Patent Publication Number: US-7590870-B2

Title: Physical presence determination in a trusted platform

Description:
BACKGROUND OF THE INVENTION 
     This invention pertains to computer systems and other information handling systems and, more particularly, to a computer system which is built on a trusted platform such as the TCPA industry standard platform. 
     There is a need in the computer industry to raise the level of confidence with which users run applications and perform network transactions. This is particularly true for electronic commerce transactions where users key in credit card and other sensitive information. Several solutions have emerged in the industry. One solution, the Smart Card, has emerged as a standard for raising the level of confidence by providing hardware which establishes a trusted user. In the Smart Card solution, the computer system is not the trusted entity. Rather, it is the smart card hardware which is the trusted entity and which is associated with a particular user. Another solution, the Trusted Computing Platform, has emerged as a standard for raising the level of confidence by providing hardware which establishes a trusted platform. With the trusted platform the user is not the trusted entity. Rather, it is the platform which is trusted. 
     Modern computer systems provide remote power-on capability. For example, the computer can be powered on when the RING signal from an incoming FAX is detected at the computer&#39;s modem. The computer can then power-on, boot, and receive the incoming fax. Likewise, the computer can be powered on when local area network activity is detected at its LAN card; it can then boot and respond to any local area network requests. Computers with this capability, however, are particularly at risk while unattended because they are vulnerable to attacks even if they are powered off. 
     SUMMARY OF THE INVENTION 
     Briefly, the invention is a method of providing a trusted platform in a computer system. A determination is made as to whether power was applied to the computer system by the activation of a power-on switch. In making the determination, a power-on status register is read which indicates the occurrence of such activation. Depending on the outcome of the determination, the operation of a trusted platform module included in the computer system is affected. 
     In another embodiment, a program product is provided on a computer readable medium having program code stored therein for providing a trusted platform in a computer system. The code is effective when executing to determine whether power was applied to the computer system by the activation of a power-on switch. In making the determination, a power-on status register is read which indicates the occurrence of such activation. Depending on the outcome of the determination, the operation of a trusted platform module included in the computer system is affected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some of the purposes of the invention having been stated, others will appear as the description proceeds, when taken in connection with the accompanying drawings, in which: 
         FIG. 1  illustrates a computer system configured in accordance with an embodiment of the present invention. 
         FIG. 2  is a detailed block diagram of the security components of an embodiment of the present invention. 
         FIG. 3  is a perspective view of the motherboard which supports and provides electrical interconnection for the security components of one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     While the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which a preferred embodiment of the present invention is shown, it is to be understood at the outset of the description which follows that persons of skill in the appropriate arts may modify the invention here described while still achieving the favorable results of this invention. Accordingly, the description which follows is to be understood as being a broad, teaching disclosure directed to persons of skill in the appropriate arts, and not as limiting upon the present invention. 
     Referring now more particularly to the accompanying drawings,  FIG. 1  illustrates an exemplary computer system  113  configured in accordance with the present invention. System  113  has a central processing unit (CPU)  110 , which is coupled to various other components by system bus  112 . The system bus  112  may be a straight bus, or it can be a hierarchal system of buses. A flash nonvolatile random access memory (“NVRAM”)  116  is coupled to the system bus  112  and includes a basic input/output system (“BIOS”) that controls certain basic functions of the computer system  113 . The function performed by NVRAM  116  of storing the basic input output system is the same as that traditionally performed by a ROM device. The flash device of the present embodiment has the advantage of being field upgradable. Random access memory (“RAM”)  114 , I/O adapter  118 , and communications adapter  134  are also coupled to the system bus  112 . I/O adapter  118  may be a small computer system interface (“SCSI”) adapter that communicates with a disk storage device  120 . Communications adapter  134  interconnects bus  112  with an outside network  160  (e.g., the Internet) enabling the computer system to communicate with other such systems. Input/Output devices are also connected to system bus  112  via user interface adapter  122  and display adapter  136 . Keyboard  124  and mouse  126  are all interconnected to bus  112  via user interface adapter  122 . Display monitor  138  is connected to system bus  112  by display adapter  136 . In this manner, a user is capable of inputting to the system  113  through the keyboard  124  or mouse  126  and receiving output from the system via display  138 . 
