[LLVMdev] Adding diversity for security (and testing)

Nick Lewycky nlewycky at google.com
Mon Sep 9 15:24:45 PDT 2013

On 29 August 2013 15:29, Stephen Crane <sjcrane at uci.edu> wrote:

> On 08/28/2013 02:37 PM, Nick Lewycky wrote:
>> 1. I'm concerned about the deployment problem. I realize that being in
>> the compiler means you can transform the program in more exciting ways, but
>> it gives you a much worse deployment story than something which modifies
>> the program on disk like "prelink".
> Yes, definitely. Deployment is an issue which users will need to consider
> when deciding whether to use compiler-based diversity. However there are
> certainly use-cases which could benefit, especially in small deployments.
> Additionally, the benefits may outweigh the increase in deployment costs
> for larger deployments.
>  2. Does this actually fill a gap in our protections? How do we ever get
>> into the situation where the user is able to deploy a ROP attack against
>> us, without tripping asan or ubsan or something caught by our warnings or
>> the static analyzer or any of the other protections offered by clang and
>> llvm? It may suffice that there exists a niche which can't afford the
>> performance penalty from asan or other things, but then we'll need to
>> discuss what the performance impact is.
> Briefly looking at ASAN again, I saw a performance penalty of 2x
> mentioned. Diversity could act as both defense in depth, and as a
> lower-impact defense for performance critical code.

Okay, I've been thinking about what I wanted answered here, and I've
decided that what I want to know is too complex for this discussion. It
boils down to: given we have all the power of clang and llvm for very
complex analysis, both static and dynamic, why is randomizing the best we
can do? Can't we somehow use all that static and dynamic analysis to shrink
the problem down to something we can solve more cleverly than randomizing
the program across a lot of axes, such as proving (or arranging for)
certain properties which reduce how much we need to randomize, etc? And if
not *why can't we*? It's that last part which I think is hardest to answer,
so I've decided I'll leave this to the security-trained folks -- if they
think this is the right approach, they're probably right.

3. Reproducible builds are a must. GCC has a -frandom-seed=text flag and
> you should use that here too. Given the same random seed, the compiler
> shall produce the same output.
> Completely agree, and that's exactly what we have done.
>  And one issue for us, the llvm developers. If we're going to accept this
>> feature, own it and maintain it, how can we test it? We know how to test
>> for correctness and measure performance, we even understand what
>> protections -fstack-protector or ASAN offer and can write spot-tests for
>> them, but how can we measure the effectiveness of probability-based
>> ROP-protections? We don't need an answer to this right now, but over time
>> we'll be maintaining it, and it seems plausible that we could accept a
>> patch which accidentally diminishes the actual security provided (image us
>> maintaining a random number generator -- it's possible to add a weakness
>> which isn't easily noticed).
> Unfortunately testing for security is a difficult problem. We have
> currently been evaluating our work by comparing a population of randomized
> variants to make sure that no gadgets are the same across variants. While
> this is reasonable for research evaluation, I'm not sure that would be
> practical (or useful) for the compiler test suite. To prevent regressions,
> I think the test suite will need to test at least 3 major areas: making
> sure that RNG results are consistent and deterministic, testing that NOPs
> are actually inserted and scheduling decisions randomized, and verifying
> that different seeds result in entirely different binaries with
> approximately equal amounts of diversifications.

I really like the idea of verifying that ROP gadgets work across different
variants. Even if it doesn't fit in the regression test suite, it should be
set up as an integration test.

The other three areas you identified largely fit in the regression suite.

Speaking of maintenance, we plan to continue work on this in our lab. We
> can certainly maintain these features at least for the next year and a
> half, and possibly longer.
>  There's a "good news" side to this too. Over lunch I talked with one of
>> our security guys, and he's excited. He tells me that diversity for
>> ROP-protection is entirely the way to go, and speculates that it ought to
>> be deployable by other vendors. Furthermore, we expect that rotating the
>> register allocation is going to be especially painful on the exploit
>> authors.
> As mentioned, we have an implementation of register randomization,
> although it will require some reworking to be ready for prime-time. We can
> definitely look into getting that polished.


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