Abstract:
Disclosed are systems and methods for protecting memory pages of a computing device using a hypervisor. An exemplary method comprises: in response to receiving a hypercall from a trusted program, detecting by the hypervisor a token associated with the trusted program; checking the token associated with the trusted program against a saved token of the hypervisor; in response to detecting that the token associated with the trusted program matches the saved token of the hypervisor, transmitting addresses of a plurality of memory pages from the hypervisor to the trusted program; and performing a checksums verification for data stored in the plurality of memory pages.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. Application of U.S. patent application Ser. No. 14/935,852 titled SYSTEM AND METHOD FOR PROTECTION OF MEMORY IN A HYPERVISOR and filed Nov. 9, 2015, which is incorporated by reference herein. 
     
    
     FIELD OF TECHNOLOGY 
       [0002]    The present disclosure relates generally to the field of computer security, and, more specifically, to systems and methods of protecting memory pages of a computing device using a hypervisor. 
       BACKGROUND 
       [0003]    Computer malware (such as Trojan horses, viruses and worms) is being developed at the ever-increasing pace and is using many methods for circumventing antivirus applications. One such method is to conceal certain resources of the computer system (such as files or registry branches) from the antivirus application, which is performing the antivirus check. By the classification of the antivirus companies, the malicious programs which make use of such a method are known as rootkits, or they make use of rootkit technology. Rootkit technologies turn out to be even more dangerous if it is possible to make use of vulnerabilities in components of the operating system (OS) which work at the kernel level. This does not allow present-day antivirus applications to detect malicious programs which use such technologies. 
         [0004]    One approach to solving such a situation is to use a hypervisor, which affords isolation of different OSs from each other, a dividing of the resources between different running OSs, and a management of resources. At the same time, the execution of code in hypervisor mode occurs on an even lower level than the execution of code at the kernel level. Not surprisingly, the companies which make antivirus applications are interested in such an approach. However, current solutions are ineffective and in some cases impossible to employ. 
       SUMMARY 
       [0005]    Disclosed are system and method for protecting memory pages of a computing device using a hypervisor. An exemplary method comprises: in response to receiving a hypercall from a trusted program, detecting by the hypervisor a token associated with the trusted program; checking the token associated with the trusted program against a saved token of the hypervisor; in response to detecting that the token associated with the trusted program matches the saved token of the hypervisor, transmitting addresses of a plurality of memory pages from the hypervisor to the trusted program; and performing a checksums verification for data stored in the plurality of memory pages. 
         [0006]    In one exemplary aspect, the saved token of the hypervisor is configured to be generated by the hypervisor in response to receiving a first hypercall from the trusted program. 
         [0007]    In one exemplary aspect, the method further comprises generating by the hypervisor a plurality of tokens, each token uniquely corresponding to an operating system present in the computing device. 
         [0008]    In one exemplary aspect, performing the checksums verification for data stored in the plurality of memory pages comprises periodically calculating checksums and comparing them with a previously saved value to detect a change in the trusted program. 
         [0009]    In one exemplary aspect, the method further comprises, in response to detecting the change in the trusted program, restoring the trusted program by the hypervisor via reloading the trusted program from a disk of the computing device. 
         [0010]    In one exemplary aspect, the method further comprises, establishing a protected communication channel between the trusted program and the hypervisor such that the trusted program is configured to make a hypercall call for a code of the hypervisor via the protected communication channel to ensure a confidentiality of the hypervisor call address by which the code is invoked. 
         [0011]    In one exemplary aspect, the method further comprises, configuring the trusted program to organize virtual memory pages of an operating system of the computing device; and configuring the hypervisor to verify the virtual memory pages of the operating system. 
         [0012]    An exemplary system for protecting memory pages of a computing device using a hypervisor comprises a hardware processor configured to: in response to receiving a hypercall from a trusted program, detect by the hypervisor a token associated with the trusted program; check the token associated with the trusted program against a saved token of the hypervisor; in response to detecting that the token associated with the trusted program matches the saved token of the hypervisor, transmit addresses of a plurality of memory pages from the hypervisor to the trusted program; and perform a checksums verification for data stored in the plurality of memory pages. 
