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Inject your code to a Portable Executable file

0. Preface
1. Prerequisite
2. Portable Executable file format
- 2.1 The MS-DOS data
- 2.2 The Windows NT data
- 2.3 The Section Headers and Sections
3. Debugger, Disassembler and some Useful Tools
- 3.1 Debuggers
  - 3.1.1 SoftICE
  - 3.1.2 OllyDbg
  - 3.1.3 Which parts are important in a debugger interface?
- 3.2 Disassembler
  - 3.2.1 Proview disassembler
  - 3.2.2 W32Dasm
  - 3.2.3 IDA Pro
- 3.3 Some Useful Tools
  - 3.3.1 LordPE
  - 3.3.2 PEiD
  - 3.3.3 Resource Hacker
  - 3.3.4 WinHex
  - 3.3.5 CFF Explorer
4. Add new section and Change OEP
- 4.1 Retrieve and Rebuild PE file
- 4.2 Create Data for new Section
- 4.3 Some notes regarding creating a new PE file
- 4.4 Some notes regarding linking this VC Project
5. Store Important Data and Reach Original OEP
- 5.1 Restore the first Registers Context
- 5.2 Restore the Original Stack
- 5.3 Approach OEP by Structured Exception Handling
  - 5.3.1 Implement Exception Handler
  - 5.3.2 Attain OEP by adjusting the Thread Context
6. Build an Import Table and Reconstruct the Original Import Table
- 6.1 Construct the Client Import Table
- 6.2 Using other API functions in run-time
- 6.3 Fix up the Original Import Table
7. Support DLL and OCX
- 7.1 Twice OEP approach
- 7.2 Implement Relocation Table
- 7.3 Build a Special Import table
8. Preserve the Thread Local Storage
9. Inject your code
10. Conclusion

0 Preface

It might be, you demand to comprehend the ways a virus program injects its procedure in to the interior of a portable executable file and corrupts it, or you are interested in implementing a packer or a protector for your specific intention to encrypt the data of your portable executable (PE) file. This article is committed to represent a brief intuition to realize the performance which is accomplished by EXE tools or some kind of mal-wares.

You can employ the source code of this article to create your custom EXE builder. It could be used to make an EXE protector in the right way, or with a wrong intention, to pullulate a virus. However, my purpose of writing this article has been to gaze on the first application, so I will not be responsible for the immoral usage of these methods.

1 Prerequisite

There are no specific mandatory prerequisites to follow the topics in this article. If you are familiar with debugger and also the portable file format, I suggest you to drop the sections 2 and 3, the whole of these sections have been made for people who don’t have any knowledge regarding the EXE file format and also debuggers.

2 Portable Executable file format

The Portable Executable file format was defined to provide the best way for the Windows Operating System to execute code and also to store the essential data which is needed to run a program, for example constant data, variable data, import library links, and resource data. It consists of MS-DOS file information, Windows NT file information, Section Headers, and Section images, Table 1.

2.1 The MS-DOS data

These data let you remember the first days of developing the Windows Operating System, the days. We were at the beginning of a way to achieve a complete Operating System like Windows NT 3.51 (I mean, Win3.1, Win95, Win98 were not perfect OSs). The MS-DOS data causes that your executable file calls a function inside MS-DOS and the MS-DOS Stub program lets it display: "This program can not be run in MS-DOS mode" or "This program can be run only in Windows mode", or some things like these comments when you try to run a Windows EXE file inside MS-DOS 6.0, where there is no footstep of Windows. Thus, this data is reserved for the code to indicate these comments in the MS-DOS operating system. The most interesting part of the MS-DOS data is "MZ"! Can you believe, it refers to the name of "Mark Zbikowski", one of the first Microsoft programmers?

To me, only the offset of the PE signature in the MS-DOS data is important, so I can use it to find the position of the Windows NT data. I just recommend you to take a look at Table 1, then observe the structure of IMAGE_DOS_HEADER in the <winnt.h> header in the <Microsoft Visual Studio .net path>/VC7/PlatformSDK/include/ folder or the <Microsoft Visual Studio 6.0 path>/VC98/include/ folder. I do not know why the Microsoft team has forgotten to provide some comment about this structure in the MSDN library!

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typedef struct _IMAGE_DOS_HEADER { // DOS .EXE header "MZ"
    WORD   e_magic;                // Magic number
    WORD   e_cblp;                 // Bytes on last page of file
    WORD   e_cp;                   // Pages in file
    WORD   e_crlc;                 // Relocations
    WORD   e_cparhdr;              // Size of header in paragraphs
    WORD   e_minalloc;             // Minimum extra paragraphs needed
    WORD   e_maxalloc;             // Maximum extra paragraphs needed
    WORD   e_ss;                   // Initial (relative) SS value
    WORD   e_sp;                   // Initial SP value
    WORD   e_csum;                 // Checksum
    WORD   e_ip;                   // Initial IP value
    WORD   e_cs;                   // Initial (relative) CS value
    WORD   e_lfarlc;               // File address of relocation table
    WORD   e_ovno;                 // Overlay number
    WORD   e_res[4];               // Reserved words
    WORD   e_oemid;                // OEM identifier (for e_oeminfo)
    WORD   e_oeminfo;              // OEM information; e_oemid specific
    WORD   e_res2[10];             // Reserved words
    LONG   e_lfanew;               // File address of the new exe header
  } IMAGE_DOS_HEADER, *PIMAGE_DOS_HEADER;

e_lfanew is the offset which refers to the position of the Windows NT data. I have provided a program to obtain the header information from an EXE file and to display it to you. To use the program, just try:

PE Viewer

Download source files - 132 Kb

This sample is useful for the whole of this article.

Table 1 - Portable Executable file format structure

MS-DOS information	`IMAGE_DOS_ HEADER`	DOS EXE Signature	Collapse Copy Code 00000000 ASCII "MZ" 00000002 DW 0090 00000004 DW 0003 00000006 DW 0000 00000008 DW 0004 0000000A DW 0000 0000000C DW FFFF 0000000E DW 0000 00000010 DW 00B8 00000012 DW 0000 00000014 DW 0000 00000016 DW 0000 00000018 DW 0040 0000001A DW 0000 0000001C DB 00 … … 0000003B DB 00 0000003C DD 000000F0
		`DOS_PartPag`
		`DOS_PageCnt`
		`DOS_ReloCnt`
		`DOS_HdrSize`
		`DOS_MinMem`
		`DOS_MaxMem`
		`DOS_ReloSS`
		`DOS_ExeSP`
		`DOS_ChkSum`
		`DOS_ExeIPP`
		`DOS_ReloCS`
		`DOS_TablOff`
		`DOS_Overlay`
		`…`Reserved words`…`
		Offset to PE signature
	MS-DOS Stub Program	Collapse Copy Code 00000040 º.´.Í!¸/LÍ!This program canno 00000060 t be run in DOS mode....$.......
Windows NT information `IMAGE_ NT_HEADERS`	Signature	PE signature (PE)	Collapse Copy Code 000000F0 ASCII "PE"
	`IMAGE_ FILE_HEADER`	`Machine`	Collapse Copy Code 000000F4 DW 014C 000000F6 DW 0003 000000F8 DD 3B7D8410 000000FC DD 00000000 00000100 DD 00000000 00000104 DW 00E0 00000106 DW 010F
		`NumberOfSections`
		`TimeDateStamp`
		`PointerToSymbolTable`
		`NumberOfSymbols`
		`SizeOfOptionalHeader`
		`Characteristics`
	`IMAGE_ OPTIONAL_ HEADER32`	`MagicNumber`	Collapse Copy Code 00000108 DW 010B 0000010A DB 07 0000010B DB 00 0000010C DD 00012800 00000110 DD 00009C00 00000114 DD 00000000 00000118 DD 00012475 0000011C DD 00001000 00000120 DD 00014000 00000124 DD 01000000 00000128 DD 00001000 0000012C DD 00000200 00000130 DW 0005 00000132 DW 0001 00000134 DW 0005 00000136 DW 0001 00000138 DW 0004 0000013A DW 0000 0000013C DD 00000000 00000140 DD 0001F000 00000144 DD 00000400 00000148 DD 0001D7FC 0000014C DW 0002 0000014E DW 8000 00000150 DD 00040000 00000154 DD 00001000 00000158 DD 00100000 0000015C DD 00001000 00000160 DD 00000000 00000164 DD 00000010
		`MajorLinkerVersion`
		`MinorLinkerVersion`
		`SizeOfCode`
		`SizeOfInitializedData`
		`SizeOfUninitializedData`
		`AddressOfEntryPoint`
		`BaseOfCode`
		`BaseOfData`
		`ImageBase`
		`SectionAlignment`
		`FileAlignment`
		`MajorOSVersion`
		`MinorOSVersion`
		`MajorImageVersion`
		`MinorImageVersion`
		`MajorSubsystemVersion`
		`MinorSubsystemVersion`
		`Reserved`
		`SizeOfImage`
		`SizeOfHeaders`
		`CheckSum`
		`Subsystem`
		`DLLCharacteristics`
		`SizeOfStackReserve`
		`SizeOfStackCommit`
		`SizeOfHeapReserve`
		`SizeOfHeapCommit`
		`LoaderFlags`
		`NumberOfRvaAndSizes`
		`IMAGE_ DATA_DIRECTORY[16]`	Export Table
			Import Table
			Resource Table
			Exception Table
			Certificate File
			Relocation Table
			Debug Data
			Architecture Data
			Global Ptr
			TLS Table
			Load Config Table
			Bound Import Table
			Import Address Table
			Delay Import Descriptor
			COM+ Runtime Header
			Reserved
Sections information	`IMAGE_ SECTION_ HEADER[0]`	`Name[8]`	Collapse Copy Code 000001E8 ASCII".text" 000001F0 DD 000126B0 000001F4 DD 00001000 000001F8 DD 00012800 000001FC DD 00000400 00000200 DD 00000000 00000204 DD 00000000 00000208 DW 0000 0000020A DW 0000 0000020C DD 60000020 CODE\|EXECUTE\|READ
		`VirtualSize`
		`VirtualAddress`
		`SizeOfRawData`
		`PointerToRawData`
		`PointerToRelocations`
		`PointerToLineNumbers`
		`NumberOfRelocations`
		`NumberOfLineNumbers`
		`Characteristics`
	`… … … IMAGE_ SECTION_ HEADER[n]`	Collapse Copy Code 00000210 ASCII".data"; SECTION 00000218 DD 0000101C ; VirtualSize = 0x101C 0000021C DD 00014000 ; VirtualAddress = 0x14000 00000220 DD 00000A00 ; SizeOfRawData = 0xA00 00000224 DD 00012C00 ; PointerToRawData = 0x12C00 00000228 DD 00000000 ; PointerToRelocations = 0x0 0000022C DD 00000000 ; PointerToLineNumbers = 0x0 00000230 DW 0000 ; NumberOfRelocations = 0x0 00000232 DW 0000 ; NumberOfLineNumbers = 0x0 00000234 DD C0000040 ; Characteristics = INITIALIZED_DATA\|READ\|WRITE 00000238 ASCII".rsrc"; SECTION 00000240 DD 00008960 ; VirtualSize = 0x8960 00000244 DD 00016000 ; VirtualAddress = 0x16000 00000248 DD 00008A00 ; SizeOfRawData = 0x8A00 0000024C DD 00013600 ; PointerToRawData = 0x13600 00000250 DD 00000000 ; PointerToRelocations = 0x0 00000254 DD 00000000 ; PointerToLineNumbers = 0x0 00000258 DW 0000 ; NumberOfRelocations = 0x0 0000025A DW 0000 ; NumberOfLineNumbers = 0x0 0000025C DD 40000040 ; Characteristics = INITIALIZED_DATA\|READ
	`SECTION[0]`	Collapse Copy Code 00000400 EA 22 DD 77 D7 23 DD 77 ê"Ýw×#Ýw 00000408 9A 18 DD 77 00 00 00 00 šÝw.... 00000410 2E 1E C7 77 83 1D C7 77 .ÇwƒÇw 00000418 FF 1E C7 77 00 00 00 00 ÿÇw.... 00000420 93 9F E7 77 D8 05 E8 77 “ŸçwØèw 00000428 FD A5 E7 77 AD A9 E9 77 ý¥çw©éw 00000430 A3 36 E7 77 03 38 E7 77 £6çw>8çw 00000438 41 E3 E6 77 60 8D E7 77 Aãæw`çw 00000440 E6 1B E6 77 2B 2A E7 77 ææw+*çw 00000448 7A 17 E6 77 79 C8 E6 77 zæwyÈæw 00000450 14 1B E7 77 C1 30 E7 77 çwÁ0çw …
	`… … … SECTION[n]`	Collapse Copy Code … 0001BF00 63 00 2E 00 63 00 68 00 c...c.h. 0001BF08 6D 00 0A 00 43 00 61 00 m...C.a. 0001BF10 6C 00 63 00 75 00 6C 00 l.c.u.l. 0001BF18 61 00 74 00 6F 00 72 00 a.t.o.r. 0001BF20 11 00 4E 00 6F 00 74 00 .N.o.t. 0001BF28 20 00 45 00 6E 00 6F 00 .E.n.o. 0001BF30 75 00 67 00 68 00 20 00 u.g.h. . 0001BF38 4D 00 65 00 6D 00 6F 00 M.e.m.o. 0001BF40 72 00 79 00 00 00 00 00 r.y..... 0001BF48 00 00 00 00 00 00 00 00 ........ 0001BF50 00 00 00 00 00 00 00 00 ........ 0001BF58 00 00 00 00 00 00 00 00 ........ 0001BF60 00 00 00 00 00 00 00 00 ........ 0001BF68 00 00 00 00 00 00 00 00 ........ 0001BF70 00 00 00 00 00 00 00 00 ........ 0001BF78 00 00 00 00 00 00 00 00 ........

