Patent Publication Number: US-2022214882-A1

Title: Stack traces using shadow stack

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of, and claims priority to, U.S. patent application Ser. No. 16/417,493, filed on May 20, 2019, and entitled “STACK TRACES USING SHADOW STACK”, the specification of which is hereby incorporated by reference in its entirety for all that it teaches and suggests. 
    
    
     BACKGROUND 
     Most programming languages support modular construction and execution of software. Typically, a program has modules or units of executable code such as methods, subroutines, functions, etc. (collectively referred to herein as “functions”). A compiled program usually includes many such functions, each having a name or handle by which it can be referenced. A function will have executable instructions, and often will have parameters for exchanging data and locally declared variables for internal computation. A program may consist of many such functions, and in many cases functions call other functions, for instance a( ) called b( ) which then called c( ). An executing program may have long chains of such function invocations. 
     To manage the invocation and execution of functions, most compilers compile programs to use some form of call stack when executing. Each time a function is called (e.g., a( ) called b( ), a corresponding new stack frame is pushed to the call stack (in this case, a frame for b( ) is pushed on top of a frame for a( )). The new frame contains a return address, i.e., the point in the program where the corresponding function was invoked (in the example, a location in a( ) where b( ) was called). Each time a function exits, the function&#39;s frame is popped from the call stack and the control flow passes to whichever return address is in the frame. A frame can also be used to store parameters passed to/from its function and local data declared in its function. 
     At any given time, the order of frames on the call stack reflects the current order of chained function calls, where each frame represents a function that has not yet exited. It is often desirable to know the current chain of function invocations when a program is executing. This information, sometimes called a stack trace or backtrace, can be useful for, among other things, debugging and performance profiling. As discussed next, there are different ways that a call stack can be used to construct a backtrace, but all prior approaches for backtracing incur computing overhead to the extent that in some scenarios backtracing is not practical. For instance, in many production systems and user devices, even a small increase in incurred latency can make backtracing prohibitive. 
     One prior approach for backtracing involves saving a caller&#39;s frame address in the callee&#39;s stack frame, and using a dedicated register (e.g., the x86 EBP register) to point to the current callee&#39;s frame. The dedicated register is used to get the current stack frame, from which the caller&#39;s stack frame&#39;s address is obtained, from which the caller&#39;s caller&#39;s stack frame address is obtained. In each frame, the return address of the corresponding function is stored at a fixed offset. Therefore, to get the stack backtrace, the return address of each frame is found and dumped. 
     Another approach involves not saving the caller&#39;s stack frame address in the callee&#39;s stack frame. Instead, the stack frame size is computed by reverse executing function prologues. A stack backtrace starts by computing the stack size, which is then subtracted from the stack pointer to get the return address. Using the return address, the caller&#39;s stack frame size can be computed, which gets the caller&#39;s return address, and so forth. Compared to the first approach, this approach is more efficient when not backtracing (lower support overhead), but is less efficient when carrying out a stack backtrace. 
     Yet another approach is to use special hardware support such as Intel&#39;s Last Branch Record (LBR). If the LBR is configured to call-stack-mode, it will record active calls. Such active call information can be dumped as a stack backtrace. 
     While these prior approaches may yield the desired output, the overhead they incur is not negligible. The first approach requires extra instructions to save the caller&#39;s stack frame address in the callee&#39;s stack frame, and the caller&#39;s frame address must be computed when “walking the stack”. The second approach requires intensive memory lookups to find the frame size information. The third approach puts stress on thread context switches because many extra registers need to be saved and restored for each context switch. 
     Discussed below are techniques for using a shadow stack to facilitate backtracing with negligible overhead. 
     SUMMARY 
     The following summary is included only to introduce some concepts discussed in the Detailed Description below. This summary is not comprehensive and is not intended to delineate the scope of the claimed subject matter, which is set forth by the claims presented at the end. 
