nevertheless, embodiments maintain compatibility with current typical real-world usage of computer systems by corporate and non-corporate users. This is so because any activity which is not previously deemed trustworthy is performed in a separate VM by certain embodiments, and so all code which may be potentially malicious is executed in its own VM that is destroyed after its immediate use is ended, thereby preventing any malicious code from effecting any lasting change to a computer system according to an embodiment of the invention. 
     The Trusted Virtual Machine—VM 0   
     In an embodiment of the invention, a special virtual machine, referred to herein as “VM 0 ,” is created to be a trusted and un-hackable portion of a computer system.  FIG. 2  depicts VM 0   230  according to an embodiment. To achieve the property of being un-hackable, VM 0   230  may be permanently disconnected from any network (i.e., VM 0   230  is not connected to any local network or the Internet). Specifically, VM 0   230  may not contain any type of networking stack, such as a TCP/IP network stack, and may not have access to any networking hardware that could allow for communication between VM 0   230  or any applications  234  executed thereby and the Internet. Thus, the only way to install software onto VM 0   230  is to have physical custody of client  200  and manually install the software on VM 0   230 . 
     Thus, in certain embodiments, one or more applications  234  executing within VM 0   230  do not have any access to a network, must be fully self contained in their functionality, and must rely only on local code and data for all their functionality. All applications that need to access the network will therefore need to run in a separate virtual machine outside of VM 0   230 , as shall be described in further detail below. It is envisioned that the software (such as one or more applications  234 ) running in VM 0  be selected at the time client  200  is manufactured or first configured for use in a controlled environment. Because VM 0   230  is never connected to any type of network, such as a TCP/IP network, all common types of network initiated attacks cannot be waged against VM 0   230 , thereby rendering VM 0   230  immune to such attacks and safe as compared to any computer or VM that is connected to the Internet. 
     In an embodiment where hypervisor  220  is a Type 2 hypervisor, when client  200  is booted, only VM 0   230  is started by the BIOS or firmware of client  200 . Once VM 0   230  is running, VM 0   230  can start hypervisor  220  immediately or on demand. In another embodiment, where hypervisor  220  is a type 1 hypervisor, hypervisor  220  is first started by the BIOS when client  200  is booted and VM 0   230  is launched by the Hypervisor  220 . Hypervisor  220  is a software component that is responsible for creating other VMs which each execute independent instances of the operating system. These additional VMs are instantiated by VM 0   230  and/or hypervisor  220  to run any untrusted code or code that needs to access the network. Untrusted code in this context is any code which has not been pre-approved as being trusted by an IT administrator of client  200 . The additional VMs are started “silently” and automatically by client  200 , e.g., these VMs are started transparently to the user and without the user having to do anything explicit. These additional VMs are also not explicitly visible to the user; instead, all the user sees on the desktop is familiar objects (such as icons, windows, and applications) without any indication that multiple VMs are executing in client  200 . Embodiments of the invention follow rules that govern what application activities are assigned to which particular VM. These rules are described below in greater detail. 
     In another embodiment (not depicted in  FIG. 2 ), VM 0   230  may have a networking stack that is firewalled off from the network using well-tested firewall software, thereby allowing VM 0   230  to have access to a computer network. Such an embodiment may only allow connections with a specific Internet system so that the software inside VM 0   230  may be updated from a designated update server. For example, the firewall software may only allow VM 0   230  to connect to one or more servers associated with the IT administrator of client  200  and may prevent VM 0   230  from establishing a connection with any other endpoint on any network. 
     Interaction with an User Interface 
     All code responsible for generating a user interface (UI) not associated with an application may be maintained in VM 0   230 . Consequently, all UI interaction activity with the desktop between a user and software executing on client  200  may take place between the user and VM 0   230 , which maintains a unified desktop for all applications running in all VMs. Interaction between the user and applications running in VMs other than VM 0   230  takes place indirectly via VM 0   230 . For example, when the user enters a password for a web site whose browser and HTML/Javascript code is running in an untrusted VM, the password is first directly provided to VM 0 , which then transfers the information to the untrusted VM. Furthermore, the untrusted VM&#39;s display is rendered on to a virtualized display, which is then composed into the VM 0  desktop (as appropriate) by controlling code running in VM 0   230 . As code executing in VM 0   230  is trusted, the user may trust any user interface controls displayed on a screen since all code responsible for rendering the user interface is trusted. 
     This approach is quite different from prior systems where often the code that controls the full desktop experience is untrusted. Consequently, if the code responsible for generating the user interface is corrupted by malware, then the user interface may be used as a tool to deceive the user. For example, malware may cause a user interface control to be displayed that requests the user to submit an authentication credential which will be used for improper purposes by the malware. However, this problem is overcome by embodiments of the invention—since all code responsible for rendering user interface controls executes in VM 0  in an embodiment, malware is prevented from hijacking or corrupting UI-rendering code. 