     Implementations of the invention include implementations as a computer system programmed to execute the method or methods described herein, and as a computer program product. According to the computer system implementation, sets of instructions or program code for executing the method or methods may be resident in the NVRAM  116 . Until required by the computer system, the program code may be stored as a computer program product in another computer memory, for example, in disk drive  120  (which may include a removable memory such as an optical disk or floppy disk for eventual use in the disk drive  120 ). In one embodiment, regardless of its source, the program code executes as the initial code which runs subsequent to any reset event in the computer system. Further, the computer program product can also be stored at another computer and transmitted when desired to the user&#39;s workstation by a network or by an external network  160 . One skilled in the art would appreciate that the physical storage of the program code physically changes the medium upon which it is stored so that the medium carries computer readable information. The change may be electrical, magnetic, chemical, biological, or some other physical change. While it is convenient to describe the invention in terms of instructions, symbols, characters, or the like, the reader should remember that all of these and similar terms should be associated with the appropriate physical elements. 
     Computer system  113  is implemented to provide a user with a trusted platform upon which certain trusted operations can be performed. The system is constructed in accordance to the Trusted Computing Platform Alliance (TCPA) specification entitled TCPA Main Specification Version 1.1b, which is hereby incorporated by reference herein. In the preferred embodiment, computer system  113  is implemented as a PC architecture system and is further adherent to the TCPA PC Specific Implementation Specification Version 1.00 which is also hereby incorporated herein by reference. Trusted platform module (TPM)  111  is a cryptographic processor which provides computer system  113  with hardware assisted cryptographic capabilities. TPM  111  can be a fully integrated security module designed to be integrated into systems. Any type of cryptographic processor can be utilized. However, in the preferred embodiment, TPM  111  implements version 1.1b of the TCPA specification for Trusted Platform Modules (TPM). The TPM  111  includes an asymmetric encryption co-processor which amongst other things performs key generation, random number generation, digital signature key generation, and hash generation functions. The TPM  111  is capable of computing a RSA signature using CRT and has an Internal EEPROM Storage for storing a predetermined number of RSA Keys. Also included are a set of 20-byte platform configuration registers (PCRs) for establishing the root of trust for the platform. One example of such a TPM device is an Atmel™ part number AT97SC320. 
     In addition to storing the BIOS code, NVRAM  116  also stores code which is used to perform power on self test (POST) routines. A portion of this POST code is responsible for establishing the root of trust for the platform. Trust is established in the platform by having the NVRAM  116  and TPM  111  physically and/or logically coupled in the computer system to form a trusted building block. 
     As will be explained in greater detail hereinafter, NVRAM  116  and TPM  111  are assembled on a circuit board, also known as a motherboard, in such a way that trusted code stored in the NVRAM  116  gains control of the computer system upon a system reset. This trusted code is known as the Core Root of Trust for Measurement (CRTM). In order to verify that the POST code being run is the code shipped by manufacturer, each section—before it is executed—is first sized by the CRTM itself. Each section of code is checked for length and check sum, and a hash is created which represents the code being run. Each hash is then stored in one of the 20 byte PCRs within the TPM  111 . Verification of these hash values can then be performed by comparing the hash values against published hash values which are published by the manufacturer for verification purposes. 
     Since it is possible to remotely power-on the computer system, for example by wake on LAN or wake on RING, it is possible to remotely power-on the system and attack it. However, such an attack can be prevented by providing a physical presence detect feature which meets the requirements of the TCPA specification. In order to maintain a secure system, the CRTM code checks for the physical presence of a person upon power-on before certain critical operations can be performed at the computer system. As will be described in further detail as the description of the present embodiment ensues, rather than implementing a physical jumper or switch as directed by the TCPA specification, the system of the present embodiment checks for physical presence by examining the core chipset registers for indication of how the computer was powered on. Based upon this examination, the computer system of the present embodiment infers the physical presence of a user. When it is inferred that there is no physical presence, the CRTM code interfaces with the TPM  111  in such a way that the TPM  111  from that point on, from that boot on, will refuse certain critical types of TPM transactions. On the other hand, when it is inferred that there is physical presence, certain critical types of TPM transactions are allowed. Avoiding a physical jumper or switch provides the present embodiment with a lower cost of manufacture. Moreover, the lack of a physical jumper or switch allows the components to be made without electrical or mechanical uniqueness. This lack of uniqueness, in turn, allows a greater ability to leverage the components of the present system as components in other systems sharing the same electrical and mechanical design, thereby further lowering overall costs for the manufacturer. 