2.2 The Windows NT data

As mentioned in the preceding section, e_lfanew storage in the MS-DOS data structure refers to the location of the Windows NT information. Hence, if you assume that the pMem pointer relates the start point of the memory space for a selected portable executable file, you can retrieve the MS-DOS header and also the Windows NT headers by the following lines, which you also can perceive in the PE viewer sample (pelib.cpp, PEStructure::OpenFileName()):

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IMAGE_DOS_HEADER        image_dos_header;
IMAGE_NT_HEADERS        image_nt_headers;
PCHAR pMem;
…
memcpy(&image_dos_header, pMem, 
       sizeof(IMAGE_DOS_HEADER));
memcpy(&image_nt_headers,
       pMem+image_dos_header.e_lfanew, 
       sizeof(IMAGE_NT_HEADERS));

It seems to be very simple, the retrieval of the headers information. I recommend inspecting the MSDN library regarding the IMAGE_NT_HEADERS structure definition. It makes comprehensible to grasp what the image NT header maintains to execute a code inside the Windows NT OS. Now, you are conversant with the Windows NT structure, it consists of the "PE" Signature, the File Header, and the Optional Header. Do not forget to take a glimpse at their comments in the MSDN Library and besides in Table 1.

One the whole, I consider merely, on the most circumstances, the following cells of the IMAGE_NT_HEADERS structure:

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FileHeader->NumberOfSections
OptionalHeader->AddressOfEntryPoint
OptionalHeader->ImageBase
OptionalHeader->SectionAlignment
OptionalHeader->FileAlignment
OptionalHeader->SizeOfImage
OptionalHeader->
  DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT]->VirtualAddress
OptionalHeader->DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT]->Size

You can observe clearly, the main purpose of these values, and their role when the internal virtual memory space allocated for an EXE file by the Windows OS is fully allocated, if you pay attention to their explanations in MSDN library, so I am not going to repeat the MSDN annotations here.

I should mention a brief comment regarding the PE data directories, or OptionalHeader-> DataDirectory[], as I think there are a few aspects of interest concerning them. When you come to survey the Optional header through the Windows NT information, you will find that there are 16 directories at the end of the Optional Header, where you can find the consecutive directories, including their Relative Virtual Address and Size. I just mention here, the notes from <winnt.h> to clarify these information:

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#define IMAGE_DIRECTORY_ENTRY_EXPORT          0   // Export Directory
#define IMAGE_DIRECTORY_ENTRY_IMPORT          1   // Import Directory
#define IMAGE_DIRECTORY_ENTRY_RESOURCE        2   // Resource Directory
#define IMAGE_DIRECTORY_ENTRY_EXCEPTION       3   // Exception Directory
#define IMAGE_DIRECTORY_ENTRY_SECURITY        4   // Security Directory
#define IMAGE_DIRECTORY_ENTRY_BASERELOC       5   // Base Relocation Table
#define IMAGE_DIRECTORY_ENTRY_DEBUG           6   // Debug Directory
#define IMAGE_DIRECTORY_ENTRY_ARCHITECTURE    7   // Architecture Specific Data
#define IMAGE_DIRECTORY_ENTRY_GLOBALPTR       8   // RVA of GP
#define IMAGE_DIRECTORY_ENTRY_TLS             9   // TLS Directory
#define IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG    10   // Load Configuration Directory
#define IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT   11   // Bound Import Directory in headers
#define IMAGE_DIRECTORY_ENTRY_IAT            12   // Import Address Table
#define IMAGE_DIRECTORY_ENTRY_DELAY_IMPORT   13   // Delay Load Import Descriptors
#define IMAGE_DIRECTORY_ENTRY_COM_DESCRIPTOR 14   // COM Runtime descriptor

The last one (15) was reserved for use in future; I have not yet seen any purpose to use it even in PE64.

For instance, if you desire to perceive the relative virtual address (RVA) and the size of the resource data, it is enough to retrieve them by:

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DWORD dwRVA = image_nt_headers.OptionalHeader->
  DataDirectory[IMAGE_DIRECTORY_ENTRY_RESOURCE]->VirtualAddress;
DWORD dwSize = image_nt_headers.OptionalHeader->
  DataDirectory[IMAGE_DIRECTORY_ENTRY_RESOURCE]->Size;

To comprehend more regarding the significance of data directories, I forward you to section 3.4.3, Microsoft Portable Executable and the Common Object File Format Specification document by Microsoft, and furthermore section 6 of this document, where you discern the various types of sections and their applications. We will discuss the section's advantage subsequently.

2.3 The Section Headers and Sections

We currently observe how the portable executable files declare the location and the size of a section on a disk storage file and inside the virtual memory space allocated for the program with IMAGE_NT_HEADERS-> OptionalHeader->SizeOfImage by the Windows task manager, as well the characteristics to demonstrate the type of the section. To understand better the Section header as my previous declaration, I suggest having a short gape on the IMAGE_SECTION_HEADER structure definition in the MSDN library. For an EXE packer developer, VirtualSize, VirtualAddress, SizeOfRawData, PointerToRawData, and Characteristics cells have significant rules. While developing an EXE packer, you should be clever enough to play with them. There are somethings to be noted while you modify them; you should take care to align the VirtualSize and VirtualAddress according to OptionalHeader->SectionAlignment, as well as SizeOfRawData and PointerToRawData in line with OptionalHeader->FileAlignment. Otherwise, you will corrupt your target EXE file and it will never run. Regarding Characteristics, I pay attention mostly to establish a section by IMAGE_SCN_MEM_READ | IMAGE_SCN_MEM_WRITE | IMAGE_SCN_CNT_INITIALIZED_DATA, I prefer my new section has ability to initialize such data during running process; such as import table; besides, I need it to be able to modify itself by the loader with my settings in the section characteristics to read- and writeable.

Moreover, you should pay attention to the section names, you can know the purpose of each section by its name. I will just forward you to section 6: Microsoft Portable Executable and the Common Object File Format Specification documents. I believe, it represents the totality of sections by their names, Table 2.

Table 2 - Section names

".text"	Code Section
"CODE"	Code Section of file linked by Borland Delphi or Borland Pascal
".data"	Data Section
"DATA"	Data Section of file linked by Borland Delphi or Borland Pascal
".rdata"	Section for Constant Data
".idata"	Import Table
".edata"	Export Table
".tls"	TLS Table
".reloc"	Relocation Information
".rsrc"	Resource Information

To comprehend the section headers and also the sections, you can run the sample PE viewer. By this PE viewer, you only can realize the application of the section headers in a file image, so to observe the main significance in the Virtual Memory, you should try to load a PE file by a debugger, and the next section represents the main idea of using the virtual address and –size in the virtual memory by using a debugger. The last note is about IMAGE_NT_HEADERS-> FileHeader-><CODE>NumberOfSections, that provides a number of sections in a PE file, do not forget to adjust it whenever you remove or add some sections to a PE file, I am talking about section injection!

3 Debugger, Disassembler and some Useful Tools

In this part, you will become familiar with the necessary and essential equipments to develop your PE tools.

3.1 Debuggers

The first essential prerequisite, to become a PE tools developer, is to have enough experience with bug tracer tools. Furthermore, you should know most of the assembly instructions. To me, the Intel documents are the best references. You can obtain them from the Intel site for IA-32, and on top of that IA-64; the future belongs to IA-64 CPUs, Windows XP 64-bit, and also PE64!

IA-32 Intel Architecture Software Developer’s Manuals.
Intel Itanium Architecture Assembly Language Reference Guide.
The Intel Itanium Processor Developer Resource Guide.

