Abstract:
The present invention provides systems and methods for applying hard-real-time capabilities in software to software security. For example, the systems and methods of the present invention allow a programmer to attach, a periodic integrity check to an application so that an attack on the application would need to succeed completely within a narrow and unpredictable time window in order to remain undetected.

Description:
[0001]    The present application claims the benefit of U.S. Provisional Patent Application No. 60/432,6.55, filed on Dec. 12, 2002, the entire contents of which are incorporated herein by this reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to systems and methods for detecting a security breach in a computer system. 
         [0004]    2. Discussion of the Background 
         [0005]    Standard methods for computer system security include passwords and other authorization tokens, use of encryption, and permission checking systems. In such standard methods, “security markers” (e.g., checksums, digital signatures, and permission levels) and “security properties” (e.g., an exact match between a data item that is supposed to be immutable and a hidden copy of that data item) can be used to validate the integrity of data and of the security system. However, all methods have flaws and costs. In practice, no security system is 100% effective. 
         [0006]    The “defense in depth” concept of computer system security provides a series of barriers and counter-checks to decrease the probability of a successful compromise and to increase the probability of early detection that can lead to some reaction—such as a system halt, safe-restart, or a counter-measure against the attacker. 
         [0007]    The more complex a security system is, the greater the difficulty in validating the implementation and design and the higher the cost in terms of computing resources and the engineering investment needed to construct and maintain the system. Different applications can realistically support different levels of security costs. For example, software controlling remote power transmission equipment is severely cost and resource constrained. 
         [0008]    What is needed are system and methods for improving security that do not impose unrealistic costs and that can be scaled to different applications. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides systems and methods for applying hard-real-time capabilities in software to software security. For example, the systems and methods of the present invention allow a programmer to attach, a periodic integrity check to an application so that an attack on the application would need to succeed completely within a narrow and unpredictable time window in order to remain undetected. 
         [0010]    Advantageously, the present invention can be adapted to a wide range of applications and computer environments. One area of intended application is in process control systems where a number of relatively small, low-cost, control devices are used to control the operation of a factory or power transmission system or warehouse and these devices may be connected by a network and where there may be real-time requirements on the correct operation of the devices. A second intended application is for computer clusters where component computers can be used to monitor the security of the other members of the cluster and where there are resources for significant cross check of security between software utilizing the present invention and traditional security software. A third intended application is for computers containing sensitive data that can be equipped with a simple test device that can communicate with software utilizing the invention to check the integrity of the application. A fourth intended application is for point-of-sale terminals where customer input of credit card or other sensitive information must be protected against snooping. This list of intended application is not meant to be exhaustive; other applications of the present invention are contemplated. 
         [0011]    In one embodiment, the present invention provides a security system for a computer system. The security system periodically, in hard real-time, checks the integrity of the computer system and/or applications running on the computer system by examining critical data structures maintained by the application code and/or the computer system and by examining the application code itself. The security system includes the following components: (1) a high priority, periodic, real-time security process or thread for (a) checking the integrity of the application code and the application&#39;s critical data structures, (b) checking the execution scheduling of the application, (c) raising an alarm in case of failure, and (d) if necessary, preempting the application, and (2) a process or routine that can atomically update an encrypted checksum and its associated data structure. The security process will raise an alarm if it finds that the application code has been tampered with, if it finds that critical data structures do not have matching checksums, or if it detects that the application is not being scheduled at the required frequency. 
         [0012]    Advantageously, the security system may further include a challenge handler and an external monitor. The external monitor may be an application running on peer computer system that is connected to the computer system by a network or it may be a security device within the computer system. The external monitor is configured to issue challenges to the challenge handler. The external monitor expects to receive from the challenge hander a response to the challenge within a predetermined time limit. If the challenge handier does not respond to the challenge within the predetermined time limit, then this is indication that the computer system may be compromised. 
         [0013]    For example, suppose that the security thread is configured to update an encrypted data item with a sequence number indicating how many cycles have passed without detection of an intruder. The external monitor can be configured to request that these data items be transmitted to the monitor using an encryption key included in the challenge sent to the challenge handler. Additionally, the monitor may require that a response to the challenge be returned within 1 millisecond. Any attacker who is not aware of this real-time challenge/response feature will not be able to produce an undetected compromise of the computer system if the integrity markers and properties have been properly selected. Moreover, an attacker who is aware of the real-time challenge/response feature must gain kernel level privileges to attack the real-time security thread while preserving the timing of the response. 