     A program is executed using a call stack and shadow stack. The call stack includes frames having respective return addresses. The frames may also store variables and/or parameters. The shadow stack stores duplicates of the return addresses in the call stack. The call stack and the shadow stack are maintained by, (i) each time a function is called, adding a corresponding stack frame to the call stack and adding a corresponding return address to the shadow stack, and (ii) each time a function is exited, removing a corresponding frame from the call stack and removing a corresponding return address from the shadow stack. A backtrace of the program&#39;s current call chain is generated by accessing the return addresses in the shadow stack. The outputted backtrace includes the return addresses from the shadow stack and/or information about the traced functions that is derived from the shadow stack&#39;s return addresses. 
     Many of the attendant features will be explained below with reference to the following detailed description considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present description will be better understood from the following detailed description read in light of the accompanying drawings, wherein like reference numerals are used to designate like parts in the accompanying description. 
         FIG. 1  shows an executing program, and a corresponding call stack and shadow stack. 
         FIG. 2  shows an example of source code compiled by a compiler to produce an executable program. 
         FIG. 3  shows details of the call stack and the shadow stack. 
         FIG. 4  shows how the call stack and shadow stack are maintained. 
         FIG. 5  shows a software-based shadow stack implementation. 
         FIG. 6  shows a hardware-based shadow stack implementation. 
         FIG. 7  shows how the shadow stack can be used to satisfy a backtrace request. 
         FIG. 8  shows a process for using the shadow stack. 
         FIG. 9  shows shadow stack content being used in combination with other information to generate a backtrace. 
         FIG. 10  shows various contexts where backtrace logic can be implemented. 
         FIG. 11  shows a backtrace output example. 
         FIG. 12  shows details of a computing device. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments discussed below relate to using a shadow stack to enable efficient stack backtracing. Discussion will begin with an overview of call stacks and shadow stacks. Various ways of implementing a shadow stack are then described, followed by details of methods for using a shadow stack to facilitate stack backtraces. 
       FIG. 1  shows an executing program  100  and a corresponding call stack  102  and shadow stack  104 . The program  100  may be compiled machine instructions, bytecode, source code being executed by an interpreter, and so forth. The program  100  includes various functions and calls to the functions. The call stack  102  may be implemented in any of many known ways. As mentioned in the Background, frames are pushed and popped in correspondence with function invocations and exits/returns, respectively. 
     The shadow stack  104  may also be implemented in known ways, using software, hardware, or both. As noted in the Background, in nearly all call stack implementations, when a function exits, its stack frame is popped off the call stack and execution control passes to whichever code location the frame&#39;s return address points to, usually, a location in the function that called the exiting function. The call stack&#39;s role in flow control—of providing return addresses—is a well-known security weakness. Using various techniques, the call stack can be altered or corrupted to change a return address, thereby directing execution to the location of malicious code. 
     Shadow stacks are one solution to the return address vulnerability. Briefly, a shadow stack is a stack that is separate from the call stack and is synchronized with the call stack so that each time a frame is added to the call stack the frame&#39;s return address is added to the shadow stack. Each time a frame is popped from the top of the call stack the return address at the top of the shadow stack is correspondingly popped off the shadow stack. If the popped return address does not match the return address in the popped frame, then an error has occurred and a remedial action can be taken. 
       FIG. 2  shows an example of source code  110  compiled by a compiler  112  to produce an executable program  114 . The source code  110  contains various functions and function calls. The compiler  112  translates the source code into the executable program  114 , which might be in the form of machine code, bytecode, intermediate code, object code, etc. The compiled executable program  114  has units of code/instructions that respectively correspond to the functions in the source code. Except perhaps in some managed runtime environments, the compiler also adds to the executable program  114  instructions for implementing the call stack. As discussed in detail below, in embodiments that use a software-based shadow stack, the compiler  112  also adds instrumentation code for implementing the shadow stack. 
       FIG. 3  shows details of the call stack  102  and the shadow stack  104 . The call stack  102  consists of frames  120 , one for each still-pending function invocation. Each frame  120  includes a return address  122 , as well as memory for local variables  124  and parameters  126 , as the case may be. The example of  FIG. 3  includes functions main( ) bar( ) and foo( ) The main( ) function is coded to call foo( ) which is coded to call bar( ) The call stack  102  shown in  FIG. 3  reflects the executable program  114  when foo( ) has been called and is still executing. As shown in  FIG. 3 , assuming that the call stack  102  has not been compromised, the shadow stack  104  should contain duplicates  126  of the return addresses in the respective frames in the call stack, and in the same order. 