To trace a PE file, SoftICE by Compuware Corporation, I knew it also as named NuMega when I was at high school, is the best debugger in the world. It implements process tracing by using kernel mode method debugging without applying Windows debugging application programming interface (API) functions. In addition, I am going to introduce one perfect debugger in user mode level. It utilizes the Windows debugging API to trace a PE file and also attaches itself to an active process. These API functions have been provided by Microsoft teams, inside the Windows Kernel32 library, to trace a specific process, by using Microsoft tools, or perhaps, to make your own debugger! Some of those API functions inlude: CreateThread(), CreateProcess(), OpenProcess(), DebugActiveProcess(), GetThreadContext(), SetThreadContext(), ContinueDebugEvent(), DebugBreak(), ReadProcessMemory(), WriteProcessMemory(), SuspendThread(), and ResumeThread().

3.1.1 SoftICE

It was in 1987; Frank Grossman and Jim Moskun decided to establish a company called NuMega Technologies in Nashua, NH, in order to develop some equipments to trace and test the reliability of Microsoft Windows software programs. Now, it is a part of Compuware Corporation and its product has participated to accelerate the reliability in Windows software, and additionally in Windows driver developments. Currently, everyone knows the Compuware DriverStudio which is used to establish an environment for implementing the elaboration of a kernel driver or a system file by aiding the Windows Driver Development Kit (DDK). It bypasses the involvement of DDK to implement a portable executable file of kernel level for a Windows system software developer. For us, only one instrument of DriverStudio is important, SoftICE, this debugger can be used to trace every portable executable file, a PE file for user mode level or a PE file for kernel mode level.

Figure 1 - SoftICE Window

EAX=00000000 EBX=7FFDD000 ECX=0007FFB0 EDX=7C90EB94 ESI=FFFFFFFF
EDI=7C919738 EBP=0007FFF0 ESP=0007FFC4 EIP=010119E0 o d i s z a p c
CS=0008 DS=0023 SS=0010 ES=0023 FS=0030 GS=0000 SS:0007FFC4=87C816D4F

0023:01013000 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0023:01013010 01 00 00 00 20 00 00 00-0A 00 00 00 0A 00 00 00 ................
0023:01013020 20 00 00 00 00 00 00 00-53 63 69 43 61 6C 63 00 ........SciCalc.
0023:01013030 00 00 00 00 00 00 00 00-62 61 63 6B 67 72 6F 75 ........backgrou
0023:01013040 6E 64 00 00 00 00 00 00-2E 00 00 00 00 00 00 00 nd..............

0010:0007FFC4 4F 6D 81 7C 38 07 91 7C-FF FF FF FF 00 90 FD 7F Om |8 ‘| .
0010:0007FFD4 ED A6 54 80 C8 FF 07 00-E8 B4 F5 81 FF FF FF FF T .
0010:0007FFE4 F3 99 83 7C 58 6D 81 7C-00 00 00 00 00 00 00 00 Xm |........
0010:0007FFF4 00 00 00 00 E0 19 01 01-00 00 00 00 00 00 00 00 .... ....

010119E0 PUSH EBP
010119E1 MOV EBP,ESP
010119E3 PUSH -1
010119E5 PUSH 01001570
010119EA PUSH 01011D60
010119EF MOV EAX,DWORD PTR FS:[0]
010119F5 PUSH EAX
010119F6 MOV DWORD PTR FS:[0],ESP
010119FD ADD ESP,-68
01011A00 PUSH EBX
01011A01 PUSH ESI
01011A02 PUSH EDI
01011A03 MOV DWORD PTR SS:[EBP-18],ESP
01011A06 MOV DWORD PTR SS:[EBP-4],0

3.1.2 OllyDbg

It was about 4 years ago, that I first saw this debugger by chance. For me, it was the best choice, I was not so wealthy to purchase SoftICE, and at that time, SoftICE only had good functions for DOS, Windows 98, and Windows 2000. I found that this debugger supported all kinds of Windows versions. Therefore, I started to learn it very fast, and now it is my favorite debugger for the Windows OS. It is a debugger that can be used to trace all kinds of portable executable files except a Common Language Infrastructure (CLI) file format in user mode level, by using the Windows debugging API. Oleh Yuschuk, the author, is one of worthiest software developers I have seen in my life. He is a Ukrainian who now lives in Germany. I should mention here that his debugger is the best choice for hacker and cracker parties around the world! It is a freeware! You can try it from OllyDbg Homepage.

Figure 2 - OllyDbg CPU Window

3.1.3 Which parts are important in a debugger interface?

I have introduced two debuggers without talking about how you can employ them, and also which parts you should pay attention more. Regarding using debuggers, I refer you to their instructions in help documents. However, I want to explain shortly the important parts of a debugger; of course, I am talking about low-level debuggers, or in other words, machine-language debuggers of the x86 CPU families.

All of low-level debuggers consist of the following subdivisions:

Registers viewer.

EAX

ECX

EDX

EBX

ESP

EBP

ESI

EDI

EIP

o d t s z a p c

Disassembler or Code viewer.

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010119E0 PUSH EBP
010119E1 MOV EBP,ESP
010119E3 PUSH -1
010119E5 PUSH 01001570
010119EA PUSH 01011D60
010119EF MOV EAX,DWORD PTR FS:[0]
010119F5 PUSH EAX
010119F6 MOV DWORD PTR FS:[0],ESP
010119FD ADD ESP,-68
01011A00 PUSH EBX
01011A01 PUSH ESI
01011A02 PUSH EDI
01011A03 MOV DWORD PTR SS:[EBP-18],ESP
01011A06 MOV DWORD PTR SS:[EBP-4],0

Memory watcher.

Stack viewer.

Command line, command buttons, or shortcut keys to follow the debugging process.

Command	SoftICE	OllyDbg
Run	F5	F9
Step Into	F11	F7
Step Over	F10	F8
Set Break Point	F8	F2

You can compare Figure 1 and Figure 2 to distinguish the difference between SoftICE and OllyDbg. When you want to trace a PE file, you should mostly consider these five subdivisions. Furthermore, every debugger comprises of some other useful parts; you should discover them by yourself.

3.2 Disassembler

We can consider OllyDbg and SoftICE as excellent disassemblers, but I also want to introduce another disassembler tool which is famous in the reverse engineering world.

3.2.1 Proview disassembler

Proview or PVDasm is an admirable disassembler by the Reverse-Engineering-Community; it is still under development and bug fixing. You can find its disassmbler source engine and employ it to create your own disassembler.

3.2.2 W32Dasm

W32DASM can disassemble both 16 and 32 bit executable file formats. In addition to its disassembling ability, you can employ it to analyze import, export and resource data directories data.

3.2.3 IDA Pro

All reverse-engineering experts know that IDA Pro can be used to investigate, not only x86 instructions, but that of various kinds of CPU types like AVR, PIC, and etc. It can illustrate the assembly source of a portable executable file by using colored graphics and tables, and is very useful for any newbie in this area. Furthermore, it has the capability to trace an executable file inside the user mode level in the same way as OllyDbg.

3.3 Some Useful Tools

A good PE tools developer is conversant with the tools which save his time, so I recommend to select some appropriate instruments to investigate the base information under a portable executable file.

3.3.1 LordPE

LordPE by y0da is still the first choice to retrieve PE file information with the possibility to modify them.

3.3.2 PEiD

PE iDentifier is valuable to identify the type of compilers, packers, and cryptors of PE files. As of now, it can detect more than 500 different signature types of PE files.

3.3.3 Resource Hacker

Resource Hacker can be employed to modify resource directory information; icon, menu, version info, string table, and etc.

3.3.4 WinHex

WinHex, it is clear what you can do with this tool.

3.3.5 CFF Explorer

Eventually, CFF Explorer by Ntoskrnl is what you wish to have as a PE Utility tool in your dream; it supports PE32/64, PE rebuild included Common Language Infrastructure (CLI) file, in other words, the .NET file, a resource modifier, and much more facilities which can not be found in others, just try and discover every unimaginable option by hand.

4 Add new section and Change OEP

We are ready to do the first step of making our project. So I have provided a library to add a new section and rebuild the portable executable file. Before starting, I like you get familiar with the headers of a PE file, by using OllyDbg. You should first open a PE file, that pops up a menu, View->Executable file, again get a popup menu Special->PE header. And you will observe a scene similar to Figure 3. Now, come to Main Menu View->Memory, try to distinguish the sections inside the Memory map window.

Figure 3

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 ASCII "MZ"
 DW 0090
 DW 0003
 DW 0000
 DW 0004
 DW 0000
 DW FFFF
 DW 0000
 DW 00B8
 DW 0000
 DW 0000
 DW 0000
 DW 0040
 DW 0000
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DB 00
 DD 000000F0

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 DOS EXE Signature
 DOS_PartPag = 90 (144.)
 DOS_PageCnt = 3
 DOS_ReloCnt = 0
 DOS_HdrSize = 4
 DOS_MinMem = 0
 DOS_MaxMem = FFFF (65535.)
 DOS_ReloSS = 0
 DOS_ExeSP = B8
 DOS_ChkSum = 0
 DOS_ExeIP = 0
 DOS_ReloCS = 0
 DOS_TablOff = 40
 DOS_Overlay = 0
































 Offset to PE signature

I want to explain how we can plainly change the Offset of Entry Point (OEP) in our sample file, CALC.EXE of Windows XP. First, by using a PE Tool, and also using our PE Viewer, we find OEP, 0x00012475, and Image Base, 0x01000000. This value of OEP is the Relative Virtual Address, so the Image Base value is used to convert it to the Virtual Address.

Virtual_Address = Image_Base + Relative_Virtual_Address

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DWORD OEP_RVA = image_nt_headers->OptionalHeader.AddressOfEntryPoint ; 
// OEP_RVA = 0x00012475
DWORD OEP_VA = image_nt_headers->OptionalHeader.ImageBase + OEP_RVA ; 
// OEP_VA = 0x01000000 + 0x00012475 = 0x01012475

PE Maker - Step 1

Download source files - 54.7 Kb

CALC.EXE - test file

Download test PE file - 49.5 Kb

DynLoader(), in loader.cpp, is reserved for the data of the new section, in other words, the Loader.

DynLoader Step 1

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__stdcall void DynLoader()
{
_asm
{
//----------------------------------
    DWORD_TYPE(DYN_LOADER_START_MAGIC)
//----------------------------------
    MOV EAX,01012475h // << Original OEP
    JMP EAX
//----------------------------------
    DWORD_TYPE(DYN_LOADER_END_MAGIC)
//----------------------------------
}
}

Unfortunately, this source can only be applied for the sample test file. We should complete it by saving the value of the original OEP in the new section, and use it to reach the real OEP. I have accomplished it in Step 2 (Section 5).

4.1 Retrieve and Rebuild PE file

I have made a simple class library to recover PE information and to use it in a new PE file.

