Patent Publication Number: US-10331891-B2

Title: Branch target computation in secure start-up using an integrity datum and an adjustment datum

Description:
BACKGROUND 
     Upon starting, computing devices typically execute a start-up process before reaching a ready state. A start-up process may include one or more stages, wherein each stage may be configured to initialize one or more components of the computing device. Further, to help prevent malicious code from being executed during start-up, one or more of the stages may include verification steps to ensure that the start-up process has not been modified. 
     SUMMARY 
     Embodiments are disclosed that relate to a secure start-up process comprising a computation of a target of a branch instruction. For example, one disclosed embodiment provides, on a computing device, a method of conducting a secure start-up process. The method comprises recognizing the branch instruction, and, in response, calculating an integrity datum of a data segment. The method further comprises obtaining an adjustment datum, and computing a branch target address based on the integrity datum and the adjustment datum. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a process flow depicting an embodiment of a method of conducting a secure start-up process comprising a branch instruction. 
         FIG. 2  schematically shows an example embodiment of a memory schematically illustrating a secure start-up process comprising a branch instruction. 
         FIG. 3  shows a process flow depicting an embodiment of a method of constructing a secure start-lip process comprising a branch instruction. 
         FIG. 4  schematically shows an example embodiment of a computing device. 
     
    
    
     DETAILED DESCRIPTION 
     As mentioned above, computing devices may employ a start-up process comprising a plurality of discrete stages, each configured to perform one or more specific jobs (e.g., initialize one or more components) before transferring control to the following stage. Each stage may include one or more verification steps to ensure that the start-up process has not been modified. Preventing such modifications may be desirable, as said modifications may allow for the execution of unauthorized code and/or access to secure memory locations. 
     As a more specific example, before jumping from one stage to the next stage, one or more verification steps may be performed on the next stage data to ensure its integrity. The jumping and verification may be accomplished, for example, via a branch instruction. The branch instruction may be configured to compare an integrity datum (e.g., checksum, digital signature, hash) computed from all or part of the next stage data with an expected value of said integrity datum stored in memory. If the next stage has been modified, the calculated integrity datum will not match the expected value, and the start-up process will fail. If the next stage has not been modified, the branch instruction may be configured to jump to a static address in the next stage, thus transferring control to the next stage (e.g., allowing execution of instructions at that next stage). 
     However, such processes may not provide sufficient security during start-up. For example, a malicious user may be able to modify the data segment (e.g., insert unauthorized code) from which the integrity datum is computed. In this instance, the integrity datum of the modified segment can be computed and stored into the expected value memory location prior to run time. Thus, upon reaching the branch instruction at run-time, the comparison between the computed (modified) integrity datum and the expected (modified) value will be successful, and the modified code may then be allowed to execute. 
     Accordingly, embodiments are disclosed herein that relate to a secure start-up process. As described in more detail below the disclosed embodiments may allow for constructing and conducting a secure start-up process comprising a branch instruction in which a target address of the branch instruction is computed from an integrity datum during execution of the branch operation. It will be understood that the term “start-up process” as used herein refers any instruction, or combination of instructions, executed by a computing device upon initialization. 
       FIG. 1  shows a process flow depicting an embodiment method  100  for conducting a secure start-up process comprising a branch instruction. At  102 , method  100  comprises recognizing a target branch instruction. At  104 , method  100  comprises calculating an integrity datum of a data segment, For example, in the case of a multi-stage staff-up process, the data segment may comprise all or part of the next stage. Said integrity datum may be substantially unique to the data segment from which it is computed such that modification of a single bit in the data segment may change the computed integrity datum. Example integrity data include, but are not limited to, checksums, digital signatures, hash functions, and/or a combination thereof. 
     At  106 , method  100  comprises evaluating a test condition of the branch instruction. The test condition may comprise a comparison operator (e.g., less than, greater than equal to) and one or more comparison operands. For example, evaluating the test condition may comprise, at  107 , comparing the computed integrity datum to an expected value of the integrity datum. In other embodiments, such as when the branch instruction is a jump instruction, the branch instruction may not include a test condition. 
     At  108 , method  100  comprises evaluating if the test condition is satisfied. If the test condition is not satisfied, method  100  ends without jumping, at  110 . On the other hand, if the test condition is satisfied, method  100  continues to  112 . At  112 , method  100  comprises obtaining an adjustment datum for use in computing the branch target address. It will be understood that the adjustment datum may be stored in any suitable location, including internally or externally to the computing system. 
     At  114 , method  100  comprises computing the branch target address based on the integrity datum and the adjustment datum. Computing may involve one or more operations acting on the integrity datum and/or the adjustment datum. Operations may include, but are not limited to, bit-wise operations (e.g. AND OR, NAND, NOR, XOR, logical shift), mathematical operations (e.g., add, subtract, multiply, divide), and/or a combination thereof. 