       FIG. 4  shows how the call stack  102  and shadow stack  104  are maintained. As discussed in detail below, the steps for shadow stack management in  FIG. 4  can be performed by special shadow stack hardware of the processor, by software, or both. When the executing program  100  calls a function at step  130 , at step  132  a new frame  134  is pushed to the call stack  102 . The new frame  134  includes a return address corresponding to the point where the called function was invoked. At step  136 , based on the same function invocation, a new return address  138  (the same return address that is in the new frame  134 ) is pushed onto the shadow stack  104 . The steps for adding to the stacks can be performed in any order or in parallel. 
     At step  140  the executing program  100  is executing a called function and the function returns (e.g., exits). At step  142  the frame corresponding to the returned-from function is popped from the call stack, and at step  144  the top return address on the shadow stack is popped from the shadow stack. As with most steps described herein, the steps for removing from the stacks can be performed in any order or in parallel. Moreover, the removal/exit process includes some logic, whether in hardware or software, to compare the return address in the popped frame with the return address popped from the shadow stack, and to respond accordingly if they do not match. However, the comparison is not necessary for backtracing and if it is possible to implement a shadow stack without any comparison/remediating steps, the small backtrace overhead may be lowered further. 
     As noted above, the shadow stack can be implemented by hardware or software.  FIG. 5  shows a software-based shadow stack implementation. In this embodiment, the compiler inserts into its translated output instrumentation code that implements the shadow stack. Generally, instrumentation code  150  will be placed near each function call  152 . The instrumentation code  150  may include entry logic  154  and exit logic  156 . The entry logic  154  may push the relevant return address onto the shadow stack, among other things. The exit logic  156  may pop the top of the shadow stack and check the popped return address against the return address in a corresponding call frame. 
       FIG. 6  shows a hardware-based shadow stack implementation. A processor  160  has circuitry that implements the various instructions in the processor&#39;s instruction set. In one embodiment, the processor&#39;s implementation of the call instruction includes both the ordinary call logic  164  as well as shadow stack logic  166 . The ordinary call logic  164  is the control flow logic of the call instruction that is found in a typical processor, regardless of whether the processor implements a shadow stack. The shadow stack logic  166  performs the shadow stack functionality discussed above, but in association with the call instruction  162 . Some processors may have a register for controlling whether the shadow stack logic is active, which may control implementation and/or backtrace-usage of the shadow stack logic  166 , among other things. The executable program may have no visibility of the shadow stack, which is implemented without instrumentation code. In other words, the same executable can run without modification on (i) one processor that lacks shadow stack support (and execution will have no shadows stack) and (ii) another processor that has shadow stack support (and will have shadow stacks). 
     In both hardware and software implementations, while a thread or program is executing, the shadow stack is assumed to be available for reading at any time, whether by the program/thread whose calls it is tracking, or by another component such as a debugger or runtime environment. 
       FIG. 7  shows how the shadow stack can be used to satisfy a backtrace request  170 . The compiled program  100  is executing. As discussed above, the program calls functions within functions, resulting in chains of function calls while corresponding state is reflected in the call stack and the shadow stack. The backtrace request  170  can be received during any arbitrary point of execution. The backtrace request  170  may be an application programming interface (API) call, an invocation of a backtrace function included in the executable program  100 , etc. The backtrace request might also be issued by an exception handler, a signal handler invoked by an external signal, and so forth. 
     Based on the backtrace request  170 , the shadow stack  104  is accessed and read, either by code in (or linked to) the program  100 , by a shadow stack instruction implemented by the processor, or a combination thereof. A full or partial copy  172  of the shadow stack  104  is captured (as used herein, a “copy” of the shadow stack refers to a full copy or a selective/partial copy). In some embodiments, the return addresses copied from the shadow stack may be limited to a top-N subset of the available addresses. Such a limit might come from a hardware constraint (e.g., a buffer size), a variable set in the program, a parameter passed into the backtrace request, etc. 
