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

[David Chisnall via llvm-dev]

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