         [0014]    In another embodiment, the present invention provides a security system for a computer system running a dual-kernel operating system having a real-time kernel and a non-real time or “general-purpose” kernel. The security system includes the following components: (1) a first real-time thread executing under the real-time kernel for checking a configurable set of integrity markers and properties of the general-purpose kernel, (2) a second real-time thread executing under the real-time kernel for checking integrity markers of the real-time kernel and the first real-time thread, (3) one or more challenge handlers executing under the real-time kernel that, provide “challenge/response” functionality when challenges are received from an external monitor, as described above, and (4) a security module executing under the general-purpose kernel that checks the integrity markers and properties of the real-time kernel and the first, and second real-time threads. 
         [0015]    In this embodiment, integrity markers checked by the first real-time thread might include (1) a checksum/digital signature on a data structure containing information about a password file used by the general-purpose kernel (such as an inode in a UNIX type environment) and (2) a checksum/digital signature on a software application running under the general-purpose kernel that is used to encrypt and decrypt passwords stored in the password file. Integrity properties checked by the first thread might include whether key applications (e.g., a web server program or a data base) had been restarted since normal operation began and whether ail of these applications appear to be getting scheduled reasonably. 
         [0016]    The challenge handler, which provide the challenge/response functionality, permit an external monitor to issue a challenge, to which the return, within a specified time limit, of an encrypted data item containing validation information is required. 
         [0017]    For example, suppose that the first two threads each update an encrypted data item with a sequence number indicating how many cycles have passed without detection of an error. An external monitor might request that these data items be transmitted to the monitor using an encryption key passed with the challenge. Additionally, the network peer might require that the response be returned within 1 millisecond. As mentioned above, any attacker who is not aware of this real-time challenge/response feature will not be able to produce an undetected compromise of the computer system if the integrity markers and properties have been properly selected. Additionally, an attacker who is aware of the real-time challenge/response feature must gain kernel level privileges to attack the real-time threads, and, in order to do that, the attacker must compromise the security components of the general purpose kernel and then defeat the three real-time threads before any one of them detects a compromise. 
         [0018]    Keeping the exact periods of these components and the selection of integrity markers and properties secret (for example, by determining them at system boot) further complicates the task of the attacker. An attack that starts from the general purpose kernel or its application is further handicapped by the nature of the separation of the real-time and general purpose kernels in that it cannot be sure of completing an action within any precise time interval. An attack that starts at the real-time kernel side is handicapped by the simpler nature of the real-time kernel, which permits a greater degree of security validation during design and implementation, and by the operation of the security module which checks the integrity of the real-time kernel and the real-time security threads. 
         [0019]    The above and other features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The accompanying drawings, which are incorporated, herein and form part of the specification, illustrate various embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. 
           [0021]      FIG. 1  is a functional block diagram of a computer system having a hard real-time operating system and a security system running under the real-time operating system. 
           [0022]      FIG. 2  is a flow chart illustrating a process according to an embodiment of the invention. 
           [0023]      FIG. 3  illustrates a computer system according to another embodiment of the invention. 
           [0024]      FIG. 4  illustrates another embodiment of a security system of the present invention. 
           [0025]      FIG. 5  is an illustration of a representative computer system. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular systems, computers, devices, components, techniques, computer languages, storage techniques, software products and systems, operating systems, interfaces, hardware, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. Detailed descriptions of well-known systems, computers, devices, components, techniques, computer languages, storage techniques, software products and systems, operating systems, interfaces, and hardware are omitted so as not to obscure the description of the present invention. 
         [0027]      FIG. 1  is a functional block diagram of a real-time computer system  100  having a hard real-time operating system  101 . In some embodiments, hard, real-time operation system  101  is the RTLinuxPro operating system available from FSMLabs of Socorro, New Mex. However, other hard, real-time operating systems can be used. Computer system  100  includes a process or thread  102  (hereafter “security process  102 ”) for detecting whether application code  104  and/or a critical data element (e.g., files, data-structures, etc.)  103  have been compromised by an intruder. 
         [0028]    In some embodiments, security process  102  determines whether application code  104  has been compromised (e.g., modified by an intruder) by (1) determining whether the code  104  has been modified unauthorizedly and/or (2) determining whether the application code  104  is executing according to a pre-determined schedule. There are a number of ways that security process  102  can determine whether code  104  has been modified unauthorizedly. For example, it could determine whether the code  104  matches a checksum associated with the code  104 . Additionally, it could have access to a past copy of code  104  and compare the past copy of the code to the current code to see if there has been any modification since the past copy was created. Other methods exist for determining whether code  104  has been modified unauthorizedly and the invention is not limited to a particular method. 
         [0029]    Similarly, security process  102  may determine whether data element  103  has been compromised by determining whether the data element has been modified by an unauthorized individual or process. There are a number of ways that security process  102  can determine whether code  104  has been modified unauthorizedly. For example, it could determine whether the code  104  matches a checksum associated with the data element  103 . 