     To illustrate an embodiment of the invention, consider  FIG. 7 , which is an illustration of exemplary desktop of client  200  according to an embodiment. As shown in  FIG. 7 , process  704  is responsible for rendering desktop  706  on a physical display of client  200 . Process  714 A runs in untrusted VM  714  and does not have complete access to the file system of client  200 . When any process inside VM  714  requests access to the file system of client  200 , it is intercepted and process  702  is responsible for rendering a window  708  depicting the contents of the file system of client  200 . Process  702  has the option of selectively displaying which contents are available to the VM  714  based on policies as set forth by the IT administrator or the user. VM  710  in  FIG. 7  that runs the solitaire game is implemented such that the display of VM  710  is a virtualized display, which is then composed into the desktop  706  (as appropriate) by controlling process  704  running in VM 0   230 . The displays of VMs  712  and  714  are rendered on the desktop  706  in a similar fashion. 
     The Legacy Virtual Machine—LVM 
       FIG. 2  depicts a legacy virtual machine (LVM)  240  according to an embodiment of the invention. LVM  240  may contain operating system  244 . LVM  240  serves as the primary entity being managed by the IT administrator of client  200 . As such, LVM  240  provides an environment that is analogous to the managed enterprise OS of corporate computer system in that an IT department may install and maintain various enterprise applications within operating system  244  of LVM  240 . In an embodiment, operating system  244  of LVM  240  may correspond to a Microsoft Windows OS or any other general purpose OS such as Linux or MacOS. 
     In an embodiment, LVM  240  is responsible for storing the main file system  242  of client  200 . File system  242  may contain the user&#39;s profile folder containing the user&#39;s settings and files. 
     LVM  240  typically only runs infrastructure OS programs and programs that are used for the purpose of managing client  200  and trusted enterprise applications. Other user programs (especially those that involve external components or consume untrusted data) do not run in LVM  240 , but instead, run elsewhere in separate VMs (such as a UCVM as described in more detail below). 
     In an embodiment, the network access of LVM  240  is restricted to just the corporate network as implemented by firewall VM  250 . Firewall VM  250  is a specialized virtual machine that comprises firewall software/applications to restrict network access of VMs running in client  200  to appropriate and/or necessary network access points. Such practice is consistent with the need for only the responsible IT administrator to be capable of connecting to LVM  240  to manage LVM  240  and processes executing therein. 
     In one embodiment, LVM  240  and VM 0   230  may be implemented in a single virtual machine. 
     Untrusted Code Virtual Machine—UCVM 
     When a user wishes to run any application that requires access to either a network or untrusted data (untrusted data is any data that originates from outside client  200 ), the application is run inside a dedicated VM that is created on-demand by hypervisor  220 . This dedicated VM is called an Untrusted Code Virtual Machine (or UCVM).  FIG. 2  depicts several UCVMs, namely UCVM  260 ,  262 ,  264 , and  266 . A UCVM operates under the assumption that, in general, any code that connects to the network and interacts with arbitrary code executing on an external device may at some point be compromised. This assumption also applies to trusted applications that interact with data originating from outside the computer system executing the trusted application, because such data may, at some point, contain embedded malicious code. To address such possibilities, such applications are executed in a UCVM to prevent any malicious code, inadvertently introduced into the UCVM, from having the capacity to affect any change outside of the UCVM. 
     In an embodiment, a UCVM is created by (a) cloning a copy of LVM  240 , or a stripped-down version of LVM  240 , in memory and (b) providing access to a restricted file system to the newly created UCVM. For example, UCVM  260  comprises restricted file system  260 A, UCVM  262  comprises restricted file system  262 A, and UCVM  264  comprises restricted file system  264 A. Each UCVM possesses its own instance or copy of the operating system, which is isolated and separate from the main operating system (including its code and data) executing within VM 0   230  or LVM  240 . For example, UCVM  260  comprises operating system  260 B, UCVM  262  comprises operating system  262 B, and UCVM  264  comprises operating system  264 B. 
     To provide a low latency user experience, UCVMs may not be booted from scratch each time an application is needed to be started. Instead, a UCVM may be created very quickly by cloning the UCVM from a template VM (with a booted OS) that has been pre-loaded in memory at system boot time. In an embodiment, the template used to clone a UCVM may be selected from templates  238  stored in VM 0   230 . A variety of techniques can be employed to make this cloning operation as fast as a few 100 milliseconds. Multiple types of templates may be used by a system to create UCVMs depending the nature and type of application(s) to be run inside the UCVM, as discussed in greater detail below in the section entitled “Cloning a UCVM from a Template.” 
     Cognitive assist module  236  is software that is responsible for implementing the rules and policies of embodiments as well as helping the user of client  200  in understanding and navigating the security model employed by client  200  on an as-needed basis. Cognitive assist module  236  helps decide what activities run in which UCVMs, including when VMs are created or destroyed, and what kind of access to network and file system resources each UCVM has. Cognitive assist module  236  also helps protect the user, e.g., when a user is fooled by malware running in a UCVM and is in the process of providing some information that they have previously provided to enterprise code running in LVM  240  (for example a password), then cognitive assist module  236  may detect this situation and prevent the user from providing the information (which may be secret corporate information) to the malware. 
     Regarding the restricted file system of each UCVM, each UCVM has access to a private copy of a subset of the files in file system  242  on client  200 . A UCVM may only have access to those files which the UCVM should need for the correct operation of the application executing therein. For example, user files are usually not required for correct operation of an application executing in a UCVM and thus are not typically exposed to a UCVM. On the other hand, if a UCVM is created as a result of the user wishing to edit a document using an application, such as MS Word, then a copy of the document the user wishes to edit will be provided to the restricted file system of the UCVM at the time the UCVM is created. Advantageously, using UCVM  260  as an example, if a process executing within UCVM  260  makes any changes to any files in restricted file system  260 A, then these changes do not impact the files stored in file system  242  maintained in LVM  240  because such changes are only made to restricted file system  260 A maintained in the UCVM and are not propagated, without express consent from the user, to file system  242  maintained by LVM  240 . 
