x86 anti DECODE (PoC)


Axel Tillequin
@bdcht


Rump SSTIC 2014

ia32 instructions


... many "oddities"
(see https://code.google.com/p/corkami/wiki/x86oddities)

prefixes



MORE PFX/SSE oddities


+
66 0F 58 /r  : ADDPD,
etc.



testing decoders...


WHAT THE CPU DOES...


what is decoded...




objdump FAIL :





WHAT IS DECODED...


IDA (6.x) FAIL :

WHAT IS DECODED...














LLVM FAIL :

WHAT IS DECODED...

capstone FAIL:

        [FIXED]


WHAT IS DECODED...

beaengine :   
Hopper : 

WHAT IS DECODED...

HTE  fail :

WTF ??!
(they got it almost right !!!)




conclusion

decoding IA32 is tricky...

  • use last prefix within each group
  • for SSE2, use pfx groups "precedence" rules  (F2/F3 > 66)
  • the decoder needs to be EASY to enhance/fix !!! 


more testings needed :
  • more CPUs (reliable behaviors ??)
  • REX/VEX things... (64 bits)
  • automated testing  ? (ptrace validation)

amoco

github.com/bdcht/amoco

(how to do it and fix it)

x86 anti DECODE

By bdcht

x86 anti DECODE

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