         [0030]    In embodiments where a checksum is utilized to determine whether data element  103  has been compromised, the first time data element  103  is created and stored a checksum for the data element is also created and stored (preferably, the stored checksum is encrypted). Additionally, whenever application code  104  needs to make a change to data element  103 , application code  104  itself or a secure update process  105  in communication with application code  104  updates the data element and the checksum associated with data element  103  so that checksum will match the modified data element  103 . Preferably, the update of the data element  103  and its checksum is done atomically (for example, by using well known methods of updating a copy, computing the checksum, and changing a pointer or index atomically). 
         [0031]    Because of the checks performed by security process  102 , in order for an intruder to compromise data element  103  or code  104  without detection, the intruder must have knowledge of the algorithm used to create the checksum. Additionally, in the embodiments where the checksum is encrypted, the intruder must encrypt the checksum using the appropriate algorithm and the appropriate key. 
         [0032]    Advantageously, in some embodiments, security process  102  and/or real-time operating system  101  is/are configured so that security process  102  executes as a high-priority task within operating system  101 . This will enable security process  102  to perform the required security checks at “precise,” regular intervals of time. This feature is illustrated in  FIG. 2 , which is a flow chart illustrating a process  200  performed by security process  102 . 
         [0033]    Process  200  may begin in step  202 , where security process  102  determines whether data element  103  has been tampered with by an intruder (e.g., modified unauthorizedly). If it has, process  202  continues to step  204 , otherwise process  200  continues to step  206 . In step  204 , security process  102  raises an alarm and/or shuts-down application process  108 . In step  206 , security process  102  determines whether code  104  has been tampered with. If it has, process  202  proceeds to step  04 , otherwise process  2   00  continues to step  208 . In step  208 , security process  102  pauses for a pre-determined amount. After step  208 , process  200  proceeds back to step  202 . Because security process  102  is a high-priority task within real-time operating system  101 , it is almost guaranteed that security process  102  will perform steps  202 - 206  at deterministic intervals determined by the pause time in step  208 . 
         [0034]    The hard, real-time feature of sp 102  is important in situations where application code  104  when executed periodically performs an important task and the consequences of it not performing this important task in an intended manner could be dire. Thus, security process  102 , which has the ability to shut down code  104  before it is scheduled to perform the important task when there is an indication, that code  104  has been tampered with, is highly valued. 
         [0035]    As a specific example, assume that code  104  is configured to output a pre-determined signal at time t=1, t=2, etc. In this example, security process  102  can be configured to check the integrity of code  104  just before code  104  is scheduled to output the signal (e.g., sp 102  can be configured to check the code&#39;s integrity at time t=0.9, t=1.9, t=2.9, . . . ). In this manner, if an intruder manages to tamper with code  104  the consequences can be minimized because sp 102  will detect, in the vast majority of cases, the tampering prior to the tampered with code  104  performing its task, and, thus, be able take some form of corrective action before the tampered with code  104  is scheduled to perform its task. 
         [0036]      FIG. 3  illustrates a computer system  300  according to another embodiment of the invention. System  300  is similar to the system shown in  FIG. 1 , with the exception that system  300  further includes a external monitor  302  that can be configured to issue challenges to a challenge handler  304 , which can be configured to respond to the challenges issued by monitor  302 . Although challenge handler is shown as being a separate process from security process  102 , this is not a limitation, as the challenge handler may be implemented as part of security process  102 . 
         [0037]    In one embodiment, the external monitor  302  is configured to present a challenge to the challenge handler  304 . The challenge may be a request for basic security information or it may contain some information about which integrity constraints and integrity properties to check. The challenge may also contain a request that a response to the challenge be transmitted at a certain point in time. In one embodiment, as soon as the challenge handler  304  receives the challenge, the challenge handler validates system security by checking integrity constraints and integrity properties. For example, this may involve examining the function of standard security components such as encryption code and password files and/or examining whether critical applications are being scheduled correctly. In all embodiments, the challenge handler  304  is configured to present a response to the external monitor  302  when the monitor  302  issues a challenge. An example of a response is a properly signed and encrypted sequence number indicating which challenge is being responded to. 
         [0038]    If the external monitor  302  does not receive a correct response from the challenge handler  304  to the appropriate time (e.g., within a specified hard time limit or at the time specified in the challenge), then external monitor  302  may declare that system  300  has been compromised. Thus, to produce an undetected compromise an attacker must not only defeat internal security, but also take over the operation of the challenge handler component  304  before expiration of the hard time limit imposed by the monitor  302 . 