     In a typical use case of a UCVM, the UCVM may run a local application or an individual web page session. When a user is done running the local application or navigates away from a web page to another page with a different Internet URL domain, the corresponding UCVM is destroyed. Any new local application or web application will be run inside a brand new, separate UCVM that is cloned again from a clean UCVM master template. Thus, if there has been any compromise to the UCVM during the course of running some malicious code that was introduced into the UCVM, then the adverse affects of the security breach are isolated to only the affected UCVM and are lost when the UCVM is destroyed. 
     For example, assume that a user double-clicks on a MS Word document icon in Windows Explorer. Embodiments create a special UCVM to run the MS Word process. In a particular embodiment, cognitive assist module  236  of VM 0   230  may dynamically create the UCVM using a template in one or more templates  238  or use a pre-existing template in memory or on the disk. The template selected by cognitive assist module  236  may be selected based on what activity is to occur within the UCVM, i.e., the selected may be designed to create a UCVM having characteristics that are optimal for running a text editor therein. The created UCVM contains a copy of the operating system as well as a restricted (local) copy of the file system. This local copy of the file system in the UCVM contains all the usual Windows and Program files; however, the user&#39;s profile folder in the local copy of the file system contains only the single target MS Word document being opened. 
     As another example, assume that three tabs are open in a web browser and further assume that each tab is open at a different web page. In consideration of the code which may be contained or embedded on a web page, each web page may be properly considered a web application. In embodiments of the invention, the code responsible for rendering the user interface (UI) of the web browser runs in VM 0   230 . On the other hand, executable code for the three web applications runs in three separate UCVMs. A core HTML/Javascript engine runs in each of the three UCVMs. A copy of the file system within each of the three separate UCVMs does not contain any part of the user&#39;s files, as they are not required for the task performed by each UCVM, namely displaying a web page. Thus, each web application (or web page in this example) is completely isolated from the rest of the system. 
     In an embodiment, a UCVM may be connected to the Internet according to an access policy determined by the nature of the code running within the UCVM. To illustrate, web pages are typically restricted as per a strict “same origin policy” similar to the rules implemented by modern web browsers. In the “same origin policy,” scripts running on web pages are permitted to access methods and properties of other scripts originating from the same site with no specific restrictions, but are prevented from accessing most methods and properties across web pages on different sites. Untrusted native applications running outside of the web browser are restricted by default to be able to connect only to the domain from which the program was downloaded (and to specific content delivery networks (CDNs) that may be in use by the domain in question). 
     This level of network access for downloaded applications can be explicitly changed (increased or decreased) by the end-user to include additional sites on the Internet. End-user control over what a UCVM can connect to may be subject to certain limitations related to corporate networks and sensitive web sites (such as a bank and web mail provider). For example, any code running in a UCVM may not, in general, access any site on a corporate Intranet to which client  200  is connected. Applications that need to connect to the corporate Intranet may need to be signed by the IT administrator of the domain. Similarly, non-web untrusted application code in a general UCVM may not connect to a web site associated with a search engine or bank or other sites that may have been previously identified as being “off limits.” These connections can only be made through a web browser (which spawns UCVMs bound to these special domains) or from a special purpose LVM called a VVM, which described in further detail below. 
     In an embodiment, there is no communication channel available for an application running in one UCVM to communicate with an application running in another UCVM. Thus, applications running in UCVMs are completely isolated from each other and from the other applications in the system. This is well suited for running downloaded third party local applications which are generally designed to be self-contained or for Internet applications (web pages are not supposed to rely on any communication between applications within the web browser). In an alternate embodiment, communication between an identified set of virtual machines can be enabled by a person with sufficient privileges, such as an IT administrator for client  200 . 
     Firewall Virtual Machine 
     In an embodiment, the implementation of the network access restrictions is done in a dedicated VM called a firewall VM.  FIG. 2  depicts an exemplary firewall VM  250  of an embodiment. Firewall VM  250  runs an isolated operating system with a dedicated and fixed set of firewall applications that implement the network access policy for all VMs in client  200  (except perhaps VM 0 , which may not have any network access). Firewall VM  250  may provide, to any virtual machine running on client  200  in which untrusted code is executed or untrusted data is being interpreted, restricted access to only those network resources deemed necessary on an as-needed basis in accordance with a policy described by policy data stored on client  200 . 
     In another embodiment of the invention, the firewall functionality of the system may be co-located and implemented inside either the hypervisor  220  of  FIG. 2 , or inside the LVM  240  of  FIG. 2  (working in conjunction with the hypervisor  220  of  FIG. 2 ), or inside VM 0   230  of  FIG. 2  (working in conjunction with the hypervisor  220  of  FIG. 2 ). 