CPELibrary Class Step 1

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//----------------------------------------------------------------
class CPELibrary 
{
private:
    //-----------------------------------------
    PCHAR                   pMem;
    DWORD                   dwFileSize;
    //-----------------------------------------
protected:
    //-----------------------------------------
    PIMAGE_DOS_HEADER       image_dos_header;
    PCHAR                   pDosStub;
    DWORD                   dwDosStubSize, dwDosStubOffset;
    PIMAGE_NT_HEADERS       image_nt_headers;
    PIMAGE_SECTION_HEADER   image_section_header[MAX_SECTION_NUM];
    PCHAR                   image_section[MAX_SECTION_NUM];
    //-----------------------------------------
protected:
    //-----------------------------------------
    DWORD PEAlign(DWORD dwTarNum,DWORD dwAlignTo);
    void AlignmentSections();
    //-----------------------------------------
    DWORD Offset2RVA(DWORD dwRO);
    DWORD RVA2Offset(DWORD dwRVA);
    //-----------------------------------------
    PIMAGE_SECTION_HEADER ImageRVA2Section(DWORD dwRVA);
    PIMAGE_SECTION_HEADER ImageOffset2Section(DWORD dwRO);
    //-----------------------------------------
    DWORD ImageOffset2SectionNum(DWORD dwRVA);
    PIMAGE_SECTION_HEADER AddNewSection(char* szName,DWORD dwSize);
    //-----------------------------------------
public:
    //-----------------------------------------
    CPELibrary();
    ~CPELibrary();
    //-----------------------------------------
    void OpenFile(char* FileName);
    void SaveFile(char* FileName);    
    //-----------------------------------------
};

By Table 1, the usage of image_dos_header, pDosStub, image_nt_headers, image_section_header [MAX_SECTION_NUM], and image_section[MAX_SECTION_NUM] is clear. We use OpenFile() and SaveFile() to retrieve and rebuild a PE file. Furthermore, AddNewSection() is employed to create the new section, the important step.

4.2 Create Data for new Section

In pecrypt.cpp, I have represented another class, CPECryptor, to comprise the data of the new section. Nevertheless, the data of the new section is created by DynLoader() in loader.cpp, DynLoader Step 1. We use the CPECryptor class to enter this data in to the new section, and also some other stuff.

CPECryptor Class Step 1

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//----------------------------------------------------------------
class CPECryptor: public CPELibrary
{
private:
    //----------------------------------------
    PCHAR pNewSection;
    //----------------------------------------
    DWORD GetFunctionVA(void* FuncName);
    void* ReturnToBytePtr(void* FuncName, DWORD findstr);
    //----------------------------------------
protected:
    //----------------------------------------
public:    
    //----------------------------------------
    void CryptFile(int(__cdecl *callback) (unsigned int, unsigned int));
    //----------------------------------------
};
//----------------------------------------------------------------

4.3 Some notes regarding creating a new PE file

Align the VirtualAddress and the VirtualSize of each section by SectionAlignment:

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image_section_header[i]->VirtualAddress=
    PEAlign(image_section_header[i]->VirtualAddress,
    image_nt_headers->OptionalHeader.SectionAlignment);

image_section_header[i]->Misc.VirtualSize=
    PEAlign(image_section_header[i]->Misc.VirtualSize,
    image_nt_headers->OptionalHeader.SectionAlignment);

Align the PointerToRawData and the SizeOfRawData of each section by FileAlignment:

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image_section_header[i]->PointerToRawData =
    PEAlign(image_section_header[i]->PointerToRawData,
            image_nt_headers->OptionalHeader.FileAlignment);

image_section_header[i]->SizeOfRawData =
    PEAlign(image_section_header[i]->SizeOfRawData,
            image_nt_headers->OptionalHeader.FileAlignment);

Correct the SizeofImage by the virtual size and the virtual address of the last section:

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image_nt_headers->OptionalHeader.SizeOfImage = 
          image_section_header[LastSection]->VirtualAddress +
          image_section_header[LastSection]->Misc.VirtualSize;

Set the Bound Import Directory header to zero, as this directory is not very important to execute a PE file:

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image_nt_headers->
  OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT].
  VirtualAddress = 0;
image_nt_headers->
  OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT].Size = 0;

4.4 Some notes regarding linking this VC Project

Set Linker->General->Enable Incremental Linking to No (/INCREMENTAL:NO).

You can comprehend the difference between incremental link and no-incremental link by looking at the following picture:

To acquire the virtual address of DynLoader(), we obtain the virtual address of JMP pemaker.DynLoader in the incremental link, but by no-incremental link, the real virtual address is gained by the following code:

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```
DWORD dwVA= (DWORD) DynLoader;
```
This setting is more critical in the incremental link when you try to find the beginning and ending of the Loader, DynLoader(), by CPECryptor::ReturnToBytePtr():

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```
void* CPECryptor::ReturnToBytePtr(void* FuncName, DWORD findstr)
{
    void* tmpd;
    __asm
   {
        mov eax, FuncName
        jmp df
hjg:    inc eax
df:     mov ebx, [eax]
        cmp ebx, findstr
        jnz hjg
        mov tmpd, eax
    }
    return tmpd;
}
```

5 Store Important Data and Reach Original OEP

Right now, we save the Original OEP and also the Image Base in order to reach to the virtual address of OEP. I have reserved a free space at the end of DynLoader() to store them, DynLoader Step 2.

PE Maker - Step 2

Download source files - 58.3 Kb

DynLoader Step 2

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__stdcall void DynLoader()
{
_asm
{
//----------------------------------
    DWORD_TYPE(DYN_LOADER_START_MAGIC)
//----------------------------------
Main_0:
    PUSHAD
    // get base ebp
    CALL Main_1
Main_1:    
    POP EBP
    SUB EBP,OFFSET Main_1
    MOV EAX,DWORD PTR [EBP+_RO_dwImageBase]
    ADD EAX,DWORD PTR [EBP+_RO_dwOrgEntryPoint]
    PUSH EAX
    RETN // >> JMP to Original OEP
//----------------------------------
    DWORD_TYPE(DYN_LOADER_START_DATA1)
//----------------------------------
_RO_dwImageBase:                DWORD_TYPE(0xCCCCCCCC)
_RO_dwOrgEntryPoint:            DWORD_TYPE(0xCCCCCCCC)
//----------------------------------
    DWORD_TYPE(DYN_LOADER_END_MAGIC)
//----------------------------------
}
}

The new function, CPECryptor::CopyData1(), will implement the copy of the Image Base value and the Offset of Entry Point value into 8 bytes of free space in the loader.

5.1 Restore the first Registers Context

It is important to recover the Original Context of the thread. We have not yet done it in the DynLoader Step 2 source code. We can modify the source of DynLoader() to repossess the first Context.

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__stdcall void DynLoader()
{
_asm
{
//----------------------------------
    DWORD_TYPE(DYN_LOADER_START_MAGIC)
//----------------------------------
Main_0:
    PUSHAD// Save the registers context in stack
    CALL Main_1
Main_1:    
    POP EBP// Get Base EBP
    SUB EBP,OFFSET Main_1
    MOV EAX,DWORD PTR [EBP+_RO_dwImageBase]
    ADD EAX,DWORD PTR [EBP+_RO_dwOrgEntryPoint]
    MOV DWORD PTR [ESP+1Ch],EAX // pStack.Eax <- EAX
    POPAD // Restore the first registers context from stack
    PUSH EAX
    XOR  EAX, EAX
    RETN // >> JMP to Original OEP
//----------------------------------
    DWORD_TYPE(DYN_LOADER_START_DATA1)
//----------------------------------
_RO_dwImageBase:                DWORD_TYPE(0xCCCCCCCC)
_RO_dwOrgEntryPoint:            DWORD_TYPE(0xCCCCCCCC)
//----------------------------------
    DWORD_TYPE(DYN_LOADER_END_MAGIC)
//----------------------------------
}
}

5.2 Restore the Original Stack

We can also recover the original stack by setting the value of the beginning stack + 0x34 to the Original OEP, but it is not very important. Nevertheless, in the following code, I have accomplished the loader code by a simple trick to reach OEP in addition to redecorating the stack. You can observe the implementation by tracing using OllyDbg or SoftICE.

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__stdcall void DynLoader()
{
_asm
{
//----------------------------------
    DWORD_TYPE(DYN_LOADER_START_MAGIC)
//----------------------------------
Main_0:
    PUSHAD // Save the registers context in stack
    CALL Main_1
Main_1:    
    POP EBP
    SUB EBP,OFFSET Main_1
    MOV EAX,DWORD PTR [EBP+_RO_dwImageBase]
    ADD EAX,DWORD PTR [EBP+_RO_dwOrgEntryPoint]
    MOV DWORD PTR [ESP+54h],EAX // pStack.Eip <- EAX
    POPAD // Restore the first registers context from stack
    CALL _OEP_Jump
    DWORD_TYPE(0xCCCCCCCC)
_OEP_Jump:
    PUSH EBP
    MOV EBP,ESP
    MOV EAX,DWORD PTR [ESP+3Ch] // EAX <- pStack.Eip
    MOV DWORD PTR [ESP+4h],EAX  // _OEP_Jump RETURN pointer <- EAX
    XOR EAX,EAX
    LEAVE
    RETN
//----------------------------------
    DWORD_TYPE(DYN_LOADER_START_DATA1)
//----------------------------------
_RO_dwImageBase:                DWORD_TYPE(0xCCCCCCCC)
_RO_dwOrgEntryPoint:            DWORD_TYPE(0xCCCCCCCC)
//----------------------------------
    DWORD_TYPE(DYN_LOADER_END_MAGIC)
//----------------------------------
}
}

5.3 Approach OEP by Structured Exception Handling

An exception is generated when a program falls into a fault code execution and an error happens, so in such a special condition, the program immediately jumps to a function called the exception handler from exception handler list of the Thread Information Block.

The next example of a try-except statement in C++ clarifies the operation of structured exception handling. Besides the assembly code of this code, it elucidates the structured exception handler installation, the raise of an exception, and the exception handler function.