     Referring to  FIG. 3 , there is illustrated a perspective view of a circuit board  301  or motherboard configured in accordance with an embodiment of the present invention. Circuit board  301  provides mechanical support and electrical interconnection between the TPM  111 , the NVRAM  116 , a core Southbridge chipset  202 , and the CPU or processor  110 . This circuit arrangement provides the basis for a trusted system platform which presents and receives information to and from the user. The platform itself is composed of the circuit arrangement shown in  FIG. 3  and primary peripheral devices (not shown) attached to the circuit board  301 . Primary peripheral devices are considered to be those devices which directly attach to and directly interact with the CPU  110 . Examples are PCI cards, LPC components, USB Host controllers and root hubs, attached serial and parallel ports, etc. However, USB and IEEE 1394 devices are not considered primary peripheral devices. 
     Referring to  FIG. 2  and  FIG. 3 , The processor  110  executes the CRTM code stored in the NVRAM  116 . As stated previously, this CRTM code interacts with the TPM  111  in such way as to provide a trusted platform. The trusted CRTM code stored in NVRAM  116  and the TPM  111  are the basic components of the trusted platform and are the only trusted components of the platform. When the proper binding is established between the CRTM code and the TPM  111 , a basis for trust is established in the platform. The binding of NVRAM  116  and TPM  111  can be physical or logical and is considered to be outside the scope of the present invention. Details concerning the binding of the CRTM to the TPM  111  are well known in the trusted computing art and are omitted so as to not obfuscate the present disclosure in unnecessary detail. In the present embodiment, the CRTM is contained within a portion of NVRAM  116 . In another embodiment, however, the CRTM code consumes the entirety of NVRAM  116 . Since the CRTM and the TPM  111  are the only trusted components of the platform and since indication of physical presence requires a trusted mechanism to be activated by the platform user, the indication of physical presence is contained within the CRTM code of NVRAM  116  and the TPM  111 . 
     The bus  112  of the preferred embodiment is a hierarchical bus having a north bus bridge (hereinafter “Northbridge,” not shown) and a south bus bridge  202  (hereinafter “Southbridge”). The Northbridge encompasses buses which are operationally closer to the processor, such as memory and caching buses. The Southbridge  202  encompasses buses which are closer to system I/O, such as X-Bus, IDE, LPC, and other buses. Note, however, that the bus  112  of the preferred embodiment need not be implemented as a hierarchical bus. Instead, a flat bus as schematically shown in  FIG. 1  can be implemented physically. Alternatively, a hierarchy involving only a single bridge chip may be used. The Southbridge  202  provides an LPC bus which, amongst other components, couples NVRAM  116 . The LPC bus is a low pin count bus based on the IBM PCAT bus and forms part of the hierarchical bus  112 . The Southbridge  202  also couples the TPM  111 . Southbridge  202  also includes a number of low-level system controllers such as an Advanced Configuration and Power Interface (ACPI) compliant power controller  204 . ACPI is an industry-standard interface for OS-directed configuration and power management. The ACPI power controller  204  within the Southbridge  202  provides a hardware interface between the operating system and the devices whose power is being controlled. Many of the functions provided by power controller  204  are accessed via registers as either enable or status registers. One such register is status register  206 . Status register  206  contains a series of bits each of which gives status as to the power configuration of the machine and as to its current and initial status. In the preferred embodiment, one of the bits, the power switch bit, is reserved to indicate whether power was last applied to the system by the activation of the system power switch housed on the front face of the system. The system power switch can be connected directly to the circuit board  301  or indirectly through the power supply. The system power switch can also be mounted on the power supply directly although mounting the power switch to the front face is preferred. When the last application of power was applied to the machine by the system power switch, the power switch bit is asserted. When the last application of power was applied to the machine by other than the system power switch, the power switch bit is de-asserted. Thus, if the system is remotely powered on, via wake on LAN or wake on RING events, for example, the power switch bit is de-asserted. In the preferred embodiment the power switch bit is implemented such that it is settable only in hardware and not by software. This is done in order to prevent spoofing by trojan or virus software attempting to breach the security of the platform. Allowing the power switch bit to be reset by software is considered an acceptable design choice since the de-assertion of the power switch bit after the operating system loads is ignored, and even if not ignored software de-assertion of the power switch bit would otherwise serve to to increase the level of security in the system. In the preferred embodiment, the power switch bit indicates whether the application of power by the system power switch was initiated at the time of the last power-on event. In an alternative embodiment, the power switch bit can be designed to indicate whether the system power switch has been depressed. In the latter case, the software which makes the determination must run sooner or otherwise take other measures to make a trusted determination. 