     Validated Virtual Machines—VVMS 
     UCVMs are not appropriate to run local applications that interact heavily with each other using local APIs such as COM, as typically there is no communication channel available for an application running in one UCVM to communicate with an application running in another UCVM. Embodiments may employ one (or more) special UCVMs called a Validated Virtual Machine (VVM) for the purpose of running relatively trusted local applications that have complex interactions between the applications. Such complex interactions are common in enterprise frameworks containing multiple applications, such as Microsoft&#39;s Office Suite and IBM&#39;s Lotus Notes. 
       FIG. 2  depicts an exemplary VVM  266  of an embodiment. Note that while  FIG. 2  depicts a single VVM for ease of explanation, other embodiments of the invention may employ two or more VVMs or no VVMs based upon the particular needs of the user and/or policies of the organization responsible for or the owner of client  200 . 
     Applications need to be signed and configured for co-location in the same VM by an administrator of client  200  before they can run in VVM  266 . Inside VVM  266 , signed applications can interact with each other using all types of APIs and frameworks supported by the OS being used. In an embodiment, the default network access policy of a VVM is to allow access to a corporate network only. The IT administrator may increase or decrease this level of access, subject to certain restrictions. 
     In an embodiment, specific signed applications or suites (groups of applications) that originate from a trusted source (other than the enterprise) may also be designated to run together in a particular VVM responsible for applications originating from that source. For example, all non-corporate applications that are signed by a specific vendor may be run together in a single VVM. These applications would then be isolated from corporate applications and general untrusted applications, but not from one another. A specific network access rule that is more permissive than the “same origin policy” used for web applications and unsigned applications may be used for a VVM. The restricted copy of file system  242  exposed to a VVM is similar to that exposed to a generic UCVM in that the restricted copy of file system  242  exposed to a VVM comprises only those files related to, or required for, performance of the applications executing within the VVM. 
     The Restricted File System Exposed to a VM 
       FIG. 3  is block diagram of the functional components involved in exposing a restricted copy of file system  242  to different UCVMs (and VVMs) according to an embodiment of the invention. File System Switch  310  is software that is configured to provide the newly created UCVM with access to a copy-on-write clone of the OS image that the UCVM was created from once the UCVM has started. The minimal operating system and program files  330  in the copy-on-write clone may be created from either the corporate LVM OS image  320  or a separate generic, stripped down OS image  322  which may be created by the IT administrator. 
     Furthermore, a newly created UCVM is provided a copy of necessary user files  340 , which are a subset of the user files in file system  242 . The composition of necessary user files  340  will be different for each user. The set of files comprising the user files in file system  242  maintained in LVM  240  are typically those files in the user&#39;s home folder, e.g., c:\Users\&lt;username&gt;. The particular copies of files that are provided to a particular UCVM as necessary user files  340  are the minimum set of files that are needed by that UCVM to accomplish what the user intended to do as captured when the target application was being invoked. For example, if the user double clicked on a specific MS Word file named ABC.docx at the location c:\Users\&lt;username&gt;\Documents in the file system  240  maintained in LVM  240 , then necessary user files  340  would only include a copy-on-write clone of the ABC.docx file and only this copy-on-write clone of the ABC.docx file is made available in the virtual c:\Users\&lt;username&gt;\Documents folder made visible to the newly created UCVM running the MS Word application. If a program (like MS Word) was started without any association with a file, then necessary user files  340  would correspond to an empty c:\Users\&lt;username&gt;\Documents virtual folder. 
     Any application running in a UCVM therefore only has access to the particular set of user files provided explicitly by the user when the program was invoked. Subsequently, if the user wants to browse file system  242  for another file from within the application (for example, by using the File-&gt;Open menu item of MS Word), then he or she will see a restricted user files directory. 
     To enable the user to select files from the user&#39;s own User Files folder in file system  242  maintained in LVM  240  using an application executing within an UCVM, a user interface may be provided to allow the user to browse his or her files in file system  242 , select one or more of the user files, and expose a copy of the selected files to the appropriate UCVM. For example,  FIG. 4  is a flowchart illustrating the steps involved in a UCVM obtaining a copy of a new user file maintained in file system  242  according to an embodiment of the invention. In step  410 , a special file is provided to each UCVM. The special file may be provided to the UCVM in a number of different ways, e.g., the special file may be inserted into each folder of the virtual C:\Users\&lt;username&gt; directory provided to each UCVM. This special file may be named something akin to “Show All My Files” or the like, as its selection will be used to trigger exposing additional copy-on-write clones of files stored in file system  242  to the UCVM. 
     In step  420 , File System Switch  310  detects when the special file is selected by the user. For example, when a program executing within a UCVM browses to the special file, presumably as a result of a user click, this action may be trapped by File System Switch  310 . 
     In step  430 , File System Switch  310  invokes a dialog with LVM  240  that allows the user to browse the full file system  242  maintained in LVM  240 . The user may then select a file or folder in file system  242 . Note that at this stage, the user may be granted read access to the full file system  242  for purposes of selecting a file or folder, but the user is not granted write access to file system  242 . Therefore, the user is prevented from modifying file system  242  maintained by LVM  240  in any way. 
     In step  440 , after the user selects a file or folder, a copy of the selected file or folder is created. The copy of the selected file or folder is then inserted into the restricted file system associated with the UCVM. As a result of inserting the copy of the selected file or folder in the restricted file system associated with the UCVM, an application executing in the UCVM may have read and write access to the copy of the selected file or folder in the virtual file system, but is prevented from effecting any change to the original copy of the selected file or folder in file system  242  maintained by LVM  240 . 
