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Re: Compiler support for erasure of sensitive data


> On Sep 9, 2015, at 1:54 PM, David Edelsohn <dje.gcc@gmail.com> wrote:
> 
> On Wed, Sep 9, 2015 at 12:36 PM, Zack Weinberg <zackw@panix.com> wrote:
> 
>> The ABI dictates basically everything you see.  The call to
>> explicit_bzero has forced the compiler to *create* a second copy of
>> the variable `k` on the stack, just so it can be erased -- and the
>> copy in registers survives (at least for a short time), which is not
>> what the programmer wanted.  With or without explicit_bzero, we have
>> no way of getting rid of the copy in registers.  More complicated
>> scenarios of course exist.
> 
>> Comments?  Please note that I do not have anything like the time
>> required to implement any of this myself (and I'm ten years out of
>> practice on GCC and have no experience whatsoever with Clang,
>> anyway).  I'm hoping this catches someone's interest.
> 
> What level of erasure of sensitive data are you trying to ensure?
> Assuming that overwriting values in the ISA registers actually
> completely clears and destroys the values is delusionally naive.

Could you point to some references about that?

> Most modern hardware architectures have hardware capabilities to
> encrypt and protect sensitive data.

I'm not sure about "most".  I haven't worked on any that could do this.

I agree it would be good to specify the threat model.  Reading between the lines, I believe it is: capture of the software-visible process state after the code is finished with the sensitive data, either via a process dump file, or a debugger.  With an explicitly stated list of goals and non-goals we can see whether a proposed solution addresses all, part, or none of the problem space, and whether it is a small solution or one much more powerful than is actually requested.

If the threat is indeed delayed process state examination in software, then I think your "dangerously naive" does not apply.  If you're talking excavating the chip and doing quantum forensics, that's a different matter.

Another threat that I don't believe is covered here is disclosure of copies of the process state held in the OS, like saved context from thread switching, copies of stuff in the page file or in now-freed memory pages, or things like that.  

	paul


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