Windows MASM / Return Address Spoofing

Code

COMMENT ~
    PUBLIC _spoofer_stub
     
    .code
     
    _spoofer_stub PROC
        pop r11 ~ poping without setting up stack frame, r11 is the return address (the one in our code)
        add rsp, 8 ~ skipping callee reserved space
        mov rax, [rsp + 24] ~ dereference shell_param
        
        mov r10, [rax] ~ load shell_param.trampoline
        mov [rsp], r10 ~ store address of trampoline as return address
        
        mov r10, [rax + 8] ~ load shell_param.function
        mov [rax + 8], r11 ~ store the original return address in shell_param.function
     
        mov [rax + 16], rbx ~ preserve rbx in shell_param.rbx
        lea rbx, fixup
        mov [rax], rbx ~ store address of fixup label in shell_param.trampoline
        mov rbx, rax ~ preserve address of shell_param in rbx
        
        jmp r10 ~ call shell_param.function
     
    fixup:
        sub rsp, 16
        mov rcx, rbx ~ restore address of shell_param
        mov rbx, [rcx + 16] ~ restore rbx from shell_param.rbx
        jmp QWORD PTR [rcx + 8] ~ jmp to the original return address
    _spoofer_stub ENDP
     
    END