         [0039]    External monitor  302  may be implemented on a computer connected to the system  300  by a deterministic network (where the time for a message to get from the monitor  302  to the secured system  300  and back is known) or it may be a specialised device within the system  300 . In the second case, the monitor  302  may be a peripheral device or even an on-chip security monitor. 
         [0040]    In other embodiments, handler  304  may be configured to transmit a validation certificate to monitor  302  according to a precise schedule, which is preferably an unpredictable schedule. For example, in embodiments where handler  304  is part of security process  102 , handier  304  may be configured to transmit to monitor  302  a validation certificate according to a predetermined schedule (e.g., every 10 milliseconds), regardless of whether monitor  302  has issued a challenge. In this way, monitor  302  will determine that there is a problem with system  100  if it does not receive a validation certificate at the scheduled time. Similarly, in embodiments where handler  304  is a separate process from security process  102 , handler  304  may be configured to validate the integrity of security process  102  on a scheduled basis and then transmit to monitor  302  a validation certificate if handler  304  validates the integrity of security process  102 . 
         [0041]      FIG. 4  illustrates another embodiment of a security system of the present invention. More specifically,  FIG. 4  is a functional block diagram, of a computer system  400  running a dual-kernel operating system  402  having a real-time kernel  404  and a non-real-time or “general-purpose” kernel  406 . The security system includes the following components: (1) a first real-time thread  411  executing under the real-time kernel for checking a configurable set of integrity markers and properties of the general-purpose kernel  406 , (2) a second real-time thread  412  executing under the real-time kernel for checking integrity markers of the real-time kernel  404  and the first real-time thread  411 , (3) at least one challenge handler  304  executing under the real-time kernel that provides “challenge/response” functionality when challenges are received from an external monitor  302 , and (4) a security module  414  executing under the general-purpose kernel that checks integrity markers and properties of the real-time kernel  404  and the first and second real-time threads  411 - 412 . 
         [0042]    In this embodiment, integrity markers checked by the first real-time thread might include (1) a checksum/digital signature on a data element  420  maintaining information about a password file used by the general-purpose kernel (such as an inode in a UNIX type environment) and (2) a checksum/digital signature on a software application  421  running under the general-purpose kernel  406  that is used to encrypt and decrypt passwords stored in the password file. Integrity properties checked by the first thread  411  might include whether key applications (e.g., a web server program or a data base) had been restarted since normal operation began and whether all of these applications appear to be getting scheduled reasonably. 
         [0043]      FIG. 5  is an illustration of a representative computer system  500  that can be used to implement the computer systems described above. Computer system  500  includes a processor or central processing unit  504  capable of executing a conventional operating systems, including dual-kernel and real-time operating systems. Central processing unit  504  communicates with a set of one or more user input/output (I/O) devices  524  over a bus  526  or other communication path. The I/O devices  524  may include a keyboard, mouse, video monitor, printer, etc. The CPU  504  also communicates with a computer readable medium (e.g., conventional volatile or non-volatile data storage devices)  528  (hereafter “storage  528 ”) over the bus  526 . The interaction between CPU  504 , I/O devices  524 , bus  526 , network interface  580 , and storage  528  are well known in the art. 
         [0044]    Storage  528  stores software  538 . Software  538  may include one or more operating system and one or more software, modules  540  for implementing the methods of the present invention. Conventional programming techniques may be used to implement software  538 . Storage  528  can also store any necessary data files. In addition, computer system  500  may be communicatively coupled to the Internet and/or other computer network through a network interface  580  to facilitate data, transfer and operator control. 
         [0045]    The systems, processes, and components set forth in the present description may be implemented using one or more general purpose computers, microprocessors, or the like programmed according to the teachings of the present specification, as will be appreciated by those skilled in the relevant art(s). Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent, to those skilled in the relevant art(s). The present invention thus also includes a computer-based product which may be hosted on a storage medium and include instructions that can be used to program a computer to perform a process in accordance with the present invention. The storage medium can include, but is not limited to, any type of disk including a floppy disk, optical disk, CDROM, magneto-optical disk, ROMs, RAMs, EPROMs, EEPROMs, flash memory, magnetic or optical cards, or any type of media suitable, for storing electronic instructions, either locally or remotely. 
         [0046]    While the processes described herein have been illustrated as a series or sequence of steps, the steps need not necessarily be performed in the order described, unless indicated otherwise. 
         [0047]    The foregoing has described the principles, embodiments, and modes of operation of the present invention. However, the invention, should not foe construed as being limited to the particular embodiments described above, as they should be regarded as being illustrative and not as restrictive. It should be appreciated that variations may be made in those embodiments by those skilled in the art without departing from the scope of the present invention. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described herein. 
         [0048]    Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.