I set a breakpoint in kernel32!LoadLibraryExW. In the calls window, I have "Source args" toggled, but the call stack still doesn't show the arguments for LoadLibraryExW when it breaks. Is there a way to easily view the arguments?
I have set the environment variable _NT_SYMBOL_PATH to SRV*c:\symbols*http://msdl.microsoft.com/download/symbols
You can't directly match the arguments to the function parameters with 'Source Args' toggled. These are available only with private PDBs.
You have to toggle 'Raw args' and make them fit with the documentation from MSDN.
If you need more than 3 arguments you must view the memory starting at esp.
This is quite simple with 32 bits, but it may be a pain with 64 bits because the arguments may not be actually written to the stack (they are passed by registers and copied to the stack only if the registers need to be overwritten and restored). For more information, you can refer to http://msdn.microsoft.com/en-us/library/ms235286(v=vs.80).aspx If you have control on the source code, compile with the /homeparams flag on the C compiler to be sure the parameters are copied on the stack to ease debugging.
In X64, the first four integer arguments go into rcx, rdx, r8 and r9 registers respectively.
The rest of the integer arguments go on the stack.
Related
Writing x64 Assembly code using MASM, we can use these directives to provide frame unwinding information. For example, from .SETFRAME definition:
These directives do not generate code; they only generate .xdata and .pdata.
Since these directives don't produce any code, I cannot see their effects in Disassembly window. So, I don't see any difference, when I write assembly function with or without these directives. How can I see the result of these directives - using dumpbin or something else?
How to write code that can test this unwinding capability? For example, I intentionally write assembly code that causes an exception. I want to see the difference in exception handling behavior, when function is written with or without these directives.
In my case caller is written in C++, and can use try-catch, SSE etc. - whatever is relevant for this situation.
Answering your question:
How can I see the result of these directives - using dumpbin or something else?
You can use dumpbin /UNWINDINFO out.exe to see the additions to the .pdata resulting from your use of .SETFRAME.
The output will look something like the following:
00000054 00001530 00001541 000C2070
Unwind version: 1
Unwind flags: None
Size of prologue: 0x04
Count of codes: 2
Frame register: rbp
Frame offset: 0x0
Unwind codes:
04: SET_FPREG, register=rbp, offset=0x00
01: PUSH_NONVOL, register=rbp
A bit of explanation to the output:
The second hex number found in the output is the function address 00001530
Unwind codes express what happens in the function prolog. In the example what happens is:
RBP is pushed to the stack
RBP is used as the frame pointer
Other functions may look like the following:
000000D8 000016D0 0000178A 000C20E4
Unwind version: 1
Unwind flags: EHANDLER UHANDLER
Size of prologue: 0x05
Count of codes: 2
Unwind codes:
05: ALLOC_SMALL, size=0x20
01: PUSH_NONVOL, register=rbx
Handler: 000A2A50
One of the main differences here is that this function has an exception handler. This is indicated by the Unwind flags: EHANDLER UHANDLER as well as the Handler: 000A2A50.
Probably your best bet is to have your asm function call another C++ function, and have your C++ function throw a C++ exception. Ideally have the code there depend on multiple values in call-preserved registers, so you can make sure they get restored. But just having unwinding find the right return addresses to get back into your caller requires correct metadata to indicate where that is relative to RSP, for any given RIP.
So create a situation where a C++ exception needs to unwind the stack through your asm function; if it works then you got the stack-unwind metadata directives correct. Specifically, try{}catch in the C++ caller, and throw in a C++ function you call from asm.
That thrower can I think be extern "C" so you can call it from asm without name mangling. Or call it via a function pointer, or just look at MSVC compiler output and copy the mangled name into asm.
Apparently Windows SEH uses the same mechanism as plain C++ exceptions, so you could potentially set up a catch for the exception delivered by the kernel in response to a memory fault from something like mov ds:[0], eax (null deref). You could put this at any point in your function to make sure the exception unwind info was correct about the stack state at every point, not just getting back into sync before a function-call.
https://learn.microsoft.com/en-us/cpp/build/exception-handling-x64?view=msvc-170&viewFallbackFrom=vs-2019 has details about the metadata.
BTW, the non-Windows (e.g. GNU/Linux) equivalent of this metadata is DWARF .cfi directives which create a .eh_frame section.
I don't know equivalent details for Windows, but I do know they use similar metadata that makes it possible to unwind the stack without relying on RBP frame pointers. This lets compilers make optimized code that doesn't waste instructions on push rbp / mov rbp,rsp and leave in function prologues/epilogues, and frees up RBP for use as a general-purpose register. (Even more useful in 32-bit code where 7 instead of 6 registers besides the stack pointer is a much bigger deal than 15 vs. 14.)