     Finally, the shadow stack copy  172  is incorporated into a trace or debug output  174 . Any of the many known means for capturing diagnostic information may be supplemented with shadow stack backtrace data in this fashion. In one embodiment, any known approach for requesting and storing a backtrace can be used by modifying such approach to obtain return address data from the shadow stack in addition to, or instead of, from the call stack. For example, a return address, which is just a number, can be converted into a function name, line number, file number etc. from symbol information. Such a full stack trace can then be stored in a log file, output to the screen, or sent over a network as telemetry for crash analysis etc. 
       FIG. 8  shows a process for using the shadow stack. At step  200 , a stack trace or backtrace is requested during execution of the program, either by the internal logic of the program or by a component communicating with the program. At step  202 , information about the shadow stack may be optionally acquired, for instance whether a shadow stack exists, its location, current size (or number of elements), the format of elements of the shadow stack, security information, attributes of the shadow stack (e.g., is it implemented in hardware or software), and so forth. 
     At step  204 , if shadow stack information was acquired, then the shadow stack information may be used to decide how or whether to carry out the backtrace request. For instance, unless a shadow stack is available (or has some specific trait), then the backtrace request may not generate a backtrace. Attributes of the shadow stack such as size, location, the size/number of elements, permissions, etc., may also be used to access and copy the shadow stack. 
     At step  206  the shadow stack is used to generate a backtrace. As discussed above, in some embodiments the backtrace is a verbatim copy of the shadow stack or a portion thereof. In other embodiments, the return addresses in the shadow stack are used to identify the relevant corresponding functions, the source code modules and line numbers where they are defined, the names of the called functions, the line numbers in functions where calls were made, information about the relevant functions (e.g., return type, parameter types/names), to name a few. 
       FIG. 9  shows shadow stack content  220  being used in combination with other information to generate a backtrace  222 . In some embodiments, the return addresses in the shadow stack content  222  can be used in combination with the call stack  102  and/or program/module metadata  224  to derive an enriched backtrace  222 . The program/module metadata  224  is metadata found in object files, symbol tables, source code files, and the like. A backtrace generating function  226  receives the shadow stack content  220 . The backtrace generating function  226  also receives or accesses the program/module metadata  226  and/or the call stack  102 . This information can be synthesized/combined in various ways. By correlating the return addresses in the shadow stack content with frames and return addresses in the call stack, the values of parameters and local variables can be obtained, the names of functions and modules can be found, line numbers of calls can be obtained, and so forth. Conceptually, the return addresses in the shadow stack content  220  can serve as an index to additional data; the shadow stack allows near zero-cost reconstruction of the call chain, and the call chain can be fleshed out with additional diagnostic information from other sources. In whatever form, the backtrace  222  is outputted, possibly through an existing diagnostic/debugging/telemetry scheme, which might involve using APIs for outputting diagnostic data, collecting formatted output in log files, sending the log files to a collection service, etc. 
     Another example is if there is data corruption on the regular stack. This is common because local variables are also stored on the regular stack, so a bug in the code can allow the return address on the regular stack to get overwritten. Because the shadow stack has no local variables, the program should need to touch any memory on the shadow stack, therefore it is much less likely to get corrupted. In the hardware case, the hardware can enforce the shadow stack to be read-only, and in the software case, the operating system can also make it read-only to everyone except for itself (for pushing and popping return addresses). In any case, if the return address is corrupted on the regular stack, the extra copy of the return address on the shadow stack could be substituted in to correct the problem. Then, the program could possibly continue to run if there are no other corruptions. If there are other corruptions, then at least we can still obtain a successful stack trace to catch the culprit of the corruption. 
     In addition, when unwinding the stack, if there is corruption in the middle of the stack, then unwinding will fail at the location of the corruption and further unwinding will be impossible since the data on the stack is wrong, therefore the location of the next return address is unknown. But if a shadow stack is available, even if the shadow stack itself has a few corrupted entries, the corrupted return address entries can be skipped and unwinding can proceed since the return addresses are known to be contiguous in memory. 