     Preferably, the processor  110  executes the CRTM code stored in NVRAM  116  as the initial code that executes after a system reset. The system enters the reset state from either a hardware or software reset event. The hardware reset state is entered upon an application of power in the computer system or it can be entered via a dedicated system reset switch. In the preferred embodiment, the CRTM code is given initial control of the computer system in order to establish trust in the platform. Once the CRTM code executes, the CRTM interacts with the TPM  111  in order to establish the root of trust for the platform. As described previously herein, the CRTM code verifies itself through the use of the hashing functions and PCR registers of the TPM  111 . In addition and amongst other things, the CRTM code reads the status register  206  for the current state of the power switch bit. The CRTM code then makes an inference as to the presence or absence of a user at the machine and based on this inference issues a command to the TPM  111  to either limit or allow certain critical TPM functions. 
     When the power switch bit of status register  206  is found to be in an asserted state, an inference is made that a user is present at the machine. In this case, the issued command allows a predetermined set of functions to execute at the TPM  111 . In the preferred embodiment, the issued command is a command which sets a physical presence flag in TPM  111 . The TPM  111  is then implemented to only allow certain functions when physical presence is indicated as per the physical presence flag. An example of such a command is a command which resets the TPM  111  to its factory default state. Such a command can only be accepted and executed by TPM  111  if physical presence has been determined. 
     Conversely, when the power switch bit of status register  206  is found to be in an de-asserted state, an inference is made that a user is not present at the machine. In this case, the issued command blocks a predetermined set of functions to execute at the TPM  111 . In the preferred embodiment, the issued command is a command which resets the physical presence flag in TPM  111  following the determination indicating lack of physical presence. The TPM  111  is then implemented to limit certain functions when physical presence is not indicated as per the physical presence flag. Given this set of circumstances, the exemplary command which attempts to reset the TPM  111  to its factory default state would be blocked by TPM  111  since no physical presence is indicated. 
     For the most part, details concerning which commands are limited and which commands are allowed by the TPM  111  have been omitted in as much as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art. Otherwise, a reader of arbitrary skill who is interested in details concerning the commands is otherwise directed toward the TCPA specifications incorporated by reference which present such details. 
     In an alternative embodiment, in addition to setting or resetting the physical presence flag in TPM  111 , an additional command is issued which sets a physical presence lock flag in TPM  111 . TPM  111  is then implemented such that the value of the physical presence flag is not changeable once the physical presence lock flag has been set. The locking of the physical presence flag has a lifetime which extends to the next platform reset. 
     Regardless of whether the physical presence flag is locked by the mechanism of the lock flag or by some other binding mechanism, once all of the CRTM metrics have been documented in the hash tables of the TPM  111  PCR&#39;s and once physical presence or lack thereof has been established at the TPM  111 , the platform is thereafter considered to be trusted and secured to the extent determined. After platform trust has been established, control can then be passed to non-secure code. 
     In one embodiment of the present invention, control is then passed to nonsecure POST code residing within NVRAM  116 . In this embodiment, the code which is given control after the platform is secured is code which accesses any computer system I/O device such as a keyboard device, a video device, or a pointing device. 
     In another embodiment, the CRTM code is considered to be the entirety of code stored within NVRAM  116 . In this embodiment, control is then passed to nonsecure code stored in other than the NVRAM  116 . Generally, this would be code which loads the operating system. In an IBM PC compatible computer system, the loading of the operating system is typically instantiated by the execution of a software INT 19 executed as the last instruction stored within the NVRAM  116 . However, one of ordinary skill in the art is able to use other methods to load the operating system and any method used would not depart from the spirit and scope of the present invention. 
     In the drawings and specifications there has been set forth a preferred embodiment of the invention and, although specific terms are used, the description thus given uses terminology in a generic and descriptive sense only and not for purposes of limitation.