The idea is that given a RIP, you can look up the offset from RSP to the return address on the stack, and the locations of any call-preserved registers. So you can restore them and continue unwinding into the parent using that return address.
The metadata indicates where each register was saved, relative to RSP or RBP, given the current RIP as a search key. In functions that use an RBP frame pointer, one piece of metadata can indicate that. (Other metadata for each push rbx / push r12 says which call-preserved regs were saved in which order).
In functions that don't use RBP as a frame pointer, every push / pop or sub/add RSP needs metadata for which RIP it happened at, so given a RIP, stack unwinding can see where the return address is, and where those saved call-preserved registers are. (Functions that use alloca or VLAs thus must use RBP as a frame pointer.)
This is the big-picture problem that the metadata has to solve. There are a lot of details, and it's much easier to leave things up to a compiler!
In this answer: https://stackoverflow.com/a/8646611/192359 , it is explained that when debugging x86 code, symbols allow the debugger to display the callstack even when FPO (Frame Pointer Omission) is used.
The given explanation is:
On the x86 PDBs contain FPO information, which allows the debugger to reliably unwind a call stack.
My question is what's this information? As far as I understand, just knowing whether a function has FPO or not does not help you finding the original value of the stack pointer, since that depends on runtime information.
What am I missing here?
Fundamentally, it is always possible to walk the stack with enough information1, except in cases where the stack or execution context has been irrecoverably corrupted.
For example, even if rbp isn't used as the frame pointer, the return address is still on the stack somewhere, and you just need to know where. For a function that doesn't modify rsp (indirectly or directly) in the body of the function it would be at a simple fixed offset from rsp. For functions that modify rsp in the body of the function (i.e., that have a variable stack size), the offset from rsp might depend on the exact location in the function.
The PDB file simply contains this "side band" information which allows someone to determine the return address for any instruction in the function. Hans linked a relevant in-memory structure above - you can see that since it knows the size of the local variables and so on it can calculate the offset between rsp and the base of the frame, and hence get at the return address. It also knows how many instruction bytes are part of the "prolog" which is important because if the IP is still in that region, different rules apply (i.e., the stack hasn't been adjusted to reflect the locals in this function yet).
In 64-bit Windows, the exact function call ABI has been made a bit more concrete, and all functions generally have to provide unwind information: not in a .pdb but directly in a section included in the binary. So even without .pdb files you should be able to unwind a properly structured 64-bit Windows program. It allows any register to be used as the frame pointer, and still allows frame-pointer omission (with some restrictions). For details, start here.
1 If this weren't true, ask yourself how the currently running function could ever return? Now, technically you could design a program which clobbers or forgets the stack in a way that it cannot return, and either never exits or uses a method like exit() or abort() to terminate. This is highly unusual and not possibly outside of assembly.
In x86 calling conventions parameters are passed on the stack and when using base pointers in a frame it is possible to reconstruct from a call stack what parameters have been passed to successive stack functions (actually the process is done in reverse order from last functioned called going back)
How can we do the same in x64 ABI considering (as per x64 ABI) that registers used for parameter passing RCX, RDX, R8, R9 -> are all volatile and thus loose their values between frames (with no stack backup). ?
I have a confusion about the system call mechanism. In X86, System Call uses eax to pass the system call number to kernel.
But what does it use to pass the parameters to kernel, at some place I am seeing it uses stack and at other places it says, it uses ebx, ecx, etc registers.
So can someone confirm which one is correct ?
Fore reference :
this link says it uses stack.
And this link says it uses registers.
Both the links tell that the parameters are passed through registers like EBX, ECX, etc to the kernel space from the user space.
In the first reference page : 35/352, System Call Implementation/wrappers task 1st point, it is given that
the parameters available in the user stack are moved to the processor registers and then this registers are used to pass parameters of the syscall to the kernel space.
I think you must be confused after seeing the word stack in that point about implementing the libc wrappers like write() which are callable from C, to interface between the system-call calling convention (6 regs) and the function-calling convention (stack args since user-space doesn't normally use -mregparm=3)
Both the links are correct.
You can see in , all system calls are declared with prefix asmlinkage. Infact when you define your system call using SYSCALL_DEFINEx macro, it defines your system call function with asmlinkage directive. asmlinkage directive directs compiler that the function should not expect any of it's parameters from CPU registers i.e. all parameters should be accessed from stack only.
