[llvm-dev] _ExtInt, LLVM integers and constant time

Sun May 3 10:54:11 PDT 2020

Hi all,

I am late to the discussion, but this is one of the topics I'm researching,
but have not been able to secure funding. It seems nobody from the LLVM
team interested in the idea.

Anyway, I'm going to explain what have in my mind.

This is a very common problem in the cryptography community. Basically,
cryptography people have said, "we are in the fight with compilers". There
are two fundamentally different approaches to this problem. The 1) shallow,
or a 2) deep theoretical approach.

In the first one [1], you can basically test and implement different
primitives in constant-time fashion and use them when the user annotate
some part of his/her program as constant time. When you see that
annotation, you will disable any optimization for that scenario and only
use the *tested* ISA instruction in each platform. There are some tools for
testing whether your code is constant time or not like Dudect and some
other tools [2,3]. This is not a guaranteed approach, but in this approach,
we are trying our best. This *only* works because the number of required
primitives is not that much. To have idea about this, look at this
Constant-time Toolkit https://github.com/pornin/CTTK (Thomas Pornin, NCC
Group).

In the second approach, the more fundamental one, your approach is to have
a theoretical concept of what is constant-time and what is not, then only
use *permitted* of optimizations for that part of the program. There is
some theoretical papers compiler team at Berkeley and French Institute for
Research in Computer Science and Automation. Integrating those concepts to
LLVM would be a huge (but extremely fun and interesting) undertaking.

This is a very interesting topic, and I had an internship offer to work on
this as a researcher, which they rescinded because of Coronavirus.
Webassembly guys would be interested in this research [6]. If anyone wants
to work on it, shoot me an email. I am really interested in collaboration.

BTW, hardware people are doing something similar too [7].

1) https://www.cl.cam.ac.uk/~rja14/Papers/whatyouc.pdf
2) https://eprint.iacr.org/2016/1123.pdf
3)
https://www.usenix.org/system/files/conference/usenixsecurity16/sec16_paper_almeida.pdf
4) https://cseweb.ucsd.edu/~dstefan/pubs/cauligi:2017:fact.pdf
5) https://hal.archives-ouvertes.fr/hal-01959560/document
6) https://dl.acm.org/doi/pdf/10.1145/3290390
7) https://www.usenix.org/system/files/sec19-gleissenthall.pdf
8) https://dl.acm.org/doi/pdf/10.1145/3371075

best wishes,
Regards.
<https://dl.acm.org/doi/pdf/10.1145/3371075>

<https://dl.acm.org/doi/pdf/10.1145/3371075>

On Thu, Apr 30, 2020 at 12:57 AM David Chisnall via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 22/04/2020 17:35, Adrien Guinet via llvm-dev wrote:
> > If we would like, at some point, to introduce such guarantees, that
> would imply adding a
> > "constant time" flag to the arithmetic operations at the LLVM IR level,
> and have the
> > backends honor it (which already seems the case), or fail if not
> possible ? .
>
> I'm really nervous about any suggestion of a constant-time flag for LLVM
> for two reasons:
>
>   1. LLVM does not have a notion of time in its abstract machine, so you
> are introducing an entirely new concept but applying it in a single
> place.  The C abstract machine does not either.  One of the most common
> flaws in papers about constant-time extensions to C is the assumption
> that they are preserving an invariant, not adding a new invariant.
>
>   2. Most ISAs do not make timing guarantees about individual
> instructions, so even if we define and preserve the guarantee throughout
> the optimisation pipeline, we can't guarantee it in the final binary.
>
> We discussed some of this on our paper on preserving security invariants
> through a compiler pipeline[1] a few years ago.
>
> In the second case, consider something as simple as an add.  There have
> been ARM implementations where a 32-bit integer add took one or two
> cycles depending on the operand values.  To guarantee constant-time
> execution, you'd need to ensure that you toggled some bits in your
> values to guarantee the slow path and then reassembled the result.
> That's a big codegen effort, but is required only for one or two our of
> hundreds of microarchitectural implementations of the AArch32 ISA.
>
> More troubling for most people should be the fact that Intel
> *explicitly* does not guarantee that CMOV is constant time.  There are
> possible microarchitectural implementations of this instruction that
> handle it entirely in the scheduler so that the instruction commits as
> soon as both the condition code and the used operand are available
> (side-effect-free instructions that lead to the unused value can be
> silently dropped).  No one implements CMOV like this, but Intel is not
> willing to commit to *never* implementing CMOV like this, so any code
> that assumes that using select instead of branches is constant time is
> not guaranteed to be.
>
> The lack of any kind of notion of time in the LLVM abstract machine
> makes plumbing this in very difficult.  Informally, a constant-time
> abstract machine has no data-dependent flow control and has no
> operations that have operand-dependent timing.  In most ISAs, this
> eliminates data-dependent loads and stores, floating point instructions,
> and integer division.
>
> If you want to support constant-time execution, I'd recommend defining
> markers for the start and end of constant-time blocks (e.g. crypto
> kernels) and explicitly annotate input values that are not secret
> dependent with metadata.  Then ensure that this metadata (insecure
> values and safe input values) is emitted in the resulting assembly.  You
> can then write a per-ISA (probably tuned per-microarchitecture)
> validator that ensures that the output is constant time according to
> your model.  You can then work backwards to find places where LLVM does
> transforms that are breaking your assumptions and work on patches to fix
> them.
>
> David
>
> [1] https://ieeexplore.ieee.org/document/8406587
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