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Fun With Thread Local Storage (part 2)

Fun With Thread Local Storage (part 2). Peter Ferrie Senior Anti-virus Researcher 26 June, 2008. 1. You Can Call Me Al. Thread Local Storage callbacks were discovered in 2000. However, widespread use didn’t occur until 2004. Now, it should be the first place to look for code,

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Fun With Thread Local Storage (part 2)

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  1. Fun With Thread Local Storage (part 2) Peter Ferrie Senior Anti-virus Researcher 26 June, 2008 1

  2. You Can Call Me Al Thread Local Storage callbacks were discovered in 2000. However, widespread use didn’t occur until 2004. Now, it should be the first place to look for code, since it runs before the main entrypoint. And that can make all the difference… 2 Peter Ferrie, Microsoft Corporation

  3. Empty! Entry Point 3 Peter Ferrie, Microsoft Corporation

  4. Empty! C3 RET 4 Peter Ferrie, Microsoft Corporation

  5. Empty! So the main file does nothing. If we assume that the structure is normal, then we could check the thread local storage table. Just in case. 5 Peter Ferrie, Microsoft Corporation

  6. Empty! TLS is present (size doesn’t matter) 6 Peter Ferrie, Microsoft Corporation

  7. Empty! Callback pointer Callback array 7 Peter Ferrie, Microsoft Corporation

  8. Empty! So the search moves to the callbacks, of which there is only one... or is there? 8 Peter Ferrie, Microsoft Corporation

  9. The One and Only 9 Peter Ferrie, Microsoft Corporation

  10. Am I Missing Something? CODE:00401010 push offset LibFileName ; "tls2" CODE:00401005 call j_LoadLibraryA CODE:0040100A mov ds:TlsCallbacksEnd, eax Hmm, LoadLibrary("tls2") Maybe DllMain contains something interesting? 10 Peter Ferrie, Microsoft Corporation

  11. I Am Missing Something No, that’s not it. 11 Peter Ferrie, Microsoft Corporation

  12. Take 2 Let’s revisit the code: CODE:00401010 push offset LibFileName ; "tls2" CODE:00401005 call j_LoadLibraryA CODE:0040100A mov ds:TlsCallbacksEnd, eax 12 Peter Ferrie, Microsoft Corporation

  13. It’s All About Image It’s the TlsCallBacks extended array trick again. Q. What value does the new callback contain? A. The DLL’s imagebase. Q. DEP won’t let that run, right? A. … 13 Peter Ferrie, Microsoft Corporation

  14. Surprise! 14 Peter Ferrie, Microsoft Corporation

  15. Not OK Of course it will. You just have to ask nicely. Or take a cue from a driver. 15 Peter Ferrie, Microsoft Corporation

  16. Chaotic-Evil When the SectionAlignment value is less than 4kb, the file header is marked Writable and Executable (unless the last section characteristics override it). That turns the ImageBase into code. 16 Peter Ferrie, Microsoft Corporation

  17. Before So we go from this… 17 Peter Ferrie, Microsoft Corporation

  18. After To this… 18 Peter Ferrie, Microsoft Corporation

  19. Presto! 19 Peter Ferrie, Microsoft Corporation

  20. Really Not OK Just a little something to add to the workload. 20 Peter Ferrie, Microsoft Corporation

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