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This 3 lines of code are used by some encoders to get EIP into ECX register:

fldz
fnstenv [esp-C]
pop ecx

The 2 first instructions push some datas on stack and we get one of this into ECX on the last line.

I have tried this code in a very basic poc. It works fine.

But in real life (in a Buffer overflow application), ecx contains 0x0000000 and I do not understand why.

The only thing I know is ESP value has been changed, but points in the middle of a big buffer.

Are there some rules on ESP ? Can ESP points anywhere (on an allocated memory region of course) ?

For example if I create a global variable in my program: unsigned char buf[1024]. Can I set ESP to buf+512 and call PUSH, fldz or fnstenv instructions ?

Thanks

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  • You might require fninit before the fldz, to initialise the FPU state explicitly. Jan 24, 2023 at 23:44

1 Answer 1

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ESP always points to the top of stack. Do you really understand stack concept? If you overwrite esp with random value, the application can crash.

fldz - Push +0.0 onto the FPU stack. fnstenv - Store FPU environment in the area which is placed on memory in your case. [esp-C] - this is memory pointer. pop ecx - just pop dword from the top of stack to the ecx

What do you want to do with this code? Describe your task.

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  • I know what is stack and how it works. The os creates a memory space for the stack. My question is will it work if i put an esp value that points on another memory space. I am not talking about a random memory space. I am talking about a buffer i control. I want to know if the cpu will detect this is not the memory space dedicated for stack
    – Bob5421
    Jan 25, 2023 at 6:07
  • You can change ESP, but after that pop/return instruction will work incorrect and call/push will store ret_addr/value outside the stack, because you change stack pointer(a location where data was stored). You can use ESP as general purpose register.
    – strannyi
    Jan 25, 2023 at 9:06

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