     The steps of  FIG. 4  ensure that files in file system  242  maintained by LVM  240  are not visible to a UCVM without explicit permission from the user. Malicious code running in a UCVM, for example, cannot programmatically access files in file system  242  in LVM  240 . Further, malicious code running in a UCVM also cannot render a false user interface to trick the user into unintentionally providing any user files to the malicious code, since all code responsible for rendering the user interface is maintained within VM 0   230 , and thus, unreachable and un-hackable by the malicious code. 
     File System Switch  310  may be implemented in a variety of ways. For example, in one embodiment, File System Switch  310  may be implemented by a network file system protocol (NFS or CIFS may be used). A special VM (or LVM  240 ) may be used as the OS serving the “User Files” shared file system. Other VMs “mount” this shared file system using NFS or CIFS (or another network file system) from the hosting VM. Application software in the hosting VM may decide what files are exposed to which VM based on instructions provided by VM 0   230 . 
     In another embodiment, File System Switch  310  may be implemented, in part, by a proprietary protocol for handling communications between the different UCVMs and File System Switch  310 . File System Switch  310 , in such an embodiment, may be implemented as part of a special VM or in LVM  240 . 
     Cloning a UCVM from a Template 
     In an embodiment of the invention, every virtual machine created in client  220  is instantiated using a template selected from one or more templates  238  stored in VM 0   230 . In an embodiment, each template in one or more templates is either immutable or may be updated in a very controlled fashion. 
     Each of one or more templates  238  may be used to instantiate or create a virtual machine with different characteristics or operational parameters. The characteristics or operational parameters described by a template may be configured, tailored, or suited for a particular context or type of processing activity. For example, each template may specify what type of code is to be run within a virtual machine created using the template, a size of the virtual machine created using the template, firewall settings for the virtual machine created using the template, what type of virtual machine (for example, a VVM, UCVM, or a LVM) is the be created using the template, how changes to a local file system within the virtual machine created using the template are to be persisted, and what portion, if any, of the network can a virtual machine created using the template access. 
     One or more devices internal to client  200  or externally connected to client  200  may interact with one or more processes executing in a virtual machine within client  200 . In an embodiment, a template may assign responsibility for a selected set of devices to a virtual machine created using the template. In other embodiments, responsibility for a selected set of devices may be assigned to a particular virtual machine by virtue of policy data stored on client  200 . Such policy data may describe one or more policies provided to client  200  from an owner or responsible organization of client  200 . Policy data of this nature may be maintained by VM 0   230  or LVM  240 , for example, in certain embodiments. 
     In an embodiment, one or more templates  238  may be arranged in a hierarchy such that there is a root node corresponding to a template having a default set of characteristics. The root node may have one or more child nodes, and each of these child nodes may be associated with a template that inherits the properties of the parent template, but contains additional or changes properties associated with that child node. Naturally, each child node may also have children, and so the hierarchy of templates may be an arbitrary number of levels deep, where each template inheriting characteristics of its parent, but yet each template is capable of further defining or changing characteristics that distinguishes the template over its parent. 
     Branches of the hierarchy of templates may be associated with, or more particularly suited, different types of activity. For example, certain templates may be associated with corporate activity, and may therefore specify characteristics related to virtual machines running corporate applications. Similarly, certain templates may be associated with the user&#39;s personal application&#39;s activity or Internet/Web related activity, and may therefore specify characteristics related to virtual machines running the user&#39;s own applications or Internet/Web applications respectively. 
       FIG. 5  is an illustration of instantiating a plurality of different virtual machines using different templates according to an embodiment of the invention. In  FIG. 5 , CVM- 0  represents a template that defines a virtual machine having characteristics suitable for running a corporate application, PVM- 0  represents a template that defines a virtual machine having characteristics suitable for running a user application (non-corporate), and WVM- 0  represents a template that defines a virtual machine having characteristics suitable for running an Internet application. Other embodiments of the invention may define a variety of other templates to define different types of templates. In the example of  FIG. 5 , cognitive assist module  236  in VM 0   230  may use CVM- 0  to instantiate one or more corporate virtual machines, such as CVM- 1 , CVM- 2 , etc. Similarly, cognitive assist module  236  may use PVM- 0  to instantiate one or more personal (non-corporate) virtual machines, such as PVM- 1 , PVM- 2 , etc., and cognitive assist module  236  may use WVM- 0  to instantiate one or more web-based virtual machines, such as WVM- 1 , WVM- 2 , etc. As depicted in  FIG. 5 , each instantiated UCVM connects to an external network through Firewall VM  250 . Cognitive assist module  236  can either create these templates on demand or create and store them while monitoring the usage of the client. 
     Installation of Software 
     In the normal operation of a typical PC, a fair amount of after-market software is installed. Such after-market software installed on a PC generally falls into one of two categories, namely (a) validated software (packages or straight executables) installed by the IT administrator of the PC or (b) end-user installed software (including web browser plugins &amp; extensions, more complex software packages that go through an explicit install phase, and straight executables that can be executed without an explicit installation phase). Note that end-user installed software may be signed (by a verifiable, known vendor) or unsigned. 
     In embodiments of the invention, installation of validated software is performed as is normally performed today. The IT administrator can manage corporate validated software using embodiments using similar procedures as performed today, except that such corporate validated software are installed in LVM  240  (or if need be, VVM  266 ). 
