Description: This article demonstrates five steps to inject your code in a portable executable (EXE, DLL, OCX,...) file without recompiling source code.
THE CODE PROJECT Prize winner: December, 2005
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.
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 dont have any knowledge regarding the EXE file format and also debuggers.
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.
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 has the performance 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!
typedef struct _IMAGE_DOS_HEADER { // DOS .EXE header
WORD e_magic; // Magic number "MZ"
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:
This sample is useful for the whole of this article.
MS-DOS information |
IMAGE_DOS_ |
DOS EXE Signature | 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 |
00000040 ..!\L!This program canno
00000060 t be run in DOS mode....$.......
| ||
Windows NT information
|
Signature | PE signature (PE) | 000000F0 ASCII "PE"
|
IMAGE_ |
Machine |
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_ |
MagicNumber |
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_ |
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_ |
Name[8] |
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 | |||
|
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] |
00000400 EA 22 DD 77 D7 23 DD 77 "w#w
00000408 9A 18 DD 77 00 00 00 00 w....
00000410 2E 1E C7 77 83 1D C7 77 .ww
00000418 FF 1E C7 77 00 00 00 00 w....
00000420 93 9F E7 77 D8 05 E8 77 ww
00000428 FD A5 E7 77 AD A9 E9 77 ww
00000430 A3 36 E7 77 03 38 E7 77 6w8w
00000438 41 E3 E6 77 60 8D E7 77 Aw`w
00000440 E6 1B E6 77 2B 2A E7 77 w+*w
00000448 7A 17 E6 77 79 C8 E6 77 zwyw
00000450 14 1B E7 77 C1 30 E7 77 w0w
| ||
|
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 ........
|
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()
):
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:
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 task manager 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:
#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:
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.
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.
".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
->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!
In this part, you will become familiar with the necessary and essential equipments to develop your PE tools.
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!
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()
.
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.
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 |
:_ |
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.
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:
EAX |
ECX |
EDX |
EBX |
ESP |
EBP |
ESI |
EDI |
EIP |
o d t s z a p c |
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 |
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 .... .... |
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.
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.
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.
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.
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.
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.
LordPE by y0da is still the first choice to retrieve PE file information with the possibility to modify them.
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.
Resource Hacker can be employed to modify resource directory information; icon, menu, version info, string table, and etc.
WinHex, it is clear what you can do with this tool.
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.
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.
00000000
00000002
00000004
00000006
00000008
0000000A
0000000C
0000000E
00000010
00000012
00000014
00000016
00000018
0000001A
0000001C
0000001D
0000001E
0000001F
00000020
00000021
00000022
00000023
00000024
00000025
00000026
00000027
00000028
00000029
0000002A
0000002B
0000002C
0000002D
0000002E
0000002F
00000030
00000031
00000032
00000033
00000034
00000035
00000036
00000037
00000038
00000039
0000003A
0000003B
0000003C
|
4D 5A
9000
0300
0000
0400
0000
FFFF
0000
B800
0000
0000
0000
4000
0000
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
F0000000
|
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
|
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 |
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
DynLoader()
, in loader.cpp, is reserved for the data of
the new section, in other words, the Loader.
__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).
I have made a simple class library to recover PE information and to use it in a new PE file.
//----------------------------------------------------------------
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.
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.
//----------------------------------------------------------------
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));
//----------------------------------------
};
//----------------------------------------------------------------
VirtualAddress
and the VirtualSize
of
each section by SectionAlignment
: 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);
PointerToRawData
and the SizeOfRawData
of each section by FileAlignment
: 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);
SizeofImage
by the virtual size and the virtual
address of the last section: image_nt_headers->OptionalHeader.SizeOfImage =
image_section_header[LastSection]->VirtualAddress +
image_section_header[LastSection]->Misc.VirtualSize;
image_nt_headers->
OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT].
VirtualAddress = 0;
image_nt_headers->
OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT].Size = 0;
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:
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()
:
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;
}
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.
__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.
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.
__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)
//----------------------------------
}
}
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.
__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)
//----------------------------------
}
}
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.
#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;
}
; 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.
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:
LEA EAX,[EBP+_except_handler1_OEP_Jump]
PUSH EAX
PUSH DWORD PTR FS:[0]
MOV DWORD PTR FS:[0],ESP
INT 3
_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.
__try // SEH installation
{
__asm
{
INT 3 // An Exception Raise
}
}
__except( ..., EXCEPTION_CONTINUE_SEARCH ){}
// Exception handler expression filter
In assembly code...
; ----------------------------------------------------
; 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.
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
.
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;
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 |
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.
MOV EAX, ContextRecord
MOV EDI, dwOEP ; EAX <- dwOEP
MOV DWORD PTR DS:[EAX+0B8h], EDI ; pContext.Eip <- EAX
#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;
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 |
ExtendedRegisters [0x200] |
By the following code, we have accomplished the main purpose of coming to OEP by the structured exception handler:
__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)
//----------------------------------
}
}
To use the Windows dynamic link library (DLL) in Windows application programming, there are two ways:
// 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.
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:
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.
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.
IMAGE_IMPORT_ |
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_ |
|||||
... |
|||||
IMAGE_IMPORT_ |
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:
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...
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.
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.
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
At this time, we can load other DLLs and find the process address of other
functions by using LoadLibrary()
and GetProcAddress()
:
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:
...
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]
.
#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.)
...
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
...
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.
...
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:
...
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.
The Offset of Entry Point of a DLL file or an OCX file is touched by the main program atleast twice:
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.
hinstDLL = LoadLibrary( "test1.dll" );
hinstOCX = LoadLibrary( "test1.ocx" );
_DllRegisterServer = GetProcAddress( hinstOCX, "DllRegisterServer" );
_DllRegisterServer(); // ocx register
When the main program frees the library usage by FreeLibrary()
, the second OEP arrival
happens.
FreeLibrary( hinstDLL );
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.
_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.
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.
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.
typedef struct _IMAGE_BASE_RELOCATION {
DWORD VirtualAddress;
DWORD SizeOfBlock;
} IMAGE_BASE_RELOCATION, *PIMAGE_BASE_RELOCATION;
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:
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.
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:
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 |
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. 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 |
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.
...
_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:
...
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:
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:
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 );
}
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.
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:
...
_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:
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;
}
We are ready to place our code inside the new section. Our code is a "Hello
World!" message by MessageBox()
from user32.dll.
...
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) ;
...
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!
Ashkbiz Danehkar