CNIT 126: 8: Debugging
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Chapter 8 of 'Practical Malware Analysis' covers the differences between disassemblers and debuggers, highlighting tools like OllyDbg for user-mode debugging and Windbg for kernel-mode debugging. It explains single-stepping, breakpoints, and exceptions, detailing how debuggers manage program execution and the various types of breakpoints that can be set. The chapter emphasizes practical applications of debugging in malware analysis, including modifying execution and handling exceptions.
CNIT 126: 8: Debugging
- 1. Practical Malware Analysis Ch 8: Debugging Rev. 9-11-18
- 2. Disassemblers v. Debuggers • A disassembler like IDA Pro shows the state of the program just before execution begins • Debuggers show – Every memory location – Register – Argument to every function • At any point during processing – And let you change them
- 3. Two Debuggers • Ollydbg – Most popular for malware analysis – User-mode debugging only – IDA Pro has a built-in debugger, but it's not as easy to use or powerful as Ollydbg • Windbg – Supports kernel-mode debugging
- 4. Source-Level v. Assembly-Level Debuggers • Source-level debugger – Usually built into development platform – Can set breakpoints (which stop at lines of code) – Can step through program one line at a time • Assembly-level debuggers (low-level) – Operate on assembly code rather than source code – Malware analysts are usually forced to use them, because they don't have source code
- 5. Windows Crashes • When an app crashes, Windows may offer to open it in a debugger • Usually it uses Windbg • Links Ch 8c, 8d
- 7. User Mode Debugging • Debugger runs on the same system as the code being analyzed • Debugging a single executable • Separated from other executables by the OS
- 8. Kernel Mode Debugging The Old Way • Requires two computers, because there is only one kernel per computer • If the kernel is at a breakpoint, the system stops • One computer runs the code being debugged • Other computer runs the debugger • OS must be configured to allow kernel debugging • Two machines must be connected
- 9. Windows 7 Advanced Boot Options • Press F8 during startup • "Debugging Mode"
- 10. Side-Effect of Debug Mode • PrntScn key causes BSOD • Created great suffering in S214 in previous semesters • Use Shift+PrntScn instead
- 11. Kernel Mode Debugging The New Way • Mark Russinovich's Livekd tool allows you to debug the kernel with only one computer! • MUCH easier :) • Tool has some limitations (Link Ch 8e)
- 12. 12 How Livekd Works • Link Ch 8f
- 13. Good Intro to OllyDbg • Link Ch 8a
- 15. Using a Debugger
- 16. Two Ways • Start the program with the debugger – It stops running immediately prior to the execution of its entry point • Attach a debugger to a program that is already running – All its threads are paused – Useful to debug a process that is affected by malware
- 17. Single-Stepping • Simple, but slow • Don't get bogged down in details
- 18. Example • This code decodes the string with XOR
- 19. Stepping-over v. Stepping-Into • Single step executes one instruction • Step-over call instructions – Completes the call and returns without pausing – Decreases the amount of code you need to analyze – Might miss important functionality, especially if the function never returns • Step-into a call – Moves into the function and stops at its first command
- 20. Pausing Execution with Breakpoints • A program that is paused at a breakpoint is called broken • Example – You can't tell where this call is going – Set a breakpoint at the call and see what's in eax
- 21. • This code calculates a filename and then creates the file • Set a breakpoint at CreateFileW and look at the stack to see the filename
- 22. WinDbg
- 23. Encrypted Data • Suppose malware sends encrypted network data • Set a breakpoint before the data is encrypted and view it
- 25. OllyDbg
- 26. Types of Breakpoints • Software execution • Hardware execution • Conditional
- 27. Software Execution Breakpoints • The default option for most debuggers • Debugger overwrites the first byte of the instruction with 0xCC – The instruction for INT 3 – An interrupt designed for use with debuggers – When the breakpoint is executed, the OS generates an exception and transfers control to the debugger
- 28. Memory Contents at a Breakpoint • There's a breakpoint at the push instruction • Debugger says it's 0x55, but it's really 0xCC
- 29. When Software Execution Breakpoints Fail • If the 0xCC byte is changed during code execution, the breakpoint won't occur • If other code reads the memory containing the breakpoint, it will read 0xCC instead of the original byte • Code that verifies integrity will notice the discrepancy
- 30. Hardware Execution Breakpoints • Uses four hardware Debug Registers – DR0 through DR3 – addresses of breakpoints – DR7 stores control information • The address to stop at is in a register • Can break on access or execution – Can set to break on read, write, or both • No change in code bytes
- 31. Hardware Execution Breakpoints • Running code can change the DR registers, to interfere with debuggers • General Detect flag in DR7 – Causes a breakpoint prior to any mov instruction that would change the contents of a Debug Register – Does not detect other instructions, however
- 32. Conditional Breakpoints • Breaks only if a condition is true – Ex: Set a breakpoint on the GetProcAddress function – Only if parameter being passed in is RegSetValue • Implemented as software breakpoints – The debugger always receives the break – If the condition is not met, it resumes execution without alerting the user
- 33. Conditional Breakpoints • Conditional breakpoints take much longer than ordinary instructions • A conditional breakpoint on a frequently- accessed instruction can slow a program down • Sometimes so much that it never finishes
- 35. Exceptions
- 36. Exceptions • Used by debuggers to gain control of a running program • Breakpoints generate exceptions • Exceptions are also caused by – Invalid memory access – Division by zero – Other conditions
- 37. First- and Second-Chance Exceptions • When a exception occurs while a debugger is attached – The program stops executing – The debugger is given first chance at control – Debugger can either handle the exception, or pass it on to the program – If it's passed on, the program's exception handler takes it
- 38. Second Chance • If the application doesn't handle the exception • The debugger is given a second chance to handle it – This means the program would have crashed if the debugger were not attached • In malware analysis, first-chance exceptions can usually be ignored • Second-chance exceptions cannot be ignored – They usually mean that the malware doesn't like the environment in which it is running
- 39. Common Exceptions • INT 3 (Software breakpoint) • Single-stepping in a debugger is implemented as an exception – If the trap flag in the flags register is set, – The processor executes one instruction and then generates an exception • Memory-access violation exception – Code tries to access a location that it cannot access, either because the address is invalid or because of access-control protections
- 40. Common Exceptions • Violating Privilege Rules – Attempt to execute privileged instruction with outside privileged mode – In other words, attempt to execute a kernel mode instruction in user mode – Or, attempt to execute Ring 0 instruction from Ring 3
- 41. List of Exceptions • Link Ch 8b
- 42. Modifying Execution with a Debugger
- 43. Skipping a Function • You can change control flags, the instruction pointer, or the code itself • You could avoid a function call by setting a breakpoint where at the call, and then changing the instruction pointer to the instruction after it – This may cause the program to crash or malfunction, or course
- 44. Testing a Function • You could run a function directly, without waiting for the main code to use it – You will have to set the parameters – This destroys a program's stack – The program won't run properly when the function completes
- 45. Modifying Program Execution in Practice
- 46. Example (A Real Virus) • Operation depends on language setting of a computer – Simplified Chinese • Uninstalls itself & does no harm – English • Display pop-up "Your luck's no good" – Japanese or Indonesian • Overwrite the hard drive with random data
- 47. Break at 1; Change Return Value