Re: Cisco IOS Shellcode Presentation

[Bart.Lansing () kohls com]

full-disclosure-bounces () lists grok org uk wrote on 07/29/2005 09:28:31 PM:

> Valdis.Kletnieks () vt edu wrote:
> > On Fri, 29 Jul 2005 15:02:51 -1000, Jason Coombs said:
> > > redesign, fundamentally, the turing machine so that before each 
> > > operation is performed a verification step is employed to ensure that 
> >
> > Ahem. No.  You *can't* "ensure" it (although you *can* do things like 
bounds
> > checking to *minimize* issues).
> >
> > It's called the Turing Halting Problem
>
>
> We're not talking about proving/disproving the result of computation 
> here, we're talking about a simple logical step inserted prior to 
> transmission of operating instructions and data to a turing machine.
>
> It does not invoke the Turing Halting Problem to ask the question 
> "should the following opcode be sent to the CPU / should the opcode be 
> read from memory and acted upon" ?
>
> The simplest solution is to duplicate the machine code, placing one copy 

> in a protected storage and requiring the CPU to confirm that both the 
> active/RAM-resident copy and the protected storage copy match before 
> proceeding with computation.
>
> This is superior to simply reading machine code from a protected storage 

> because the point is that malicious arbitrary code that overwrites or 
> reprograms or inserts itself into the runtime memory space of an active 
> process would easily defeat a volatile copy of a non-volatile protected 
> storage image of some machine code. Only by requiring the CPU to perform 

> a validation of each opcode instruction but allowing the CPU to continue 

> to behave in all other respects as it behaves today does the protection 
> arise. Other approaches are possible, but the basic idea of a separate 
> supply of bits useful for the runtime authentication of opcodes remains 
> the same.
>
> Turing has nothing to say on this subject because he never contemplated 
> it, to the best of my knowledge. Turing never tried to defend against 
> buffer overflows back in the 1930s, yet people invoke him as a sage 
> unerring philosopher of our time. Why?
>
> Regards,
>
> Jason Coombs
> jasonc () science org
> _______________________________________________
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Pardon my intrusion (or don't, either way...)

It occurs to me that your solution is flawed as well.  What assurance do 
we have that your "protected storage" is future-proof (i.e. unbreachable 
by an means whatsoever)?  Assuming it is not, you'll need to be prepared 
to have the "protected storage" verify itself against the "really 
protected storage", which has validated itself against the "exceptionally 
well looked after" storage, which was tested against the "superbly vaulted 
super-secret storage"...ad nauseum...before you can send instructions to 
the cpu with any absolute guarantee that the code it wants to run is 
legit.  As the ability to break into/compromise your vaulted storage and 
its children improves, one can logically project a situation where your 
proposed system is burning far more cycles validating itself than it can 
possibly spend doing its job.

Jason, et al, I appreciate that from a theoretical standpoint I am wildly 
out of my depth...but the underlying flaw in the logic of your premise has 
nothing to do with the technologies and everything to do with your basic 
assumptions. 


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