         [0013]    An exemplary non-transitory computer readable medium storing computer executable instructions for protecting memory pages of a computing device using a hypervisor, includes instructions for: in response to receiving a hypercall from a trusted program, detecting by the hypervisor a token associated with the trusted program; checking the token associated with the trusted program against a saved token of the hypervisor; in response to detecting that the token associated with the trusted program matches the saved token of the hypervisor, transmitting addresses of a plurality of memory pages from the hypervisor to the trusted program; and performing a checksums verification for data stored in the plurality of memory pages. 
         [0014]    The above simplified summary of example aspects serves to provide a basic understanding of the present disclosure. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects of the present disclosure. Its sole purpose is to present one or more aspects in a simplified form as a prelude to the more detailed description of the disclosure that follows. To the accomplishment of the foregoing, the one or more aspects of the present disclosure include the features described and particularly pointed out in the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more example aspects of the present disclosure and, together with the detailed description, serve to explain their principles and implementations. 
           [0016]      FIG. 1  illustrates an exemplary system in which a mechanism of a trusted call for code in hypervisor mode is realized. 
           [0017]      FIG. 2  illustrates an exemplary method of providing a hypervisor call address for a trusted program. 
           [0018]      FIG. 3  illustrates an example of a memory page containing the hypervisor address for a call from the trusted program. 
           [0019]      FIG. 4  illustrates an exemplary method of protecting memory pages using a hypervisor. 
           [0020]      FIG. 5  illustrates an example of a general-purpose computer system by means of which the disclosed aspects of systems and method can be implemented. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Example aspects are described herein in the context of a system, method, and computer program product for protecting memory pages of a computing device using a hypervisor. Those of ordinary skill in the art will realize that the following description is illustrative only and is not intended to be in any way limiting. Other aspects will readily suggest themselves to those skilled in the art having the benefit of this disclosure. Reference will now be made in detail to implementations of the example aspects as illustrated in the accompanying drawings. The same reference indicators will be used to the extent possible throughout the drawings and the following description to refer to the same or like items. 
         [0022]    A hypervisor is a program enabling a simultaneous, parallel execution of several operating systems (OS) on the same computer. Hypervisors are of two types: the first has its own built-in device drivers and scheduler, and therefore does not depend on a particular OS, while the second type works in the same ring as the kernel of the main OS (kernel mode or ring 0). The first type of hypervisors is also known as bare-metal and is the preferred example for realization of the hypervisor in the present invention. The execution of code in hypervisor mode occurs at an even lower level than the execution of code in kernel mode or ring 0. 
         [0023]    A call for code being executed in hypervisor mode (a hypercall) is a transition to the execution of code in hypervisor mode (also simply called ‘execution’), which requires hardware support of the virtualization technology on the part of the processor. 
         [0024]    A trusted OS is an operating system using adequate hardware and software to enable a simultaneous processing of information of different degrees of secrecy by a group of users without violating the access rights. Generally, a trusted OS is able to provide confidentiality and integrity of the user data. One can learn more about trusted systems in A Guide to Understanding Configuration Management in Trusted Systems (1988). 
         [0025]    A trusted call for code is a call for outside code during which it is guaranteed that the call comes from a trusted source (a memory page belonging to the process of a trusted application), which excludes the possibility of a call for outside code from malicious or untrusted programs. A trusted application is a program whose executable file has a digital signature and is not malicious. 