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#include "stdafx.h"
#include "windows.h"

void RAISE_AN_EXCEPTION()
{    
_asm
{
    INT 3
    INT 3
    INT 3
    INT 3
}
}

int _tmain(int argc, _TCHAR* argv[])
{
    __try
    {
        __try{
            printf("1: Raise an Exception/n");
            RAISE_AN_EXCEPTION();
        }
        __finally
        {
            printf("2: In Finally/n");
        }
    }
    __except( printf("3: In Filter/n"), EXCEPTION_EXECUTE_HANDLER )
    {
        printf("4: In Exception Handler/n");
    }
    return 0;
}

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; main()
00401000: PUSH EBP
00401001: MOV EBP,ESP
00401003: PUSH -1
00401005: PUSH 00407160
; __try {
; the structured exception handler (SEH) installation 
0040100A: PUSH _except_handler3  
0040100F: MOV EAX,DWORD PTR FS:[0]
00401015: PUSH EAX
00401016: MOV DWORD PTR FS:[0],ESP
0040101D: SUB ESP,8
00401020: PUSH EBX
00401021: PUSH ESI
00401022: PUSH EDI
00401023: MOV DWORD PTR SS:[EBP-18],ESP
;     __try {
00401026: XOR ESI,ESI
00401028: MOV DWORD PTR SS:[EBP-4],ESI
0040102B: MOV DWORD PTR SS:[EBP-4],1
00401032: PUSH OFFSET "1: Raise an Exception"
00401037: CALL printf
0040103C: ADD ESP,4
; the raise a exception, INT 3 exception
; RAISE_AN_EXCEPTION()
0040103F: INT3      
00401040: INT3
00401041: INT3
00401042: INT3
;     } __finally {
00401043: MOV DWORD PTR SS:[EBP-4],ESI
00401046: CALL 0040104D
0040104B: JMP 00401080
0040104D: PUSH OFFSET "2: In Finally"
00401052: CALL printf
00401057: ADD ESP,4
0040105A: RETN
;     }
; }
; __except( 
0040105B: JMP 00401080
0040105D: PUSH OFFSET "3: In Filter"
00401062: CALL printf
00401067: ADD ESP,4
0040106A: MOV EAX,1 ; EXCEPTION_EXECUTE_HANDLER = 1
0040106F: RETN
;     , EXCEPTION_EXECUTE_HANDLER )
; {
; the exception handler funtion
00401070: MOV ESP,DWORD PTR SS:[EBP-18]
00401073: PUSH OFFSET "4: In Exception Handler"
00401078: CALL printf
0040107D: ADD ESP,4
; }
00401080: MOV DWORD PTR SS:[EBP-4],-1
0040108C: XOR EAX,EAX
; restore previous SEH
0040108E: MOV ECX,DWORD PTR SS:[EBP-10]
00401091: MOV DWORD PTR FS:[0],ECX
00401098: POP EDI
00401099: POP ESI
0040109A: POP EBX
0040109B: MOV ESP,EBP
0040109D: POP EBP
0040109E: RETN

Make a Win32 console project, and link and run the preceding C++ code, to perceive the result:

1: Raise an Exception
3: In Filter
2: In Finally
4: In Exception Handler
_

This program runs the exception expression, printf("3: In Filter/n");, when an exception happens, in this example the INT 3 exception. You can employ other kinds of exception too. In OllyDbg, Debugging options->Exceptions, you can see a short list of different types of exceptions.

5.3.1 Implement Exception Handler

We desire to construct a structured exception handler in order to reach OEP. Now, I think you have distinguished the SEH installation, the exception raise, and the exception expression filter, by foregoing the assembly code. To establish our exception handler approach, we need to comprise the following codes:

SEH installation:

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    LEA EAX,[EBP+_except_handler1_OEP_Jump]
    PUSH EAX
    PUSH DWORD PTR FS:[0]
    MOV DWORD PTR FS:[0],ESP

