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Intro to Reverse Engineering

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  1. Intro to Reverse Engineering ~ intropy ~

  2. Intro

  3. Why do we reverse engineer? • Closed source software • Vulnerability Research • Product verification • Proprietary formats • Interoperability • SMB on UNIX • Word compatible editors • Virus research

  4. Why should you give a fuck? • Basis of computing • Reverse engineering teaches the inner workings of any processor • Learning how the processor handles data helps in understanding many other aspects of computer security • All the cool kids are doing it (not really)

  5. Real Time RCE (Debugging) • Debuggers that disassemble • OllyDbg • WinDbg • SoftIce • Code actually runs • The application actually executes all instructions as if it was ran normally • Uses interrupts to control execution of the program • Swaps out the current instruction with an interrupt instruction code • Swaps it back when the execution is continued

  6. Static Analysis (Dead Listing) • Traditional disassemblers • IDA Pro • W32Dasm • objdump • Code does not execute • The disassembler parses the file format and related code sections • Good disassemblers do deep recursive analysis to ensure proper instruction disassembly • Allows the user the ability to look at what code will do without actually running it • Does not allow the ease of live disassembly/debugging • Viewing registers • Inspecting the contents of memory

  7. File Formats

  8. What are file formats? • Files that adhere to a specific format often being executable by an operating system • Executable files are created from source code and libraries by a compiler • Data files can be created by anything from a text editor to an mp3 encoder

  9. Executable Contents • Machine code • Instructions the program will run • Memory locations • code addresses • function addresses • Program data • Static variables • Strings • Loader data • Imports • Exports

  10. Sections • Allows the loader to find various information • Not finite, executables can have user defined sections

  11. Executable Formats • ELF – Executable and Linker Format • History Originally published by UNIX system laboratories as a dynamic, linkable format to be used in various UNIX platforms • What uses ELF • Linux • Solaris • Most modern BSD based unix’s • Dissection • Header • Sections

  12. ELF Header • The header contains various information the operating system loading needs e_ident – Contains various identification fields including Endianess, ELF version, Operating System e_type – Identifies the object file type including relocatable, executable, or core file e_machine – Contains the processor type including Intel 80386, HPPA, PowerPC e_version – Contains the file version information e_entry -Contains the entry point for the executable e_phoff – Contains the program files header offset in bytes e_shoff – Contains the section header offset e_flags – Contains the processor specific flags e_ehsize – Contains the ELF header size in bytes

  13. ELF Sections • Each section of an ELF executable contain various information needed to execute .bss - This section holds uninitialized data that contributes to the program's memory image. By definition, the system initializes the data with zeros when the program begins to run. .comment - This section holds version control information. .ctors - This section holds initialized pointers to the C++ constructor functions. .data - This section holds initialized data that contribute to the program's memory image. .data1 - This section holds initialized data that contribute to the program's memory image. .debug - This section holds information for symbolic debugging. The contents are unspecified. .dtors - This section holds initialized pointers to the C++ destructor functions. .dynamic - This section holds dynamic linking information.

  14. ELF Sections Cont… .dynstr - This section holds strings needed for dynamic linking, most commonly the strings that represent the names associated with symbol table entries. .dynsym - This section holds the dynamic linking symbol table. .fini - This section holds executable instructions that contribute to the process termination code. When a program exits normally the system arranges to execute the code in this section. .got - This section holds the global offset table. .hash - This section holds a symbol hash table. .init - This section holds executable instructions that contribute to the process initialization code. When a program starts to run the system arranges to execute the code in this section before calling the main program entry point. .interp - This section holds the pathname of a program interpreter. If the file has a loadable segment that includes the section, the section's attributes will include the SHF_ALLOC bit. Otherwise, that bit will be off. .line - This section holds line number information for symbolic debugging, which describes the correspondence between the program source and the machine code. The contents are unspecified.

  15. ELF Sections Cont… .note - This section holds information in the ``Note Section'' format described below. .plt - This section holds the procedure linkage table. .relNAME - This section holds relocation information. By convention, ``NAME'' is supplied by the section to which the relocations apply. Thus a relocation section for .text normally would have the name .rel.text .rodata - This section holds read-only data that typically contributes to a non- writable segment in the process image. .rodata1 - This section holds read-only data that typically contributes to a non- writable segment in the process image. .shstrtab - This section holds section names. .strtab - This section holds strings, most commonly the strings that represent the names associated with symbol table entries. .symtab - This section holds a symbol table. If the file has a loadable segment that includes the symbol table, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off. .text - This section holds the ``text'' or executable instructions, of a program.