         [0026]      FIG. 1  shows an example of a system in which a mechanism of a trusted call for code in hypervisor mode is realized. The operating system  110 , executes various applications  130 , as well as a trusted program  150 , and a malicious program  140 . The applications  130  include various user applications such as a browser, a word processor, and others. The malicious program  140 , such as a virus, a worm, a Trojan or other, can use different ways of concealing its presence in the operating system (e.g., using rootkit technologies), making it possible to avoid detection by the antivirus applications (not shown in  FIG. 1 , but may be one of the applications  130 ). This fact means that the OS  110  cannot be considered to be trusted and it is a threat to the user data. One can read about ways of concealing the presence of malicious programs in an operating system in various sources, such as https://en.wikipedia.org/wiki/Rootkit. 
         [0027]    In order to detect the presence of a malicious program  140 , as well as to prevent the possibility of access by the malicious program  140  to the user information, a trusted program  150  and hypervisor  120  may be used. The trusted program  150  may be realized both in the form of a separate application and in the form of a component of the antivirus application. The key characteristic of the trusted program  150  is its ability to make calls for execution of code of the hypervisor  120 . As already noted above, the execution of code in hypervisor mode occurs on an even lower level than the execution of code in kernel mode, which makes it possible to ignore possible rootkit technologies used by the malicious program  140 . The code being executed in kernel mode (ring 0) does not have access to the code, which will be executed in hypervisor mode. In the same fashion, the code being executed in user mode (ring 3) does not have access to the code, which is executed in kernel mode. More details can be found about protection rings in various publications such as Russinovich, Mark E.; David A. Solomon (2005).  Microsoft Windows Internals  (4 ed.). Microsoft Press. It should be noted that an execution of code in ring 0 is also known as an execution on the kernel level, and an execution of code in ring 3 is an execution on the user level. 
         [0028]    As already mentioned, the trusted program  150  makes calls for execution of code of the hypervisor  150  (whose code will be executed in hypervisor mode), i.e., it makes a hypercall. It is assumed that the OS  110  is initially trusted, but the malicious program  140  compromises the trust level of the OS. It is also presumed that the malicious program  140  has a complex logic (such as a rootkit functionality), making it possible to avoid its detection by the antivirus application installed in the OS  110  (not shown in  FIG. 1 ). Thus, it is necessary to provide protection for the confidential user information from the malicious program  140 . 
         [0029]    In a modern OS, information can be protected in several ways: using encryption, controlling access to data storage, and providing protection of the virtual memory of those processes that work with user data. The present invention generally relates to the systems and methods for protection of the virtual memory of processes. 
         [0030]    The methods of protecting the virtual memory of processes are known. For example, the NX bit makes it possible to set a bit forbidding the execution for a memory page, in order to achieve the possibility of preventing the execution of data as executable code. For example, a commonly owned U.S. Pat. No. 8,990,934, which is incorporated by reference herein, describes a technique of controlling the mutually exclusive setting of an execute bit and a write bit for a memory page in order to prevent the writing and execution of an exploit. 
         [0031]    However, known technologies have a drawback in that they work on the level of the OS kernel, which leaves the possibility of execution of malicious code on the same level of privileges, but with an earlier onset of working during the startup (initialization) of the OS after the computer is turned on. Such a malicious code may disconnect or, even worse, control the above-described memory protection algorithms, which again does not afford protection for the user&#39;s information in these cases. More details on the startup (initialization) of the OS after turning on the computer can be found in various publications, such as Russinovich, Mark E.; David A. Solomon (2005); Microsoft Windows Internals (4 ed.); Microsoft Press. 
         [0032]    The execution of code in hypervisor mode makes it possible to check for memory changes on the part of a malicious code, even if the latter is executed in kernel mode. However, a call for the execution of code in hypervisor mode—the making of a hypercall—requires a separate application, whose code will make such hypercalls. In the present disclosure, that application is the trusted program  150 , which may be an antivirus application. 
         [0033]    It is important to note that it is generally very difficult to make code in hypervisor  120  mode in the form of a monolithic code sector which will contain verifications of virtual memory pages of the processes running in the OS  110 , for several reasons:
       the hypervisor code becomes “overloaded” (it proves to be too complex to write and debug), it begins to consume too may system resources, especially processor time;   the hypervisor code works too “low” (on a hypervisor level), i.e., it does not “know” the mechanism of working of the OS (kernel level), which works “higher” than the hypervisor, this problem being characteristic of the bare-metal hypervisors.       