     At  115 , if the computed branch target address is correct, method  100  comprises, at  116 , jumping to the correct branch target address. Modification to the data segment from which the integrity datum is computed, and/or modification to the stored adjustment datum, may result in the computation of an incorrect branch target address. Accordingly, if the computed branch target address is incorrect, method  100  comprises, at  118 , jumping to the incorrect computed branch target address. Such a jump may result in an unknown or unstable computing device state. In other embodiments, such a jump may result in the computing device going into a “locked” mode. In yet other embodiments, upon determining an incorrect computed branch target address, method  100  may comprise not jumping. For example, one or more mechanisms may be configured to detect the incorrect computed branch target address and to halt further code execution (e.g., resulting in a “locked” mode). It will be understood that these scenarios are presented for the purpose of example, and are not intended to be limiting in any manner. 
     As mentioned above, a secure start-up process may involve one or more discrete stages, and each stage may include a branch instruction by which the integrity of the following stage is confirmed before proceeding to said following stage.  FIG. 2  schematically shows an example embodiment of a memory  200  for conducting a secure start-up process comprising a branch instruction. Memory  200  comprises a plurality of stages, such as stage  1   202  beginning at address ADDR S 1  and stage  2   204  beginning at ADDR S 2 , up to stage N beginning at ADDR SN. 
     First instruction  206 , located at ADDR T 0 , may be accessed via an external instruction  208 . It will be understood that the term “external instruction” refers to an instruction located externally to memory  200 . For example, first instruction  206  may be located at a reset vector that is automatically accessed upon reset of a computing device comprising memory  200 , 
     Stage  1   202  further comprises a plurality of instructions, such as branch instruction  210  located at ADDR B 1 . Upon the test condition of branch instruction  210  being satisfied, branch instruction  210  may be configured to jump to a computed target address. For example, if memory  200  is unmodified, branch instruction  210  may be configured to jump  212  to first instruction  214  of stage  2   204  located at ADDR T 1 , and the start-up process will execute the instructions beginning at ADDR T 1 . 
     However, if memory  200  is modified, branch instruction  210  may be configured to jump  216  to a different, incorrect memory location. For example, as illustrated, branch instruction  210  may jump  216  to a location in stage  2   204  after branch instruction  218 . In other instances, the incorrect memory location may be located external to memory  200  and/or may be an invalid address. Said incorrect jump may result in an undefined, unstable, or “locked” system state. 
     Although first instructions  206  and  214  are illustrated as being offset from the starting address of their respective stage (e.g., stage  1  begins at ADDR S 1  while first instruction  206  is located at ADDR T 0 ), it will be understood that such an offset may not exist in other examples. Furthermore, although branch instructions  210  and  218  are illustrated as the last instruction in their respective stages, it will be understood that in some instances, additional instructions may follow. For example, said additional instructions may be included to “lock” the system comprising memory  200  in the event the branch instruction test condition is not satisfied. 
     A secure start-up process such as that illustrated in  FIG. 1  may be constructed in any suitable manner.  FIG. 3  shows a process flow depicting an example embodiment of a method  300  for constructing a secure start-up process. At  302 , method  300  comprises recognizing a branch target address. The branch target address represents the address that will be computed and subsequently accessed upon satisfaction of a branch instruction test condition. For example, the branch target address may point to a first instruction of a next stage of the secure start-up process. It will be understood that the branch target address may he any static address known during all or part of the construction process, either directly (e.g., pre-defined memory address) or indirectly (e.g., a label defining a location in source code). 
     At  304 , method  300  comprises calculating an integrity datum of a data segment. As mentioned above, example integrity data may include, but are not limited to, checksums, digital signatures, hash functions, or a combination thereof. Furthermore, the integrity datum may be substantially unique to the data segment such that modification of a single bit in the data segment may effect change in the integrity datum. 
     At  306 , method  300  comprises calculating an adjustment datum based on the integrity datum and the branch target address. Said calculation may utilize one or more reversible algorithms such that known values (e.g., integrity datum and branch target address) may be used to compute the unknown value (e.g., adjustment datum). Said algorithms may include one or more bit-wise operations, one or more mathematical operations, and/or a combination thereof. 
     At  308 , method  300  comprises defining a branch instruction. The branch instruction may be configured to alter the control flow based on a test condition comparison  310 . Comparison  310  may comprise one or more comparison values and a comparison operator (e.g., less than, greater than, equal to) operating on the one or more comparison operands. In sonic embodiments, such as when the branch instruction is a jump instruction, the branch instruction may not include comparison  310 . The branch instruction may further include a branch target address computation  312 . Computation  312  may utilize, for example, the reverse of the algorithm used at  306  to compute the adjustment datum. 
     At  314 , method  300  comprises producing a program image. It will be understood that “producing” may comprise one or more mechanisms resulting in one or more instructions, defined in a target ISA or HDE, for conducting a secure start-up process (e.g., method  100  of  FIG. 1 ). Such mechanisms may include, but are not limited to, compiling, translating, optimizing, and combinations thereof. 