     With respect to end-user installed software, IT administrators have two choices for how they would like to handle this type of installation using embodiments of the invention. The first choice is for the IT administrator to lock down client  200  by disallowing any installation of end-user installed software. While this is a safer operating decision, this approach may reduce the end-user&#39;s productivity because the end user cannot take advantage of applications that may be otherwise useful that have not yet been validated by the IT administrator. The IT administrator may provide installation support on an individual and as-needed basis whenever a user wishes to install any end-user installed software; however, doing so will increase the cost of support by the IT administrator. 
     The second choice is for the IT administrator to allow the user to install end-user installed software him or herself using features provided by embodiments of the invention. End-user installed software may include browser plugins, browser extensions, signed and unsigned installation packages, and straight executables. Browser plugins are installed into an installed browser plugin database that is maintained in a particular UCVM. The installed browser plugin database may be implemented, in an embodiment, using file and registry diff store  820  shown in  FIG. 8 , which is an illustration of safely installing an untrusted application according to an embodiment of the invention of the invention. During installation of a plugin, the installed browser plugin database is also updated to record the domain that was used to initiate the plugin install. Presumably, this is the web page that contains an element or component that requires the plugin to render the complete content in the web page. Subsequently, the web browser loads an installed plugin into a web HTML/JS engine instance (which runs inside a UCVM) only if the domain of the web page to be displayed by the UCVM matches a domain, recorded in the plugin database, associated with the installed plugin. A plugin that is used by multiple sites is installed only once, but is associated with multiple domains. Popular plugins like Flash may be pre-installed in certain embodiments. 
     Browser extensions may be installed into a web browser&#39;s extension database that is maintained in a particular UCVM. During runtime, browser extensions are treated like web applications in that each browser extension is run inside its own UCVM. In an embodiment, the web browser extension database and the installed browser plugin database may be implemented in the same database in a single UCVM. 
     Signed installation packages may be run and the resulting installation may update either the LVM image or the Generic Windows image based on a policy set by the IT administrator. 
     Unsigned installation packages go through a virtual install. The virtual installation of unsigned installation packages will be described with reference to  FIG. 8 , which is an illustration of safely installing an untrusted application according to an embodiment of the invention of the invention. A registry and program files change set is created and stored in file and registry diff store  820 . Start-menu and desktop changes by the installer are captured in a special folder which contains desktop and start menu items for all user-installed applications. Subsequently, if an unsigned application is run, it is run in a UCVM cloned from the Generic Windows image all by itself. Virtual disk  810  in  FIG. 8  is the normal virtual disk of the UCVM. DiffStore  820 , which further virtualizes the file system and the registry as seen by the applications of UCVM, is typically implemented as a separate module outside of the normal block level virtual disk store. 
     Signed and unsigned executables may be run in a UCVM. Such a UCVM may be created on demand and destroyed after its use is ended by embodiments. 
     Managing Web Cookies and Caches 
     A web cookie (or simply “cookie”) is a piece of text stored on a user&#39;s computer by their web browser. A cookie can be used for authentication, storing web site preferences, shopping cart contents, the identifier for a server-based session, or anything else that can be accomplished through storing text data. 
     While the actual cookie itself is not visible to the user, the user would notice a difference in the user experience of interacting with a web site if cookies could not be saved between visits to the web site. Accordingly, embodiments of the invention provide mechanism to store cookies before a UCVM is destroyed, so that the next time the user visits the web site using a web browser running in a different UCVM, any cookies that have been stored and are associated with that web site may be injected into the new UCVM. 
     Similarly, to provide the best user experience, it would be advantageous to carry over the cache of a web browser for a particular web domain from one UCVM to the next, so that the next time the user visits the web domain using a different UCVM, there is no a delay in displaying content due to an unpopulated cache. Thus, embodiments of the invention provide mechanism to store the web cache of a web browser for a web domain before a UCVM is destroyed, so that the next time the user visits the web site using a web browser running in a different UCVM, the cache of the web browser need not be warmed (i.e., repopulated), as the cache in the new UCVM has been updated to contain all the objects the cache previously contained in the prior, and now destroyed, UCVM used to visit the web domain. 
     To provide a concrete example with reference to the example of  FIG. 2 , assume that a user initially transparently uses UCVM  260  to run a web browser to visit web site A. When UCVM  260  is destroyed, any cookies and cache files are extracted and saved. Thereafter, assume the user transparently uses UCVM  262  to run a web browser to visit web site B. As web site B is hosted by a different web domain than web site A, the previously stored cookies and cache files associated with web site A will not injected into UCVM  262 . Thereafter, if UCVM  262  is destroyed, then any cookies and cache files are extracted and saved. At a later point in time, if the user thereafter transparently uses UCVM  264  to run a web browser to visit web site A, then the previously stored cookies and cache files associated with the web domain of web site A will be injected into UCVM  264 . This allows the web browser running in UCVM  264  to visit web site A to appear, to the user, to have the same state of the prior web browser used to visit web site A, even through different virtual machines are used between visits. Note that no portions of the file system are saved between visits to a web site; only the state of the web session is saved. 