 
         [0036]    Therefore, it may be desirable to move some of the functionality out of the hypervisor  120  into the trusted program  150 , while leaving in the hypervisor  120  only the memory page verification functionality. The trusted program  150  may be implemented to allow for the specifics of the realization of the OS  110  (for example, allowing for the virtual memory page organization of the Windows OS), making it possible to have this hypervisor  120  as a cross-platform hypervisor  120 . 
         [0037]    In one exemplary aspect, there should be a protected communication channel between the trusted program  150  and the hypervisor  120 . In order to provide such a communication channel, it is necessary that only the trusted program  150  can make a call for code of the hypervisor  120  (a hypercall). Considering that the hypervisor  120  code is stored in RAM, it is necessary to ensure confidentiality of the address by which this code is invoked. 
         [0038]      FIG. 2  exemplary method of providing a hypervisor  120  call address for a trusted program  150 . In step  205 , immediately after the boot-up of the computer system, the hypervisor  120  is loaded even before the initialization of the OS  110 . As a rule, hypervisors of bare-metal type are initialized prior to the starting of the host OSs. A host OS means any OS which is started after the initialization of the hypervisor—in the given case, this is the OS  110 . In step  207 , the trusted program  150  is loaded, which is generally realized in the form of a driver of the OS  110 , and it is loaded as soon as possible. This requirement is necessary in order to still consider the OS  110  to be trusted in step  207 , since the early start-up of the trusted program  150  makes it possible to start it before the malicious program  140  is started. In step  210 , the trusted program  150  performs the first call of the hypervisor  120  code (a hypercall), after which the address of the hypervisor  120  is returned to memory (the so-called safe hypercall address). Next, this address needs to be protected against unauthorized access, for which, in step  220 , a memory page  300  is created, whose exemplary structure is shown in  FIG. 3 . The memory page itself consists of a set of memory addresses by which hypercalls can be done. In one exemplary aspect, only one call will result immediately in the hypercall, while the others will cause an exception and an error in the working of the OS  110  with subsequent rebooting thereof. This memory page  300  may be created either by the trusted program  150  or directly by the hypervisor  120  during its first call. The filling of the page by code with calls is done by the hypervisor  120  in step  230 . The memory page may be additionally protected by, for example, setting, in step  240 , a security parameter (e.g., a security bit) indicating that this memory page cannot be overwritten. In various aspects, the security parameter may be set either by the trusted program  150  or by the hypervisor  120 . 
         [0039]    Let us consider the mechanism of sending the correct call address of the hypervisor  120  to the trusted program  150 . Since the storing of the address itself in the memory of the process of the trusted program  150  might not be safe, since one cannot rule out an intervention by the malicious program  140  in order to get it, the trusted program  150  also stores a token in the form of a randomly generated key, which can be used to make hypercalls. The token itself is generated by the hypervisor  120  at the time of its first call (step  210  in  FIG. 2 ) and sent to the trusted program  150  in order to compare the value of the token during the hypercall, thereby preventing situations with a malicious call of the hypervisor  120 . The token is generated once during the working of the computer system (after turning on the power supply) and it is unique to the OS  110 . Thus, if other host OSs are present in the computer system, the hypervisor  120  generates an individual token for each of them. 