  16. Executable Formats Cont… • PE – Portable Executable • History Microsoft migrated to the PE format with the introduction of the Windows NT 3.1 operating system. It is based of a modified form of the UNIX COFF format • What uses PE • Windows NT • Window 2000 • Windows XP • Windows 2003 • Windows CE • Dissection • DOS Stub • The DOS stub contains a message that the executable will not run in DOS mode • Optional Header (Not optional] • RVA • Relative virtual addressing • Sections

  17. Optional Header • The optional header in a PE executable contains various information regarding the executable contents needed for the OS loader SizeOfCode - Size of the code (text) section, or the sum of all code sections if there are multiple sections. AddressOfEntryPoint – Address of the entry function to start execution from BaseOfCode - RVA of the start of the code relative to the base address BaseOfData – RVA of the start of the data relative to the base address SectionAlignment – Alignment of sections when loaded into memory FileAlignment – Alignment of section on disk SizeOfImage - Size, in bytes, of image, including all headers; must be a multiple of Section Alignment SizeOfHeaders - Combined size of MS-DOS stub, PE Header, and section headers rounded up to a multiple of FileAlignment. NumberOfRvaAndSizes - Number of data-dictionary entries in the remainder of the Optional Header. Each describes a location and size.

  18. Sections • The sections in a PE file contain various pieces of the executable needed to run including various RVA’s and offsets .text – Contains all executable code .idata – Contains imported data such as dll addresses .edata – Contains any exported data .data – Contains initialized data like global variables and string literals .bss – Contains un-initialized data .rsrc – Contains all module resources .reloc – Contains relocation data for the OS loader

  19. Data Formats • Different than executable formats • Doesn’t usually contain machine code • Has structure but not always defined sections • A reverser often needs to reverse how a file format functions • Proprietary formats are not always published • Reversing allows compatibility (i.e. Microsoft doc) • Data rights management • Often the only way to get what you pay for is to take action

  20. Assembly Language

  21. What is it • Lowest level of programming (besides microcode) • Direct processor register access utilizing architecture defined instructions • Output of most compilers

  22. How is it used • Directly using an assembler • NASM • ml • as • Output by a high level compiler • GCC • cl

  23. What does it looks like • Depends on the instruction set • IA32 • mov eax, 0x1 • PA-RISC • copy %r14,%r25 • ARM • LDR r0,[r8]

  24. Instruction Sets • The mneumonics for the opcodes handled by the processor • Minimal set of “commands” that achieve a programming goal

  25. Different Instruction Set Architectures • RISC - Reduced Instruction Set Computing • Fixed length 32 bit instructions • 32 general purpose registers • Vendors • IBM (PowerPC) • HP (PA-RISC) • Apple (PowerPC) • CISC - Complex Instruction Set Computing • Multibyte instructions • Multiple synonymous opcodes • 16 registers • Vendors • Intel (IA-32) • DEC [PDP-11] • Motorola (m68K)

  26. Registers and the Stack

  27. Overview • Purpose • Registers are used to store temporary data • Pointers • Computations • The stack is used to manage data • Variables • Data

  28. Stack Layout • Stack is dynamic but builds as it goes • Addresses start at a higher address and builds to lower addresses • The stack is generally allocated in 4 byte chunks

  29. Register sizes • Register sizes depend on the supported architecture • 32 bit • 64 bit • IA32 • 16 registers 32 bits (4 bytes) each • RISC • 32 general purpose registers 64 bits [8 bytes] each

  30. IA32 Registers • EBP – Stack frame base pointer • Points to the start of the functions stack frame • ESP – Stack source pointer • Points to the current (top) location on the stack • EIP – Instruction pointer • Points to the next executable instruction

  31. IA32 Registers Cont… • General Purpose registers • Used in general computation and control flow • EAX – Accumulator register • EBX – General data register • ECX – Counter register • EDX – General data register • ESI – Source index register • EDI – Destination index register • Segment registers • Used to segment memory and compute addresses • CS – Code segment register • SS - Stack segment register • DS - Data segment register • ES - Extra (More data) segment register • FS - Third data segment register • GS – Fourth data segment register • EFLAGS • CF – Carry Flag • SF – Signed Flag • ZF – Zero Flag

  32. Overview of IA-32 Instruction Set • mov – Moves source to destination • lea – Loads effective address • jmp – Jump • jne – Jump if not equal • jg – Jump if greater than • call – Unconditional function call • ret – Returns from a function to the caller • add – Adds two values • sub – subtracts two values • xor – XORs two values • cmp – Compares two registers

  33. Calling conventions Calling conventions define how the callers data is arranged on the stack • cdecl • Most common calling convention • Dynamic parameters • Caller unwinds stack • pop ebp • ret • fastcall • Higher performance • First two parameters are passed over registers • stdcall • Common in Windows • Parameters are received in reverse order • Function unwinds stack • ret 0x16

  34. Example PUSH EBP ; Pushes the contents of EBP onto the stack MOV EBP, ESP ; Moves the address of ESP to EBP CMP DWORD PTR [EBP+C], 111 ; Subtract what is at EBP+12 with 111 JNZ 00401054 ; If previous compare is not zero jump to 00401054 MOV EAX, DWORD PTR [EBP+10] ; Move what is at EBP+16 to EAX CMP AX, 64 ; Subtract what we moved to EAX with 64 JNZ 00401068 ; If the comparison does not equal 0 jump to address POP EBP ; Store the current value on the stack in EBP RET ; Return to the caller