     In one embodiment, the cookies and cache information is captured in Diff Store  820  associated with the URL of the website. In each visit to the same URL, the UCVM utilizes the same Diff Store presenting the cookies and caches to the UCVM. In another embodiment, the cookies and cache files can be captured at the end of the session and saved to the client system&#39;s core file system in a special folder. On visiting the same URL again, the cookies and cache can be re-injected into the file system of the UCVM. 
     Efficient Physical-to-Virtual Disk Conversion 
     Platform virtualization is performed on a given hardware platform by host software (a control program), which creates a simulated computer environment, a virtual machine, for its guest software. A hypervisor, also called virtual machine manager (VMM), is one of many hardware virtualization techniques that allow multiple operating systems, termed guests, to run concurrently on a host computer. The hypervisor presents to the guest operating systems a virtual operating platform and manages the execution of the guest operating systems. A guest OS executes as if it was running directly on the physical hardware. Access to physical system resources such as the network access, display, keyboard, and disk storage is suitably virtualized so that guest OS does not know these are virtual devices. 
     Generally, there are two types of hypervisors. Type 1 (or native, bare metal) hypervisors run directly on the host&#39;s hardware to control the hardware and to manage guest operating systems. A guest operating system thus runs on another level above the hypervisor. Type 2 (or hosted) hypervisors run within a conventional operating system environment. With the hypervisor layer as a distinct second software level, guest operating systems run at the third level above the hardware. In other words, Type 1 hypervisor runs directly on the hardware; a Type 2 hypervisor runs on another operating system, such as Windows. Embodiments of the invention may use any type of hypervisor. Thus, hypervisor  220  in  FIG. 2  may either be a Type 1 or a Type 2 hypervisor. 
     A virtual disk image is a file on a physical disk, which has a well-defined (published or proprietary) format and is interpreted by a hypervisor as a hard disk. In terms of naming, a virtual disk image may have a specific file type extension, e.g., .vmdk for VMware VMDK, .vhd for Xen and Microsoft Hyper-V, and .vdi for Oracle VM VirtualBox. 
     There are two approaches employed for storage allocation by prior hypervisors, namely, (1) pre-allocate the entire storage for the virtual disk upon creation and (2) dynamically grow the storage on demand. In the former approach involving pre-allocation, the virtual disk may be implemented as either split over a collection of flat files (typically one is 2 GB in size) or as a single, large monolithic flat file. In the latter approach involving on-demand growth, the virtual disk may also be implemented using split or monolithic files, except that storage is allocated on demand. 
     There are two modes in which a disk can be mapped for use by a virtual machine. In a virtual mode, the mapped disk is presented as if it is a logical volume, or a virtual disk file, to the guest operating system and its real hardware characteristics are hidden. In a physical mode, also called the pass through mode, the hypervisor bypasses the I/O virtualization layer and passes all I/O commands directly to the disk. 
     A virtual machine (VM) is a software implementation of a machine (i.e. a computer) that executes programs like a physical machine. Virtual machines allow the sharing of the underlying physical machine resources between different virtual machines, each running its own operating system. The software layer providing the virtualization is called a hypervisor, such as hypervisor  220  in  FIG. 2 . 
     Virtual machines each require their own image of the operating system. The guest OS and host OS typically do not share the OS image, even if they are the same OS. This is problematic for several reasons. First, if a user wishes to run 10 different virtual machines, then she will require 10 different copies of the OS for the guest OSs, which requires an undesirable amount of storage to maintain. As she is already running one virtual machine at the host, the total number of different copies of the OS required would be 11. Second, the OS for a VM has to be created either by installing a new OS or shipping a copy of the OS from somewhere else, which is burdensome for those who do not have access to OS images. Further, it is also time consuming to install a new OS or ship an OS image, which is typically quite large. A third problem is that any software present in the host OS (such as a printer driver) will not be available in a guest OS unless it is installed again. 
     Shadow Copy (Volume Snapshot Service or Volume Shadow Copy Service or VSS) is a technology included in Microsoft Windows that allows taking manual or automatic backup copies or snapshots of data (termed “shadow copies”), even if it has a lock, on a specific volume at a specific point in time over regular intervals. VSS operates at the block level of the file system. Shadow Copy is implemented as a Windows service called the Volume Shadow Copy service. Software VSS provider service is also included as part of the Microsoft Windows OS to be used by Windows applications. Shadow Copy technology requires that the file system to be NTFS to be able to create and store shadow copies. Shadow Copies can be created on local and external (removable or network) volumes by any Windows component that uses this technology, such as when creating a scheduled Windows Backup or automatic System Restore point. 
     Snapshots have two primary purposes. First, they allow the creation of consistent backups of a volume, ensuring that the contents cannot change while the backup is being made. Second, they avoid problems with file locking. By creating a read-only copy of the volume, backup programs are able to access every file without interfering with other programs writing to those same files. Through the integration between the Volume Shadow Copy Service, hardware or software VSS providers, application level writers and backup applications, VSS enables integral backups that are point in time and application level consistent without the backup tool having knowledge about the internals of each application. The end result is similar to a versioning file system, allowing any file to be retrieved as it existed at the time any of the snapshots was made. Unlike a true versioning file system, however, users cannot trigger the creation of new versions of an individual file, only the entire volume. 