         [0040]      FIG. 4  illustrates an exemplary method of protecting memory pages using a hypervisor. In step  410 , the trusted program  150  makes hypercalls, after which, in step  420 , the token is checked and, if the token of the trusted program  150  matches the saved token of the hypervisor  120  (check in step  430 ), then in step  450  the trusted program  150  sends the addresses of the memory pages which require protection. The memory protection involves setting of bits to forbid/allow various operations—Read, Write, and eXecute code. In step  460 , checksums verification is performed (cyclic redundancy check, CRC) for the data which is being stored in the protected memory pages. In step  470 , the hypervisor  120  orders a periodic verification by the calculation of the checksums for the protected memory pages because an attack is possible involving operation by the linear addresses. The periodic verification can be initialized by either the hypervisor  120  or the trusted program  150  by using hypercalls at equal intervals of time. The hypervisor  120  can also check the integrity of the trusted program  150  in order to make sure it has not been altered by the malicious program  140 . The integrity check includes computing a checksum and comparing it with a previously saved value to determine a change in the trusted program  150 . This check occurs in step  420 . If the code of the trusted program  150  has been changed, the hypervisor can restore it in memory by loading it from disk. 
         [0041]    Below is an example of an attack involving operation by linear addresses. For example, the IDT (Interrupt Dispatch Table) of the OS  110  is located at linear address 0xF1D10000, which is a mapping (i.e., a mapping of the address of the virtual memory page onto the physical address) of the physical address 0x123000. The hypervisor  120  can establish the above-described protection (with no loss of performance) for the physical page at address 0x123000. But the malicious program  140  can pick out a physical page, such as 0x321000, copy therein the contents of the original page 0x123000, having replaced the necessary elements there (in the given example, this will be the interrupt handler), and establish the mapping 0xF1D10000- &gt;0x321000, so that the hypervisor  120  cannot detect the change, not having information on the logic of the working of the virtual memory in the OS  110 . Therefore, the trusted program  150  should periodically check from within the OS  110  the correctness of the page tables, such that, as in the given example, 0xF1D1000 indeed corresponds to 0x123000 and not something else. 
         [0042]    Let us consider examples of the storage of the hypervisor  120  prior to its loading. The code of the hypervisor  120  may be stored either as in a UEFI (Unified Extensible Firmware Interface) service or in a separate device on the PCI or PCIe board, which device is not defined in the OS  110  (e.g., the hypervisor  120  or the trusted program  150  independently processes interrupts from this device), or by using a disk virtualization (the hypervisor  120  excludes sectors of a disk on which its code is stored, changing the disk driver basic functionality). 
         [0043]      FIG. 5  illustrates an example of a general-purpose computer system (which may be a personal computer or a server) on which the disclosed systems and method can be implemented. As shown, the computer system includes a central processing unit  21 , a system memory  22  and a system bus  23  connecting the various system components, including the memory associated with the central processing unit  21 . The system bus  23  is realized like any bus structure known from the prior art, containing in turn a bus memory or bus memory controller, a peripheral bus and a local bus, which is able to interact with any other bus architecture. The system memory includes permanent memory (ROM)  24  and random-access memory (RAM)  25 . The basic input/output system (BIOS)  26  includes the basic procedures ensuring the transfer of information between elements of the personal computer  20 , such as those at the time of loading the operating system with the use of the ROM  24 . 
         [0044]    The personal computer  20 , in turn, includes a hard disk  27  for reading and writing of data, a magnetic disk drive  28  for reading and writing on removable magnetic disks  29  and an optical drive  30  for reading and writing on removable optical disks  31 , such as CD-ROM, DVD-ROM and other optical information media. The hard disk  27 , the magnetic disk drive  28 , and the optical drive  30  are connected to the system bus  23  across the hard disk interface  32 , the magnetic disk interface  33  and the optical drive interface  34 , respectively. The drives and the corresponding computer information media are power-independent modules for storage of computer instructions, data structures, program modules and other data of the personal computer  20 . 
         [0045]    The present disclosure provides the implementation of a system that uses a hard disk  27 , a removable magnetic disk  29  and a removable optical disk  31 , but it should be understood that it is possible to employ other types of computer information media  56  which are able to store data in a form readable by a computer (solid state drives, flash memory cards, digital disks, random-access memory (RAM) and so on), which are connected to the system bus  23  via the controller  55 . 