  35. OllyDbg

  36. Overview • Purpose • OllyDbg is a general purpose win32 user land debugger. The great thing about it is the intuitive UI and powerful disassembler • Licensing • OllyDbg is free (shareware), however it is not open source and the source code is not available • Extensibility • OllyDbg has defined a plugin architecture allowing extensibility via powerful plugins

  37. Window Layouts • Window layouts are the various parts of the UI that contain pertinent information • Code window – Displays the executable machine code • Register window – Allows the user to watch the contents of each register during execution • Memory window – Allows the user to view the contents of various memory locations • Stack window – Displays the stack, including memory addresses and values

  38. Working in OllyDbg • Navigation • Moving • Searching • Commenting • Can be entered in the code window with the ; or : keys • Listing Names • The names window displays all functions or imported functions used in the program • Listing them is easy via the shortcut Ctrl + N • Showing Memory • Displaying memory can be useful when looking for strings or other important data • Displaying the memory map window can be achieved via Alt + M

  39. Working in OllyDbg Cont… • Breakpoints • Breakpoints allow the debugger to stop at a specified address or instruction • There are two types of breakpoints in general • Software breakpoints • Handled by the operating system • Set by navigating to the specified address and hitting F2 • Hardware breakpoints • Handled by the processor • Set by finding a place in memory you want to break on access and right clicking selecting the proper option • Olly also provides a way to view and turn on and off breakpoints via the breakpoints window with Alt + B

  40. Working in OllyDbg Cont… • Controlling Execution • Starting the process • Once the target program is either loaded or attached in Olly you can start execution. This will actually set up an initial breakpoint at the application entry point • There are several ways you can proceed from the entry point • Single stepping • Executes one instruction at a time and can be achieved by hitting F7 • Steps into every function • Tedious as fuck • Execute until return • Executes until the ret instuction is encoutered which can be achieved by hitting Ctrl + F9 • Executes all instructions in the current function • Faster than single stepping but not as comprehensive

  41. Working in OllyDbg Cont… • Watching execution • Registers • Handled in the register window • Red highlighting indicates a register has changed • Stack • Handled in the stack window • Display can be address or relative address from ebp • Call stack • Displays the functions the current function has been called from • Can be displayed with the shortcut Alt + K

  42. OllyDbg Case Study*(smarty word for demo) • Example • Program displays a popup box • Goal is to make the proper box show and exit • Patching • Allows us to modify the executable assembly code and save it to a new file with the changes

  43. OllyDbg Plugins • OllyDbg provides a downloadable PDK for plugin development • Several plugins exist that provide extra usability • Heap Vis • Breakpoint manager • Ollyscript

  44. IDA Pro

  45. Overview • IDA Pro was originally designed as a powerful disassembler • Supports 30+ processors • It has since been broadened to include a built in debugger • Designed for reverse engineers with quickness and robustness in mind • This sometimes makes the learning curve step • Extensible plugin architecture and scripting language

  46. Window Layouts • Customizing window layouts • Each saved session will store any customized layouts • A default layout can also be saved • Customized layouts are provided to help the user with workflow and can consist of any combination or number of windows

  47. Navigation • Shortcuts • Most actions have equivalent shortcuts associated with them • Some of the most used • [Enter] – Jumps into the function under the cursor • [Esc] – Returns to the previous cursor position • Jumping • IDA allows the user to jump to various parts of a binary file easily • Some of the jumps • Entry point – Jumps to the entry point of the binary • By name – Allows the user to jump to a specific function or string in the binary • By address – Allows the user to jump to a specific address • Markers • Markers can be used to tag locations in the binary for future reference • Markers are set using Alt + M and naming • Jumping to a marker is easily achieved with Ctrl + M

  48. Editing • Comments • Comments allow you to organize and document important parts of the binary • Comments can be entered using the shortcut keys ; or : • Function names can be renamed to something more descriptive • Often times symbols are not available for the binary and naming each functions allows you to understand and track your work • Functions can be renamed using the shortcut Alt + P

  49. Windows • IDA View • Displays the disassembled binary • Hex View • Display the hex view of the current cursor position • Names • The names windows displays textual names and addresses in the binary • Strings • The strings window contains any ascii strings present in the executable • Imports • The imports window contains the imported functions from dll’s • Functions • The functions window allows you to view all functions and their addresses

  50. Graphing • IDA Pro has a powerful graphing engine that allows a user to visualize call graphs and xrefs • Flow chart graphs display the current functions machine code and any branches • Function call graph will display the call flow of all the functions in the executable (Can be large) • Xref graphs display the to and from xrefs with machine code