     The above described methods and processes may be tied to a computing system including one or more computers, in particular, the methods and processes described herein may be implemented as a computer application, computer service, computer API, computer library, and/or other computer program product. 
       FIG. 4  schematically shows a nonlimiting computing system  400  that may perform one or more of the above described methods and processes. Computing system  400  is shown in simplified form, it is to be understood that virtually any computer architecture may be used without departing from the scope of this disclosure. In different embodiments, computing system  400  may take the form of a mainframe computer, server computer, desktop computer, laptop computer, tablet computer, home entertainment computer, network computing device, mobile computing device, mobile communication device, gaming device, etc. 
     Computing system  400  includes a logic subsystem  402  and a data-holding subsystem  404 . Computing system  400  may optionally include a display subsystem  406 , communication subsystem  408 , and/or other components not shown in  FIG. 4 . Computing system  400  may also optionally include user input devices such as keyboards, mice, game controllers, cameras, microphones, and/or touch screens, for example. 
     Logic subsystem  402  may include one or more physical devices configured to execute one or more instructions. For example, the logic subsystem may be configured to execute one or more instructions that are part of one or more applications, services, programs, routines, libraries, objects, components, data structures, or other logical constructs. Such instructions may be implemented to perform a task, implement a data type, transform the state of one or more devices, or otherwise arrive at a desired result. 
     The logic subsystem may include one or more processors that are configured to execute software instructions. Additionally or alternatively, the logic subsystem may include one or more hardware or firmware logic machines configured to execute hardware or firmware instructions. Processors of the logic subsystem may be single core or multicore, and the programs executed thereon may be configured for parallel or distributed processing. The logic subsystem may optionally include individual components that are distributed throughout two or more devices, which may be remotely located and/or configured for coordinated processing. One or more aspects of the logic subsystem may be virtualized and executed by remotely accessible networked computing devices configured in a cloud computing configuration. 
     Data-holding subsystem  404  may include one or more physical, non-transitory, devices configured to hold data and/or instructions executable by the logic subsystem to implement the herein described methods and processes. When such methods and processes are implemented, the state of data-holding subsystem  404  may be transformed (e.g., to hold different data). 
     Data-holding subsystem  404  may include removable media and/or built-in devices. Data-holding subsystem  404  may include optical memory devices (e.g., CD, DVD, HD-DVD, Blu-Ray Disc, etc.), semiconductor memory devices (e.g., RAM, EPROM, EEPROM, etc.) and/or magnetic memory devices (e.g., hard disk drive, floppy disk drive, tape drive, MRAM, etc.), among others. Data-holding subsystem  404  may include devices with one or more of the following characteristics: volatile, nonvolatile, dynamic, static, read/write, read-only, random access, sequential access, location addressable, file addressable, and content addressable. In some embodiments, logic subsystem  402  and data-holding subsystem  404  may be integrated into one or more common devices, such as an application specific integrated circuit or a system on a chip. 
       FIG. 4  also shows an aspect of the data-holding subsystem in the form of removable computer-readable storage media  410 , which may be used to store and/or transfer data and/or instructions executable to implement the herein described methods and processes. Removable computer-readable storage media  410  may take the form of CDs, DVDs, HD-DVDs, Blu-Ray Discs, EEPROMs, and/or floppy disks, among others. 
     It is to be appreciated that data-holding subsystem  404  includes one or more physical, non-transitory devices. In contrast, in some embodiments aspects of the instructions described herein may he propagated in a transitory fashion by a pure signal (e.g., an electromagnetic signal, an optical signal, etc.) that is not held by a physical device for at least a finite duration. Furthermore, data and/or other forms of information pertaining to the present disclosure may be propagated by a pure signal. 
     When included, display subsystem  406  may be used to present a visual representation of data held by data-holding subsystem  404 . As the herein described methods and processes change the data held by the data-holding subsystem, and thus transform the state of the data-holding subsystem, the state of display subsystem  406  may likewise be transformed to visually represent changes in the underlying data. Display subsystem  406  may include one or more display devices utilizing virtually any type of technology. Such display devices may be combined with logic subsystem  402  and/or data-holding subsystem  404  in a shared enclosure, or such display devices may be peripheral display devices. 
     When included, communication subsystem  408  may be configured to communicatively couple computing system  408  with one or more other computing devices. Communication subsystem  408  may include wired and/or wireless communication devices compatible with one or more different communication protocols. As nonlimiting examples, the communication subsystem may be configured for communication via a wireless telephone network, a wireless local area network, a wired local area network, a wireless wide area network, a wired wide area network, etc. In some embodiments, the communication subsystem may allow computing system  400  to send and/or receive messages to and/or from other devices via a network such as the Internet. 
     It is to be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated may be performed in the sequence illustrated, in other sequences, in parallel, or in some cases omitted. Likewise, the order of the above-described processes may be changed. 
     The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.