When called from user space each parameters are pushed to CPU registers, during user to kernel transition, kernel needs to save all the registers onto stack (in order to restore the environment before returning to the user space) when handling the system call requests from user space, so after that the parameters are available on stack for kernel space system call function.
I would like to test a buffer-overflow by writing "Hello World" to console (using Windows XP 32-Bit). The shellcode needs to be null-free in order to be passed by "scanf" into the program I want to overflow. I've found plenty of assembly-tutorials for Linux, however none for Windows. Could someone please step me through this using NASM? Thxxx!
Assembly opcodes are the same, so the regular tricks to produce null-free shellcodes still apply, but the way to make system calls is different.
In Linux you make system calls with the "int 0x80" instruction, while on Windows you must use DLL libraries and do normal usermode calls to their exported functions.
For that reason, on Windows your shellcode must either:
Hardcode the Win32 API function addresses (most likely will only work on your machine)
Use a Win32 API resolver shellcode (works on every Windows version)
If you're just learning, for now it's probably easier to just hardcode the addresses you see in the debugger. To make the calls position independent you can load the addresses in registers. For example, a call to a function with 4 arguments:
PUSH 4 ; argument #4 to the function
PUSH 3 ; argument #3 to the function
PUSH 2 ; argument #2 to the function
PUSH 1 ; argument #1 to the function
MOV EAX, 0xDEADBEEF ; put the address of the function to call
CALL EAX
Note that the argument are pushed in reverse order. After the CALL instruction EAX contains the return value, and the stack will be just like it was before (i.e. the function pops its own arguments). The ECX and EDX registers may contain garbage, so don't rely on them keeping their values after the call.
A direct CALL instruction won't work, because those are position dependent.
To avoid zeros in the address itself try any of the null-free tricks for x86 shellcode, there are many out there but my favorite (albeit lengthy) is encoding the values using XOR instructions:
MOV EAX, 0xDEADBEEF ^ 0xFFFFFFFF ; your value xor'ed against an arbitrary mask
XOR EAX, 0xFFFFFFFF ; the arbitrary mask
You can also try NEG EAX or NOT EAX (sign inversion and bit flipping) to see if they work, it's much cheaper (two bytes each).
You can get help on the different API functions you can call here: http://msdn.microsoft.com
The most important ones you'll need are probably the following:
WinExec(): http://msdn.microsoft.com/en-us/library/ms687393(VS.85).aspx
LoadLibrary(): http://msdn.microsoft.com/en-us/library/windows/desktop/ms684175(v=vs.85).aspx
GetProcAddress(): http://msdn.microsoft.com/en-us/library/ms683212%28v=VS.85%29.aspx
The first launches a command, the next two are for loading DLL files and getting the addresses of its functions.
Here's a complete tutorial on writing Windows shellcodes: http://www.codeproject.com/Articles/325776/The-Art-of-Win32-Shellcoding
Assembly language is defined by your processor, and assembly syntax is defined by the assembler (hence, at&t, and intel syntax) The main difference (at least i think it used to be...) is that windows is real-mode (call the actual interrupts to do stuff, and you can use all the memory accessible to your computer, instead of just your program) and linux is protected mode (You only have access to memory in your program's little cubby of memory, and you have to call int 0x80 and make calls to the kernel, instead of making calls to the hardware and bios) Anyway, hello world type stuff would more-or-less be the same between linux and windows, as long as they are compatible processors.
To get the shellcode from your program you've made, just load it into your target system's
debugger (gdb for linux, and debug for windows) and in debug, type d (or was it u? Anyway, it should say if you type h (help)) and between instructions and memory will be the opcodes.
Just copy them all over to your text editor into one string, and maybe make a program that translates them all into their ascii values. Not sure how to do this in gdb tho...
Anyway, to make it into a bof exploit, enter aaaaa... and keep adding a's until it crashes
from a buffer overflow error. But find exactly how many a's it takes to crash it. Then, it should tell you what memory adress that was. Usually it should tell you in the error message. If it says '9797[rest of original return adress]' then you got it. Now u gotta use ur debugger to find out where this was. disassemble the program with your debugger and look for where scanf was called. Set a breakpoint there, run and examine the stack. Look for all those 97's (which i forgot to mention is the ascii number for 'a'.) and see where they end. Then remove breakpoint and type the amount of a's you found out it took (exactly the amount. If the error message was "buffer overflow at '97[rest of original return adress]" then remove that last a, put the adress you found examining the stack, and insert your shellcode. If all goes well, you should see your shellcode execute.
Happy hacking...