An Exception Raise:
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```
    INT 3
```

Exception handler expression filter:

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_except_handler1_OEP_Jump:
    PUSH EBP
    MOV EBP,ESP
    ...
    MOV EAX, EXCEPTION_CONTINUE_SEARCH // EXCEPTION_CONTINUE_SEARCH = 0
    LEAVE
    RETN

So we yearn for making the ensuing C++ code in assembly language to inaugurate our engine to approach the Offset of Entry Point by SEH.

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__try // SEH installation
{
    __asm 
    {
        INT 3 // An Exception Raise
    }
}
__except( ..., EXCEPTION_CONTINUE_SEARCH ){}
// Exception handler expression filter

In assembly code...

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    ; ----------------------------------------------------
    ; the structured exception handler (SEH) installation
    ; __try {
    LEA EAX,[EBP+_except_handler1_OEP_Jump]
    PUSH EAX
    PUSH DWORD PTR FS:[0]
    MOV DWORD PTR FS:[0],ESP
    ; ----------------------------------------------------
    ; the raise a INT 3 exception
    INT 3
    INT 3
    INT 3
    INT 3
    ; }     ; __except( ... 
    ; ----------------------------------------------------
    ; exception handler expression filter
_except_handler1_OEP_Jump:
    PUSH EBP
    MOV EBP,ESP
    ... 
    MOV EAX, EXCEPTION_CONTINUE_SEARCH ; EXCEPTION_CONTINUE_SEARCH = 0
    LEAVE
    RETN
    ; , EXCEPTION_CONTINUE_SEARCH ) { }

The exception value, __except(..., Value), determines how the exception is handled, it can have three values, 1, 0, -1. To understand them, refer to the try-except statement description in the MSDN library. We set it to EXCEPTION_CONTINUE_SEARCH (0), not to run the exception handler function, therefore by this value, the exception is not recognized, is simply ignored, and the thread continues its code-execution.

How the SEH installation is implemented

As you perceived from the illustrated code, the SEH installation is done by the FS segment register. Microsoft Windows 32 bit uses the FS segment register as a pointer to the data block of the main thread. The first 0x1C bytes comprise the information of the Thread Information Block (TIB). Therefore, FS:[00h] refers to ExceptionList of the main thread, Table 3. In our code, we have pushed the pointer to _except_handler1_OEP_Jump in the stack and changed the value of ExceptionList, FS:[00h], to the beginning of the stack, ESP.

Thread Information Block (TIB)

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typedef struct _NT_TIB32 {
    DWORD ExceptionList;
    DWORD StackBase;
    DWORD StackLimit;
    DWORD SubSystemTib;
    union {
        DWORD FiberData;
        DWORD Version;
    };
    DWORD ArbitraryUserPointer;
    DWORD Self;
} NT_TIB32, *PNT_TIB32;

Table 3 - FS segment register and Thread Information Block

DWORD PTR FS:[00h]	ExceptionList
DWORD PTR FS:[04h]	StackBase
DWORD PTR FS:[08h]	StackLimit
DWORD PTR FS:[0Ch]	SubSystemTib
DWORD PTR FS:[10h]	FiberData / Version
DWORD PTR FS:[14h]	ArbitraryUserPointer
DWORD PTR FS:[18h]	Self

5.3.2 Attain OEP by adjusting the Thread Context

In this part, we effectuate our performance by accomplishing the OEP approach. We change the Context of the thread and ignore every simple exception handling, and let the thread continue the execution, but in the original OEP!

When an exception happens, the context of the processor during the time of the exception is saved in the stack. By EXCEPTION_POINTERS, we have access to the pointer of ContextRecord. The ContextRecord has the CONTEXT data structure, Table 4, this is the thread context during the exception time. When we ignore the exception by EXCEPTION_CONTINUE_SEARCH (0), the instruction pointer as well the context will be set to ContextRecord in order to return to the previous condition. Therefore, if we change the Eip of the Win32 Thread Context to the Original Offset of Entry Point, it will come clearly into OEP.

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    MOV EAX, ContextRecord
    MOV EDI, dwOEP                   ; EAX <- dwOEP
    MOV DWORD PTR DS:[EAX+0B8h], EDI ; pContext.Eip <- EAX

Win32 Thread Context structure

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#define MAXIMUM_SUPPORTED_EXTENSION     512

typedef struct _CONTEXT {
    //-----------------------------------------
    DWORD ContextFlags;
    //-----------------------------------------
    DWORD   Dr0;
    DWORD   Dr1;
    DWORD   Dr2;
    DWORD   Dr3;
    DWORD   Dr6;
    DWORD   Dr7;
    //-----------------------------------------
    FLOATING_SAVE_AREA FloatSave;
    //-----------------------------------------
    DWORD   SegGs;
    DWORD   SegFs;
    DWORD   SegEs;
    DWORD   SegDs;
    //-----------------------------------------
    DWORD   Edi;
    DWORD   Esi;
    DWORD   Ebx;
    DWORD   Edx;
    DWORD   Ecx;
    DWORD   Eax;
    //-----------------------------------------
    DWORD   Ebp;
    DWORD   Eip;
    DWORD   SegCs;
    DWORD   EFlags;
    DWORD   Esp;
    DWORD   SegSs;
    //-----------------------------------------
    BYTE    ExtendedRegisters[MAXIMUM_SUPPORTED_EXTENSION];
    //----------------------------------------
} CONTEXT, 
*LPCONTEXT;

Table 4 - CONTEXT

Context Flags	0x00000000	`ContextFlags`
Context Debug Registers	0x00000004	`Dr0`
	0x00000008	`Dr1`
	0x0000000C	`Dr2`
	0x00000010	`Dr3`
	0x00000014	`Dr6`
	0x00000018	`Dr7`
Context Floating Point	0x0000001C	`FloatSave`	`StatusWord`
	0x00000020		`StatusWord`
	0x00000024		`TagWord`
	0x00000028		`ErrorOffset`
	0x0000002C		`ErrorSelector`
	0x00000030		`DataOffset`
	0x00000034		`DataSelector`
	0x00000038 ... 0x00000087		`RegisterArea` [0x50]
	0x00000088		`Cr0NpxState`
Context Segments	0x0000008C	`SegGs`
	0x00000090	`SegFs`
	0x00000094	`SegEs`
	0x00000098	`SegDs`
Context Integer	0x0000009C	`Edi`
	0x000000A0	`Esi`
	0x000000A4	`Ebx`
	0x000000A8	`Edx`
	0x000000AC	`Ecx`
	0x000000B0	`Eax`
Context Control	0x000000B4	`Ebp`
	0x000000B8	`Eip`
	0x000000BC	`SegCs`
	0x000000C0	`EFlags`
	0x000000C4	`Esp`
	0x000000C8	`SegSs`
Context Extended Registers	0x000000CC ... 0x000002CB	`ExtendedRegisters`[0x200]

By the following code, we have accomplished the main purpose of coming to OEP by the structured exception handler:

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__stdcall void DynLoader()
{
_asm
{
//----------------------------------
    DWORD_TYPE(DYN_LOADER_START_MAGIC)
//----------------------------------
Main_0:
    PUSHAD  // Save the registers context in stack
    CALL Main_1
Main_1:    
    POP EBP
    SUB EBP,OFFSET Main_1 // Get Base EBP
    MOV EAX,DWORD PTR [EBP+_RO_dwImageBase]
    ADD EAX,DWORD PTR [EBP+_RO_dwOrgEntryPoint]
    MOV DWORD PTR [ESP+10h],EAX    // pStack.Ebx <- EAX
    LEA EAX,[EBP+_except_handler1_OEP_Jump]
    MOV DWORD PTR [ESP+1Ch],EAX    // pStack.Eax <- EAX
    POPAD  // Restore the first registers context from stack
    //----------------------------------------------------
    // the structured exception handler (SEH) installation 
    PUSH EAX
    XOR  EAX, EAX
    PUSH DWORD PTR FS:[0]        // NT_TIB32.ExceptionList
    MOV DWORD PTR FS:[0],ESP    // NT_TIB32.ExceptionList <-ESP
    //----------------------------------------------------
    // the raise a INT 3 exception
    DWORD_TYPE(0xCCCCCCCC)
    //--------------------------------------------------------
// -------- exception handler expression filter ----------
_except_handler1_OEP_Jump:
    PUSH EBP
    MOV EBP,ESP
    //------------------------------
    MOV EAX,DWORD PTR SS:[EBP+010h]    // PCONTEXT: pContext <- EAX
    //==============================
    PUSH EDI
    // restore original SEH
    MOV EDI,DWORD PTR DS:[EAX+0C4h]    // pContext.Esp
    PUSH DWORD PTR DS:[EDI]
    POP DWORD PTR FS:[0]
    ADD DWORD PTR DS:[EAX+0C4h],8    // pContext.Esp
    //------------------------------
    // set the Eip to the OEP
    MOV EDI,DWORD PTR DS:[EAX+0A4h] // EAX <- pContext.Ebx
    MOV DWORD PTR DS:[EAX+0B8h],EDI // pContext.Eip <- EAX
    //------------------------------
    POP EDI
    //==============================
    MOV EAX, EXCEPTION_CONTINUE_SEARCH
    LEAVE
    RETN
//----------------------------------
    DWORD_TYPE(DYN_LOADER_START_DATA1)
//----------------------------------
_RO_dwImageBase:                DWORD_TYPE(0xCCCCCCCC)
_RO_dwOrgEntryPoint:            DWORD_TYPE(0xCCCCCCCC)
//----------------------------------
    DWORD_TYPE(DYN_LOADER_END_MAGIC)
//----------------------------------
}
}

6 Build an Import Table and Reconstruct the Original Import Table

To use the Windows dynamic link library (DLL) in Windows application programming, there are two ways:

Using Windows libraries by additional dependencies:

Using Windows dynamic link libraries in run-time:

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// DLL function signature
typedef HGLOBAL (*importFunction_GlobalAlloc)(UINT, SIZE_T);
...
importFunction_GlobalAlloc __GlobalAlloc;

// Load DLL file
HINSTANCE hinstLib = LoadLibrary("Kernel32.dll");
if (hinstLib == NULL)
{
    // Error - unable to load DLL
}

// Get function pointer
__GlobalAlloc = 
    (importFunction_GlobalAlloc)GetProcAddress(hinstLib,  
                                         "GlobalAlloc");
if (addNumbers == NULL) 
{
     // Error - unable to find DLL function
}

FreeLibrary(hinstLib);

When you make a Windows application project, the linker includes at least kernel32.dll in the base dependencies of your project. Without LoadLibrary() and GetProcAddress() of Kernel32.dll, we can not load a DLL in run-time. The dependencies information is stored in the import table section. By Dependency Walker, it is not so difficult to observe the DLL module and the functions which are imported into a PE file.

We attempt to establish our custom import table to conduct our project. Furthermore, we have to fix up the original import table at the end in order to run the real code of the program.

PE Maker - Step 3

Download source files - 65.4 Kb

6.1 Construct the Client Import Table

I strongly advise you to read the section 6.4 of the Microsoft Portable Executable and the Common Object File Format Specification document. This section contains the principal information to comprehend the import table performance.

The import table data is accessible by a second data directory of the optional header from PE headers, so you can access it by using the following code:

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DWORD dwVirtualAddress = image_nt_headers->
  OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT].VirtualAddress;
DWORD dwSize = image_nt_headers->
  OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT].Size;

The VirtualAddress refers to structures by IMAGE_IMPORT_DESCRIPTOR. This structure contains the pointer to the imported DLL name and the relative virtual address of the first thunk.

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typedef struct _IMAGE_IMPORT_DESCRIPTOR {
    union {
        DWORD   Characteristics;
        DWORD   OriginalFirstThunk;
    };
    DWORD   TimeDateStamp;
    DWORD   ForwarderChain;
    DWORD   Name;         // the imported DLL name
    DWORD   FirstThunk;   // the relative virtual address of the first thunk
} IMAGE_IMPORT_DESCRIPTOR, *PIMAGE_IMPORT_DESCRIPTOR;

When a program is running, the Windows task manager sets the thunks by the virtual address of the function. The virtual address is found by the name of the function. At first, the thunks hold the relative virtual address of the function name, Table 5; during execution, they are fixed up by the virtual address of the functions, Table 6.

Table 5 - The Import Table in file image

`IMAGE_IMPORT_ DESCRIPTOR[0]`	`OriginalFirstThunk`
	`TimeDateStamp`
	`ForwarderChain`
	`Name_RVA`	------>	"kernel32.dll",0
	`FirstThunk_RVA`	------>	`proc_1_name_RVA`	------>	0,0,"LoadLibraryA",0
			`proc_2_name_RVA`	------>	0,0,"GetProcAddress",0
			`proc_3_name_RVA`	------>	0,0,"GetModuleHandleA",0
			...
`IMAGE_IMPORT_ DESCRIPTOR[1]`
`...`
`IMAGE_IMPORT_ DESCRIPTOR[n]`

Table 6 - The Import Table in virtual memory

`IMAGE_IMPORT_DESCRIPTOR[0]`	`OriginalFirstThunk`
	`TimeDateStamp`
	`ForwarderChain`
	`Name_RVA`	`------>`	"kernel32.dll",0
	`FirstThunk_RVA`	`------>`	`proc_1_VA`
			`proc_2_VA`
			`proc_3_VA`
			`...`
`IMAGE_IMPORT_DESCRIPTOR[1]`
`...`
`IMAGE_IMPORT_DESCRIPTOR[n]`

We want to make a simple import table to import LoadLibrary(), and GetProcAddress() from Kernel32.dll. We need these two essential API functions to cover other API functions in run-time. The following assembly code shows how easily we can reach our solution:

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0101F000: 
00000000 ; OriginalFirstThunk
0101F004: 00000000 ; TimeDateStamp
0101F008: 00000000 ; ForwarderChain
0101F00C: 0001F034 ; Name;       ImageBase + 0001F034 -> 0101F034 -> "Kernel32.dll",0
0101F010: 0001F028 ; FirstThunk; ImageBase + 0001F028 -> 0101F028
0101F014: 00000000
0101F018: 00000000
0101F01C: 00000000
0101F020: 00000000
0101F024: 00000000
0101F028: 0001F041 ; ImageBase + 0001F041 -> 0101F041 -> 0,0,"LoadLibraryA",0
0101F02C: 0001F050 ; ImageBase + 0001F050 -> 0101F050 -> 0,0,"GetProcAddress",0
0101F030: 00000000
0101F034: 'K' 'e' 'r' 'n' 'e' 'l' '3' '2' '.' 'd' 'l' 'l' 00
0001F041: 00 00 'L' 'o' 'a' 'd' 'L' 'i' 'b' 'r' 'a' 'r' 'y' 'A' 
00
0001F050: 00 00 'G' 'e' 't' 'P' 'r' 'o' 'c' 'A' 'd' 'd' 'r' 'e' 's' 's'
 00

After running...

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0101F000: 
00000000 ; OriginalFirstThunk
0101F004: 00000000 ; TimeDateStamp
0101F008: 00000000 ; ForwarderChain
0101F00C: 0001F034 ; Name;       ImageBase + 0001F034 -> 0101F034 -> "Kernel32.dll",0
0101F010: 0001F028 ; FirstThunk; ImageBase + 0001F028 -> 0101F028
0101F014: 00000000
0101F018: 00000000
0101F01C: 00000000
0101F020: 00000000
0101F024: 00000000
0101F028: 7C801D77 ; -> Kernel32.LoadLibrary()
0101F02C: 7C80AC28 ; -> Kernel32.GetProcAddress()
0101F030: 00000000
0101F034: 'K' 'e' 'r' 'n' 'e' 'l' '3' '2' '.' 'd' 'l' 'l' 
00
0001F041: 00 00 'L' 'o' 'a' 'd' 'L' 'i' 'b' 'r' 'a' 'r' 'y' 'A'
 00
0001F050: 00 00 'G' 'e' 't' 'P' 'r' 'o' 'c' 'A' 'd' 'd' 'r' 'e' 's' 's'
 00

I have prepared a class library to make every import table by using a client string table. The CITMaker class library in itmaker.h, it will build an import table by sz_IT_EXE_strings and also the relative virtual address of the import table.

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static const char *sz_IT_EXE_strings[]=
{
    "Kernel32.dll",
    "LoadLibraryA",
    "GetProcAddress",
    0,,
    0,
};

We subsequently employ this class library to establish an import table to support DLLs and OCXs, so this is a general library to present all possible import tables easily. The next step is clarified in the following code.

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CITMaker *ImportTableMaker = new CITMaker( IMPORT_TABLE_EXE );
...
pimage_section_header=AddNewSection( ".xxx", dwNewSectionSize );
// build import table by the current virtual address
ImportTableMaker->Build
( pimage_section_header->VirtualAddress ); 
memcpy( pNewSection, ImportTableMaker->pMem, 
ImportTableMaker->dwSize );
...
memcpy( image_section[image_nt_headers->FileHeader.NumberOfSections-1], 
        pNewSection, 
        dwNewSectionSize );
...
image_nt_headers->OptionalHeader.
  DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT].VirtualAddress 
  = pimage_section_header->VirtualAddress;
image_nt_headers->OptionalHeader.
  DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT].Size 
  = ImportTableMaker->dwSize;
...
delete ImportTableMaker;

The import table is copied at the beginning of the new section, and the relevant data directory is adjusted to the relative virtual address of the new section and the size of the new import table

6.2 Using other API functions in run-time

At this time, we can load other DLLs and find the process address of other functions by using LoadLibrary() and GetProcAddress():

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lea edi, @"Kernel32.dll"
//-------------------
push edi
mov eax,offset _p_LoadLibrary
call [ebp+eax] //LoadLibrary(lpLibFileName);
//-------------------
mov esi,eax    // esi -> hModule
lea edi, @"GetModuleHandleA"
//-------------------
push edi
push esi
mov eax,offset _p_GetProcAddress
call [ebp+eax] //GetModuleHandle=GetProcAddress(hModule, lpProcName);
//--------------------

I want to have a complete imported function table similar in performance done in a real EXE file. If you look inside a PE file, you will discover that an API call is done by an indirection jump through the virtual address of the API function:

JMP DWORD PTR [XXXXXXXX]

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...
0101F028: 7C801D77      ; Virtual Address of kernel32.LoadLibrary()
...
0101F120: JMP DWORD PTR [0101F028]
...
0101F230: CALL 0101F120 ;  JMP to kernel32.LoadLibrary
...

It makes it easy to expand the other part of our project by this performance, so we construct two data tables: first for API virtual addresses, and the second for the JMP [XXXXXXXX].

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#define __jmp_api               byte_type(0xFF) byte_type(0x25)
__asm
{
...
//----------------------------------------------------------------
_p_GetModuleHandle:             dword_type(0xCCCCCCCC)
_p_VirtualProtect:              dword_type(0xCCCCCCCC)
_p_GetModuleFileName:           dword_type(0xCCCCCCCC)
_p_CreateFile:                  dword_type(0xCCCCCCCC)
_p_GlobalAlloc:                 dword_type(0xCCCCCCCC)
//----------------------------------------------------------------
_jmp_GetModuleHandle:           __jmp_api   dword_type(0xCCCCCCCC)
_jmp_VirtualProtect:            __jmp_api   dword_type(0xCCCCCCCC)
_jmp_GetModuleFileName:         __jmp_api   dword_type(0xCCCCCCCC)
_jmp_CreateFile:                __jmp_api   dword_type(0xCCCCCCCC)
_jmp_GlobalAlloc:               __jmp_api   dword_type(0xCCCCCCCC)
//----------------------------------------------------------------
...
}

In the succeeding code, we has concluded our ambition to install a custom internal import table! (We can not call it import table.)

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    ...
    lea edi,[ebp+_p_szKernel32]
    lea ebx,[ebp+_p_GetModuleHandle]
    lea ecx,[ebp+_jmp_GetModuleHandle]
    add ecx,02h
_api_get_lib_address_loop:
        push ecx
        push edi
        mov eax,offset _p_LoadLibrary
        call [ebp+eax]    //LoadLibrary(lpLibFileName);
        pop ecx
        mov esi,eax       // esi -> hModule
        push edi
        call __strlen
        add esp,04h
        add edi,eax
_api_get_proc_address_loop:
            push ecx
            push edi
            push esi
            mov eax,offset _p_GetProcAddress
            call [ebp+eax]//GetModuleHandle=GetProcAddress(hModule, lpProcName);
            pop ecx
            mov [ebx],eax
            mov [ecx],ebx // JMP DWORD PTR [XXXXXXXX] 
            add ebx,04h
            add ecx,06h
            push edi
            call __strlen
            add esp,04h
            add edi,eax
            mov al,byte ptr [edi]
        test al,al
        jnz _api_get_proc_address_loop
        inc edi
        mov al,byte ptr [edi]
    test al,al
    jnz _api_get_lib_address_loop
    ...

6.3 Fix up the Original Import Table

In order to run the program again, we should fix up the thunks of the actual import table, otherwise we have a corrupted target PE file. Our code must correct all of the thunks the same as Table 5 to Table 6. Once more, LoadLibrary() and GetProcAddress() aid us in our effort to reach our intention.

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    ...
    mov ebx,[ebp+_p_dwImportVirtualAddress]
    test ebx,ebx
    jz _it_fixup_end
    mov esi,[ebp+_p_dwImageBase]
    add ebx,esi                   // dwImageBase + dwImportVirtualAddress
_it_fixup_get_lib_address_loop:
        mov eax,[ebx+00Ch]        // image_import_descriptor.Name
        test eax,eax
        jz _it_fixup_end
        
        mov ecx,[ebx+010h]        // image_import_descriptor.FirstThunk
        add ecx,esi
        mov [ebp+_p_dwThunk],ecx // dwThunk
        mov ecx,[ebx]             // image_import_descriptor.Characteristics
        test ecx,ecx
        jnz _it_fixup_table
            mov ecx,[ebx+010h]
_it_fixup_table:
        add ecx,esi
        mov [ebp+_p_dwHintName],ecx // dwHintName
        add eax,esi  // image_import_descriptor.Name + dwImageBase = ModuleName
        push eax  // lpLibFileName
        mov eax,offset _p_LoadLibrary
        call [ebp+eax]             // LoadLibrary(lpLibFileName);

        test eax,eax
        jz _it_fixup_end
        mov edi,eax
_it_fixup_get_proc_address_loop:
            mov ecx,[ebp+_p_dwHintName] // dwHintName
            mov edx,[ecx]            // image_thunk_data.Ordinal
            test edx,edx
            jz _it_fixup_next_module
            test edx,080000000h      // .IF( import by ordinal )
            jz _it_fixup_by_name
                and edx,07FFFFFFFh   // get ordinal
                jmp _it_fixup_get_addr
_it_fixup_by_name:
            add edx,esi  // image_thunk_data.Ordinal + dwImageBase = OrdinalName
            inc edx
            inc edx                  // OrdinalName.Name
_it_fixup_get_addr:
            push edx //lpProcName
            push edi                 // hModule                        
            mov eax,offset _p_GetProcAddress
            call [ebp+eax] // GetProcAddress(hModule, lpProcName);

            mov ecx,[ebp+_p_dwThunk] // dwThunk
            mov [ecx],eax  // correction the thunk
            // dwThunk => next dwThunk
            add dword ptr [ebp+_p_dwThunk], 004h
            // dwHintName => next dwHintName
            add dword ptr [ebp+_p_dwHintName],004h
        jmp _it_fixup_get_proc_address_loop
_it_fixup_next_module:
        add ebx,014h      // sizeof(IMAGE_IMPORT_DESCRIPTOR)
    jmp _it_fixup_get_lib_address_loop
_it_fixup_end:
    ...

7 Support DLL and OCX

Now, we intend to include the dynamic link library (DLL) and OLE-ActiveX Control in our PE builder project. Supporting them is very easy if we pay attention to the two time arrival into the Offset of Entry Point, the relocation table implementation, and the client import table.

PE Maker - Step 4

Download source files - 68.6 Kb

7.1 Twice OEP approach

The Offset of Entry Point of a DLL file or an OCX file is touched by the main program atleast twice:

Constructor:
When a DLL is loaded by LoadLibrary(), or an OCX is registered by using LoadLibrary() and GetProcAddress() through calling DllRegisterServer(), the first of the OEP arrival is done.

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```
hinstDLL = LoadLibrary( "test1.dll" );
```
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```
hinstOCX = LoadLibrary( "test1.ocx" );
_DllRegisterServer = GetProcAddress( hinstOCX, "DllRegisterServer" );
_DllRegisterServer(); // ocx register
```
Destructor:
When the main program frees the library usage by FreeLibrary(), the second OEP arrival happens.

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```
FreeLibrary( hinstDLL );
```
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```
FreeLibrary( hinstOCX );
```

To perform this, I have employed a trick, that causes in the second time again, the instruction pointer (EIP) traveling towards the original OEP by the structured exception handler.

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_main_0:
    pushad  // save the registers context in stack
    call _main_1
_main_1:    
    pop ebp
    sub ebp,offset _main_1 // get base ebp
    //---------------- support dll, ocx  -----------------
_support_dll_0: 
    jmp _support_dll_1 // nop; nop; // << trick // in the second time OEP
    jmp _support_dll_2
_support_dll_1:
    //----------------------------------------------------
    
    ...
    
    //---------------- support dll, ocx  1 ---------------
    mov edi,[ebp+_p_dwImageBase]
    add edi,[edi+03Ch]// edi -> IMAGE_NT_HEADERS
    mov ax,word ptr [edi+016h]// edi -> image_nt_headers->FileHeader.Characteristics
    test ax,IMAGE_FILE_DLL
    jz _support_dll_2
        mov ax, 9090h // << trick
        mov word ptr [ebp+_support_dll_0],ax
_support_dll_2:
    //----------------------------------------------------
    ...
    into OEP by SEH ...

I hope you have caught the trick in the preceding code, but this is not all of it, we have problem in ImageBase, when the library has been loaded in different image bases by the main program. We should write some code to find the real image base and store it to use forward.

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    mov eax,[esp+24h] // the real imagebase
    mov ebx,[esp+30h] // oep
    cmp eax,ebx
    ja _no_dll_pe_file_0
        cmp word ptr [eax],IMAGE_DOS_SIGNATURE
        jne _no_dll_pe_file_0
            mov [ebp+_p_dwImageBase],eax
_no_dll_pe_file_0:

This code finds the real image base by investigating the stack information. By using the real image base and the formal image base, we should correct all memory calls inside the image program!! Don't be afraid, it will be done simply by the relocating the table information.

