GetSyscallStub.nim

#[
    S3cur3Th1sSh1t, Twitter: @Shitsecure
    License: BSD 3-Clause
]#

import winim
import strutils
import ptr_math

type
  PS_ATTR_UNION* {.pure, union.} = object
    Value*: ULONG
    ValuePtr*: PVOID
  PS_ATTRIBUTE* {.pure.} = object
    Attribute*: ULONG 
    Size*: SIZE_T
    u1*: PS_ATTR_UNION
    ReturnLength*: PSIZE_T
  PPS_ATTRIBUTE* = ptr PS_ATTRIBUTE
  PS_ATTRIBUTE_LIST* {.pure.} = object
    TotalLength*: SIZE_T
    Attributes*: array[2, PS_ATTRIBUTE]
  PPS_ATTRIBUTE_LIST* = ptr PS_ATTRIBUTE_LIST
  KNORMAL_ROUTINE* {.pure.} = object
    NormalContext*: PVOID
    SystemArgument1*: PVOID
    SystemArgument2*: PVOID
  PKNORMAL_ROUTINE* = ptr KNORMAL_ROUTINE

var SYSCALL_STUB_SIZE: int = 23

proc RVAtoRawOffset(RVA: DWORD_PTR, section: PIMAGE_SECTION_HEADER): PVOID =
    return cast[PVOID](RVA - section.VirtualAddress + section.PointerToRawData)

proc toString(bytes: openarray[byte]): string =
  result = newString(bytes.len)
  copyMem(result[0].addr, bytes[0].unsafeAddr, bytes.len)

proc GetSyscallStub*(functionName: LPCSTR, syscallStub: LPVOID): BOOL =
    var
        file: HANDLE
        fileSize: DWORD
        bytesRead: DWORD
        fileData: LPVOID
        ntdllString: LPCSTR = "C:\\windows\\system32\\ntdll.dll"
        nullHandle: HANDLE

    file = CreateFileA(ntdllString, cast[DWORD](GENERIC_READ), cast[DWORD](FILE_SHARE_READ), cast[LPSECURITY_ATTRIBUTES](NULL), cast[DWORD](OPEN_EXISTING), cast[DWORD](FILE_ATTRIBUTE_NORMAL), nullHandle)
    fileSize = GetFileSize(file, nil)
    fileData = HeapAlloc(GetProcessHeap(), 0, fileSize)
    ReadFile(file, fileData, fileSize, addr bytesRead, nil)

    var
        dosHeader: PIMAGE_DOS_HEADER = cast[PIMAGE_DOS_HEADER](fileData)
        imageNTHeaders: PIMAGE_NT_HEADERS = cast[PIMAGE_NT_HEADERS](cast[DWORD_PTR](fileData) + dosHeader.e_lfanew)
        exportDirRVA: DWORD = imageNTHeaders.OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_EXPORT].VirtualAddress
        section: PIMAGE_SECTION_HEADER = IMAGE_FIRST_SECTION(imageNTHeaders)
        textSection: PIMAGE_SECTION_HEADER = section
        rdataSection: PIMAGE_SECTION_HEADER = section

    let low: uint16 = 0

    for Section in low ..< imageNTHeaders.FileHeader.NumberOfSections:
        var ntdllSectionHeader = cast[PIMAGE_SECTION_HEADER](cast[DWORD_PTR](IMAGE_FIRST_SECTION(imageNTHeaders)) + cast[DWORD_PTR](IMAGE_SIZEOF_SECTION_HEADER * Section))
        if ".rdata" in toString(ntdllSectionHeader.Name):
            rdataSection = ntdllSectionHeader
        
    var exportDirectory: PIMAGE_EXPORT_DIRECTORY = cast[PIMAGE_EXPORT_DIRECTORY](RVAtoRawOffset(cast[DWORD_PTR](fileData) + exportDirRVA, rdataSection))

    var addressOfNames: PDWORD = cast[PDWORD](RVAtoRawOffset(cast[DWORD_PTR](fileData) + cast[DWORD_PTR](exportDirectory.AddressOfNames), rdataSection))
    var addressOfFunctions: PDWORD = cast[PDWORD](RVAtoRawOffset(cast[DWORD_PTR](fileData) + cast[DWORD_PTR](exportDirectory.AddressOfFunctions), rdataSection))
    var stubFound: BOOL = 0
    var oldProtection: DWORD = 0
    let low2: int = 0
    for low2 in 0 ..< exportDirectory.NumberOfNames:
        var functionNameVA: DWORD_PTR = cast[DWORD_PTR](RVAtoRawOffset(cast[DWORD_PTR](fileData) + addressOfNames[low2], rdataSection))
        var functionVA: DWORD_PTR = cast[DWORD_PTR](RVAtoRawOffset(cast[DWORD_PTR](fileData) + addressOfFunctions[low2 + 1], textSection))
        var functionNameResolved: LPCSTR = cast[LPCSTR](functionNameVA)
        var compare: int = lstrcmpA(functionNameResolved,functionName)
        if (compare == 0):
            copyMem(syscallStub, cast[LPVOID](functionVA), SYSCALL_STUB_SIZE)
            stubFound = 1
            return stubFound
    return stubFound