       FIG. 10  shows various contexts where backtrace logic can be implemented. In one implementation, shown at the top of  FIG. 10 , the program  100  includes calls to an API for capturing backtraces. The API may be any known API for debugging or capturing backtraces. However, the logic implementing the API uses the shadow stack. The API may also have additional functions/methods that are relevant to using a shadow stack. For example, the API may define functions for setting prerequisites for honoring backtrace requests (e.g., shadow stack is available, shadow stack is available and supported by hardware, etc.). The API may also facilitate backward compatibility. The backtracing functionality can also be fully contained within the program, as shown in the middle of  FIG. 10 . In another embodiment, an environment  230  such as a debugger, managed code environment, interpreter, or the like both executes the program and provides the backtracing functionality. 
       FIG. 11  shows a backtrace output example  240 . The example includes strings  242  generated based on shadow stack data. Any known style of backtrace formatting and content can be used. Settings, different backtrace functions (e.g., backtrace_raw( ) or backtrace_symbols( )), or backtrace function parameters can be used to control the backtrace style and content, which can range from a list of bare memory addresses to detailed information and graphics about the functions that appear in the backtrace. 
     Although embodiments above involve compiled programs, most of the techniques can be readily applied to interpreted programs. In that case, the interpreter can be configured to use the shadow stack in similar fashion. The interpreter should be the same from the program&#39;s point of view; both compiled and interpreted programs have call/ret instructions that get executed by the processor. 
     Some call stack implementations use multiple call stacks that are linked together. In such cases, corresponding shadow stacks are linked, and unwinding the current call chain for a backtrace may involve unwinding the linked shadow stacks. 
     The term “program” as used herein is also considered to refer to threads. Usually, each thread has its own call stack and shadow stack. 
       FIG. 12  shows details of a computing device  300  on which embodiments described above may be implemented. The technical disclosures herein will suffice for programmers to write software, and/or configure reconfigurable processing hardware (e.g., field-programmable gate arrays (FPGAs)), and/or design application-specific integrated circuits (ASICs), etc., to run on the computing device or host  300  (possibly via cloud APIs) to implement the embodiments described herein. 
     The computing device or host  300  may have one or more displays  322 , a network interface  324  (or several), as well as storage hardware  326  and processing hardware  328 , which may be a combination of any one or more of: central processing units, graphics processing units, analog-to-digital converters, bus chips, FPGAs, ASICs, Application-specific Standard Products (ASSPs), or Complex Programmable Logic Devices (CPLDs), etc. The storage hardware  326  may be any combination of magnetic storage, static memory, volatile memory, non-volatile memory, optically or magnetically readable matter, etc. The meaning of the term “storage”, as used herein does not refer to signals or energy per se, but rather refers to physical apparatuses and states of matter used thereby to read and/or store information. The hardware elements of the computing device or host  300  may cooperate in ways well understood in the art of machine computing. In addition, input devices may be integrated with or in communication with the computing device or host  300 . The computing device or host  300  may have any form-factor or may be used in any type of encompassing device. The computing device or host  300  may be in the form of a handheld device such as a smartphone, a tablet computer, a gaming device, a server, a rack-mounted or backplaned computer-on-a-board, a system-on-a-chip, or others. 
     Embodiments and features discussed above can be realized in the form of information stored in volatile or non-volatile computer or device readable media. This is deemed to include at least media such as optical storage (e.g., compact-disk read-only memory (CD-ROM)), magnetic media, flash read-only memory (ROM), or any current or future means of storing digital information. The stored information can be in the form of machine executable instructions (e.g., compiled executable binary code), source code, bytecode, or any other information that can be used to enable or configure computing devices to perform the various embodiments discussed above. This is also deemed to include at least volatile memory such as random-access memory (RAM) and/or virtual memory storing information such as central processing unit (CPU) instructions during execution of a program carrying out an embodiment, as well as non-volatile media storing information that allows a program or executable to be loaded and executed. The embodiments and features can be performed on any type of computing device, including portable devices, workstations, servers, mobile wireless devices, and so on.