7.2 Implement Relocation Table

To understand the relocation table better, you can take a look at the section 6.6 of Microsoft Portable Executable and Common Object File Format Specification document. The relocation table contains many packages to relocate the information related to the virtual address inside the virtual memory image. Each package comprise of a 8 bytes header to exhibit the base virtual address and the number of data, demonstrated by the IMAGE_BASE_RELOCATION data structure.

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typedef struct _IMAGE_BASE_RELOCATION {
    DWORD   VirtualAddress;
    DWORD   SizeOfBlock;
} IMAGE_BASE_RELOCATION, *PIMAGE_BASE_RELOCATION;

Table 7 - The Relocation Table

Block[1]	VirtualAddress
	SizeOfBlock
	type:4	offset:12	type:4	offset:12
	type:4	offset:12	type:4	offset:12
	type:4	offset:12	type:4	offset:12
	...	...	...	...
	type:4	offset:12	00	00
Block[2]	VirtualAddress
	SizeOfBlock
	type:4	offset:12	type:4	offset:12
	type:4	offset:12	type:4	offset:12
	type:4	offset:12	type:4	offset:12
	...	...	...	...
	type:4	offset:12	00	00
...	...
Block[n]	VirtualAddress
	SizeOfBlock
	type:4	offset:12	type:4	offset:12
	type:4	offset:12	type:4	offset:12
	type:4	offset:12	type:4	offset:12
	...	...	...	...
	type:4	offset:12	00	00

Table 7 illustrates the main idea of the relocation table. Furthermore, you can upload a DLL or an OCX file in OllyDbg to observe the relocation table, the ".reloc" section through Memory map window. By the way, we find the position of the relocation table by using the following code in our project:

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DWORD dwVirtualAddress = image_nt_headers->
  OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BASERELOC].
  VirtualAddress;
DWORD dwSize = image_nt_headers->
  OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BASERELOC].Size;

By OllyDbg, we have the same as the following for the ".reloc" section, by using the Long Hex viewer mode. In this example, the base virtual address is 0x1000 and the size of the block is 0x184.

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008E1000 : 00001000  00000184  30163000  30403028
008E1010 : 30683054  308C3080  30AC309C  30D830CC
008E1020 : 30E030DC  30E830E4  30F030EC  310030F4
008E1030 : 3120310D  315F3150  31A431A0  31C031A8
008E1040 : 31D031CC  31F431EC  31FC31F8  32043200
008E1050 : 320C3208  32143210  324C322C  32583254
008E1060 : 3260325C  32683264  3270326C  32B03274

It relocates the data in the subsequent virtual addresses:

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0x1000 + 0x0000 = 0x1000
0x1000 + 0x0016 = 0x1016
0x1000 + 0x0028 = 0x1028
0x1000 + 0x0040 = 0x1040
0x1000 + 0x0054 = 0x1054
...

Each package performs the relocation by using consecutive 4 bytes form its internal information. The first byte refers to the type of relocation and the next three bytes are the offset which must be used with the base virtual address and the image base to correct the image information.

type	offset
03	00	00	00

What is the type

The type can be one of the following values:

IMAGE_REL_BASED_ABSOLUTE (0): no effect.
IMAGE_REL_BASED_HIGH (1): relocate by the high 16 bytes of the base virtual address and the offset.
IMAGE_REL_BASED_LOW (2): relocate by the low 16 bytes of the base virtual address and the offset.
IMAGE_REL_BASED_HIGHLOW (3): relocate by the base virtual address and the offset.

What is done in the relocation?

By relocation, some values inside the virtual memory are corrected according to the current image base by the ".reloc" section packages.

delta_ImageBase = current_ImageBase - image_nt_headers->OptionalHeader.ImageBase

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mem[ current_ImageBase + 0x1000 ] = 
   mem[ current_ImageBase + 0x1000 ] + delta_ImageBase ;
mem[ current_ImageBase + 0x1016 ] = 
   mem[ current_ImageBase + 0x1016 ] + delta_ImageBase ;
mem[ current_ImageBase + 0x1028 ] = 
   mem[ current_ImageBase + 0x1028 ] + delta_ImageBase ;
mem[ current_ImageBase + 0x1040 ] = 
   mem[ current_ImageBase + 0x1040 ] + delta_ImageBase ;
mem[ current_ImageBase + 0x1054 ] = 
  mem[ current_ImageBase + 0x1054 ] + delta_ImageBase ;
...

I have employed the following code from Morphine packer to implement the relocation.

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    ...
_reloc_fixup:
    mov eax,[ebp+_p_dwImageBase]
    mov edx,eax
    mov ebx,eax
    add ebx,[ebx+3Ch] // edi -> IMAGE_NT_HEADERS
    mov ebx,[ebx+034h]// edx ->image_nt_headers->OptionalHeader.ImageBase
    sub edx,ebx // edx -> reloc_correction // delta_ImageBase
    je _reloc_fixup_end
    mov ebx,[ebp+_p_dwRelocationVirtualAddress]
    test ebx,ebx
    jz _reloc_fixup_end
    add ebx,eax
_reloc_fixup_block:
    mov eax,[ebx+004h]          //ImageBaseRelocation.SizeOfBlock
    test eax,eax
    jz _reloc_fixup_end
    lea ecx,[eax-008h]
    shr ecx,001h
    lea edi,[ebx+008h]
_reloc_fixup_do_entry:
        movzx eax,word ptr [edi]//Entry
        push edx
        mov edx,eax
        shr eax,00Ch            //Type = Entry >> 12
        mov esi,[ebp+_p_dwImageBase]//ImageBase
        and dx,00FFFh
        add esi,[ebx]
        add esi,edx
        pop edx
_reloc_fixup_HIGH:              // IMAGE_REL_BASED_HIGH  
        dec eax
        jnz _reloc_fixup_LOW
            mov eax,edx
            shr eax,010h        //HIWORD(Delta)
            jmp _reloc_fixup_LOW_fixup        
_reloc_fixup_LOW:               // IMAGE_REL_BASED_LOW 
            dec eax
        jnz _reloc_fixup_HIGHLOW
        movzx eax,dx            //LOWORD(Delta)
_reloc_fixup_LOW_fixup:
            add word ptr [esi],ax// mem[x] = mem[x] + delta_ImageBase
        jmp _reloc_fixup_next_entry
_reloc_fixup_HIGHLOW:           // IMAGE_REL_BASED_HIGHLOW
            dec eax
        jnz _reloc_fixup_next_entry
        add [esi],edx           // mem[x] = mem[x] + delta_ImageBase
_reloc_fixup_next_entry:
        inc edi
        inc edi                 //Entry++
        loop _reloc_fixup_do_entry
_reloc_fixup_next_base:
    add ebx,[ebx+004h]
    jmp _reloc_fixup_block
_reloc_fixup_end:
    ...

7.3 Build a Special Import table

In order to support the OLE-ActiveX Control registration, we should present an appropriate import table to our target OCX and DLL file.

Therefore, I have established an import table by the following string:

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const char *sz_IT_OCX_strings[]=
{
    "Kernel32.dll",
    "LoadLibraryA",
    "GetProcAddress",
    "GetModuleHandleA",
    0,
    "User32.dll",
    "GetKeyboardType",
    "WindowFromPoint",
    0,
    "AdvApi32.dll",
    "RegQueryValueExA",
    "RegSetValueExA",
    "StartServiceA",
    0,
    "Oleaut32.dll",
    "SysFreeString",
    "CreateErrorInfo",
    "SafeArrayPtrOfIndex",
    0,
    "Gdi32.dll",
    "UnrealizeObject",
    0,
    "Ole32.dll",
    "CreateStreamOnHGlobal",
    "IsEqualGUID",
    0,
    "ComCtl32.dll",
    "ImageList_SetIconSize",
    0,
    0,
};

Without these API functions, the library can not be loaded, and moreover the DllregisterServer() and DllUregisterServer() will not operate. In CPECryptor::CryptFile, I have distinguished between EXE files and DLL files in the initialization of the new import table object during creation:

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if(( image_nt_headers->FileHeader.Characteristics 
             & IMAGE_FILE_DLL ) == IMAGE_FILE_DLL )
{
    ImportTableMaker = new CITMaker( IMPORT_TABLE_OCX );
}
else
{
    ImportTableMaker = new CITMaker( IMPORT_TABLE_EXE );
}

8 Preserve the Thread Local Storage

By using Thread Local Storage (TLS), a program is able to execute a multithreaded process, this performance mostly is used by Borland linkers: Delphi and C++ Builder. When you pack a PE file, you should take care to keep clean the TLS, otherwise, your packer will not support Borland Delphi and C++ Builder linked EXE files. To comprehend TLS, I refer you to section 6.7 of the Microsoft Portable Executable and Common Object File Format Specification document, you can observe the TLS structure by IMAGE_TLS_DIRECTORY32 in winnt.h.

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typedef struct _IMAGE_TLS_DIRECTORY32 {
    DWORD   StartAddressOfRawData;
    DWORD   EndAddressOfRawData;
    DWORD   AddressOfIndex;
    DWORD   AddressOfCallBacks;
    DWORD   SizeOfZeroFill;
    DWORD   Characteristics;
} IMAGE_TLS_DIRECTORY32, * PIMAGE_TLS_DIRECTORY32;

To keep safe the TLS directory, I have copied it in a special place inside the loader:

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...
_tls_dwStartAddressOfRawData:   dword_type(0xCCCCCCCC)
_tls_dwEndAddressOfRawData:     dword_type(0xCCCCCCCC)
_tls_dwAddressOfIndex:          dword_type(0xCCCCCCCC)
_tls_dwAddressOfCallBacks:      dword_type(0xCCCCCCCC)
_tls_dwSizeOfZeroFill:          dword_type(0xCCCCCCCC)
_tls_dwCharacteristics:         dword_type(0xCCCCCCCC)
...

It is necessary to correct the TLS directory entry in the Optional Header:

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if(image_nt_headers->
  OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_TLS].
  VirtualAddress!=0)
{
    memcpy(&pDataTable->image_tls_directory,
           image_tls_directory,
           sizeof(IMAGE_TLS_DIRECTORY32));    
    dwOffset=DWORD(pData1)-DWORD(pNewSection);
    dwOffset+=sizeof(t_DATA_1)-sizeof(IMAGE_TLS_DIRECTORY32);
    image_nt_headers->
      OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_TLS].
      VirtualAddress=dwVirtualAddress + dwOffset;
}

9 Inject your code

We are ready to place our code inside the new section. Our code is a "Hello World!" message by MessageBox() from user32.dll.

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    ...
    push MB_OK | MB_ICONINFORMATION
    lea eax,[ebp+_p_szCaption]
    push eax
    lea eax,[ebp+_p_szText]
    push eax
    push NULL
    call _jmp_MessageBox
    // MessageBox(NULL, szText, szCaption, MB_OK | MB_ICONINFORMATION) ;
    ...

PE Maker - Step 5

Download source files - 71.7 Kb

10 Conclusion

By this article, you have perceived how easily we can inject code to a portable executable file. You can complete the code by using the source of other packers, create a packer in the same way as Yoda's Protector, and make your packer undetectable by mixing up with Morphine source code. I hope that you have enjoyed this brief discussion of one part of the reverse engineering field. See you again in the next discussion!

Inject your code to a Portable Executable file

Contents

0 Preface

1 Prerequisite

2 Portable Executable file format

2.1 The MS-DOS data

PE Viewer

Table 1 - Portable Executable file format structure

2.2 The Windows NT data

2.3 The Section Headers and Sections

Table 2 - Section names

3 Debugger, Disassembler and some Useful Tools

3.1 Debuggers

3.1.1 SoftICE

Figure 1 - SoftICE Window

3.1.2 OllyDbg

Figure 2 - OllyDbg CPU Window

3.1.3 Which parts are important in a debugger interface?

3.2 Disassembler

3.2.1 Proview disassembler

3.2.2 W32Dasm

3.2.3 IDA Pro

3.3 Some Useful Tools

3.3.1 LordPE

3.3.2 PEiD

3.3.3 Resource Hacker

3.3.4 WinHex

3.3.5 CFF Explorer

4 Add new section and Change OEP

Figure 3

PE Maker - Step 1

CALC.EXE - test file

DynLoader Step 1

4.1 Retrieve and Rebuild PE file

CPELibrary Class Step 1

4.2 Create Data for new Section

CPECryptor Class Step 1

4.3 Some notes regarding creating a new PE file

4.4 Some notes regarding linking this VC Project

5 Store Important Data and Reach Original OEP

PE Maker - Step 2

DynLoader Step 2

5.1 Restore the first Registers Context

5.2 Restore the Original Stack

5.3 Approach OEP by Structured Exception Handling

5.3.1 Implement Exception Handler

How the SEH installation is implemented

Thread Information Block (TIB)

Table 3 - FS segment register and Thread Information Block

5.3.2 Attain OEP by adjusting the Thread Context

Win32 Thread Context structure

Table 4 - CONTEXT

6 Build an Import Table and Reconstruct the Original Import Table

PE Maker - Step 3

6.1 Construct the Client Import Table

Table 5 - The Import Table in file image

Table 6 - The Import Table in virtual memory

6.2 Using other API functions in run-time

JMP DWORD PTR [XXXXXXXX]

6.3 Fix up the Original Import Table

7 Support DLL and OCX

PE Maker - Step 4

7.1 Twice OEP approach

7.2 Implement Relocation Table

Table 7 - The Relocation Table

What is the type

What is done in the relocation?

7.3 Build a Special Import table

8 Preserve the Thread Local Storage

9 Inject your code

PE Maker - Step 5

10 Conclusion

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