         [0046]    The computer  20  has a file system  36 , where the recorded operating system  35  is kept, and also additional program applications  37 , other program modules  38  and program data  39 . The user is able to enter commands and information into the personal computer  20  by using input devices (keyboard  40 , mouse  42 ). Other input devices (not shown) can be used: microphone, joystick, game controller, scanner, and so on. Such input devices usually plug into the computer system  20  through a serial port  46 , which in turn is connected to the system bus, but they can be connected in other ways, for example, with the aid of a parallel port, a game port or a universal serial bus (USB). A monitor  47  or other type of display device is also connected to the system bus  23  across an interface, such as a video adapter  48 . In addition to the monitor  47 , the personal computer can be equipped with other peripheral output devices (not shown), such as loudspeakers, a printer, and so on. 
         [0047]    The personal computer  20  is able to operate in a network environment, using a network connection to one or more remote computers  49 . The remote computer (or computers)  49  are also personal computers or servers having the majority or all of the aforementioned elements in describing the nature of a personal computer  20 , as shown in  FIG. 5 . Other devices can also be present in the computer network, such as routers, network stations, peer devices or other network nodes. 
         [0048]    Network connections can form a local-area computer network (LAN)  50  and a wide-area computer network (WAN). Such networks are used in corporate computer networks and internal company networks, and they generally have access to the Internet. In LAN or WAN networks, the personal computer  20  is connected to the local-area network  50  across a network adapter or network interface  51 . When networks are used, the personal computer  20  can employ a modem  54  or other modules for providing communications with a wide-area computer network such as the Internet. The modem  54 , which is an internal or external device, is connected to the system bus  23  by a serial port  46 . It should be noted that the network connections are only examples and need not depict the exact configuration of the network, i.e., in reality there are other ways of establishing a connection of one computer to another by technical communication modules. 
         [0049]    In various aspects, the systems and methods described herein may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the methods may be stored as one or more instructions or code on a non-transitory computer-readable medium. Computer-readable medium includes data storage. By way of example, and not limitation, such computer-readable medium can comprise RAM, ROM, EEPROM, CD-ROM, Flash memory or other types of electric, magnetic, or optical storage medium, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a processor of a general purpose computer. 
         [0050]    In various aspects, the systems and methods described in the present disclosure can be addressed in terms of modules. The term “module” as used herein refers to a real-world device, component, or arrangement of components implemented using hardware, such as by an application specific integrated circuit (ASIC) or field-programmable gate array (FPGA), for example, or as a combination of hardware and software, such as by a microprocessor system and a set of instructions to implement the module&#39;s functionality, which (while being executed) transform the microprocessor system into a special-purpose device. A module can also be implemented as a combination of the two, with certain functions facilitated by hardware alone, and other functions facilitated by a combination of hardware and software. In certain implementations, at least a portion, and in some cases, all, of a module can be executed on the processor of a general purpose computer (such as the one described in greater detail in  FIG. 5  above). Accordingly, each module can be realized in a variety of suitable configurations, and should not be limited to any particular implementation exemplified herein. 
         [0051]    In the interest of clarity, not all of the routine features of the aspects are disclosed herein. It would be appreciated that in the development of any actual implementation of the present disclosure, numerous implementation-specific decisions must be made in order to achieve the developer&#39;s specific goals, and these specific goals will vary for different implementations and different developers. It is understood that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art, having the benefit of this disclosure. 
         [0052]    Furthermore, it is to be understood that the phraseology or terminology used herein is for the purpose of description and not of restriction, such that the terminology or phraseology of the present specification is to be interpreted by the skilled in the art in light of the teachings and guidance presented herein, in combination with the knowledge of the skilled in the relevant art(s). Moreover, it is not intended for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. 
         [0053]    The various aspects disclosed herein encompass present and future known equivalents to the known modules referred to herein by way of illustration. Moreover, while aspects and applications have been shown and described, it would be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts disclosed herein.