     Embodiments of the invention overcome this limitation by creating virtual disks based on VSS shadow copies.  FIG. 6  is an illustration of a virtual disk based on VSS shadow copies according to an embodiment. The virtual disk of  FIG. 6  allows for many guest OSs running on the same host to share the same OS copy with the host OS. VSS shadow copies may be created fast and efficiently. Creating virtual disks on top of VSS is also a very fast operation, which means that VMs (with same OS as host OS) can be created very efficiently. Shadow copies are also maintained cheaply by windows OS by keeping the changes since the time shadow was created. Hence, the disk usage of multiple VMs is reduced substantially. VMs can also be maintained very efficiently since VSS snapshots can be updated once and have the changes reflected in all VMs. Since a VSS shadow copy contains all the software the user has installed on the machine at the time of the VSS shadow copy creation, virtual disks also receive access to all the software. Moreover, the version of the software, including any patches installed, is exactly the same. In addition to all the software, user documents are also visible to virtual machines. A virtual disk of an embodiment is an accurate point-in-time copy of host physical disk. 
     In an embodiment where VSS snapshots are read-only, a ‘Delta Store Disk’ may be attached to the virtual disk. The Delta Store disk is used to capture all the changes being made to the virtual disk. 
     Security Afforded by Embodiments 
     Embodiments of the invention provide a secure environment to prevent malicious code from affecting any lasting change in a computer system. Arbitrary code (either a web application or a native executable) runs inside an isolated operating system running on an isolated virtual machine. This code has no access to any other application (either a native application or a web application) being run by the user because those applications run in other operating systems running in separate virtual machines. Moreover, arbitrary code has access to only the specific part of the file system that is needed for correct execution of the code. Access to additional parts of the file system has to be provided by code that runs in VM 0  (which is secure and fortified against unauthorized intrusion) and any increased access needs explicit authorization from the human user. 
     Specific trusted code that needs to interact in a complex way with other applications may be explicitly designated to run together inside the same designated VM. This type of VM also has limited access to the file system. 
     All code has limited network access to just what that code needs for its correct execution. All virtual machines are created from templates stored in VM 0  which are either immutable or can be updated in a very controlled fashion. Consequently, if a security bug exists in a piece of code, the effect of the security bug is isolated (“space limited”) because the compromised code has access to only a limited part of the file system, the network, devices, etc. Moreover, the effect of the security bug is “time limited” because the virtual machine that has been compromised will be subsequently discarded and a new virtual machine is created for future application instances from a clean immutable VM template. 
     Hardware Mechanisms 
     In an embodiment, client  200  of  FIG. 2  may be implemented on, include, or correspond to a computer system.  FIG. 9  is a block diagram that illustrates a computer system  900  upon which an embodiment of the invention may be implemented. In an embodiment, computer system  900  includes processor  904 , main memory  906 , ROM  908 , storage device  910 , and communication interface  918 . Computer system  900  includes at least one processor  904  for processing information. Computer system  900  also includes a main memory  906 , such as a random access memory (RAM) or other dynamic storage device, for storing information and instructions to be executed by processor  904 . Main memory  906  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  904 . Computer system  900  further includes a read only memory (ROM)  908  or other static storage device for storing static information and instructions for processor  904 . A storage device  910 , such as a magnetic disk or optical disk, is provided for storing information and instructions. 
     Computer system  900  may be coupled to a display  912 , such as a cathode ray tube (CRT), a LCD monitor, and a television set, for displaying information to a user. An input device  914 , including alphanumeric and other keys, is coupled to computer system  900  for communicating information and command selections to processor  904 . Other non-limiting, illustrative examples of input device  914  include a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  904  and for controlling cursor movement on display  912 . While only one input device  914  is depicted in  FIG. 9 , embodiments of the invention may include any number of input devices  914  coupled to computer system  900 . 
     Embodiments of the invention are related to the use of computer system  900  for implementing the techniques described herein. According to one embodiment of the invention, those techniques are performed by computer system  900  in response to processor  904  executing one or more sequences of one or more instructions contained in main memory  906 . Such instructions may be read into main memory  906  from another machine-readable medium, such as storage device  910 . Execution of the sequences of instructions contained in main memory  906  causes processor  904  to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement embodiments of the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software. 
     The term “machine-readable storage medium” as used herein refers to any tangible medium that participates in storing instructions which may be provided to processor  904  for execution. Such a medium may take many forms, including but not limited to, non-volatile media and volatile media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  910 . Volatile media includes dynamic memory, such as main memory  906 . 
     Non-limiting, illustrative examples of machine-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, or any other medium from which a computer can read. 
     Various forms of machine readable media may be involved in carrying one or more sequences of one or more instructions to processor  904  for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a network link  920  to computer system  900 . 
     Communication interface  918  provides a two-way data communication coupling to a network link  920  that is connected to a local network. For example, communication interface  918  may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  918  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface  918  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  920  typically provides data communication through one or more networks to other data devices. For example, network link  920  may provide a connection through a local network to a host computer or to data equipment operated by an Internet Service Provider (ISP). 
     Computer system  900  can send messages and receive data, including program code, through the network(s), network link  920  and communication interface  918 . For example, a server might transmit a requested code for an application program through the Internet, a local ISP, a local network, subsequently to communication interface  918 . The received code may be executed by processor  904  as it is received, and/or stored in storage device  910 , or other non-volatile storage for later execution. 
     In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicants to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.