[cfe-dev] Clang insertion of fence instructions to mitigate Spectre variant 1?

[Chandler Carruth via cfe-dev]

I've posted an RFC about this mitigation approach now:
http://lists.llvm.org/pipermail/llvm-dev/2018-March/122085.html

My patch also happens to implement fence insertion, but the costs seem
prohibitive.

On Thu, Mar 22, 2018 at 3:25 PM Reid Kleckner <rnk at google.com> wrote:

> I know Chandler is working on a different, hopefully less expensive,
> mitigation based on load masking. He has a document that he is polishing
> that should be sent for RFC soon.
>
> On Wed, Mar 21, 2018, 5:59 AM Dallman, John via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> As far as I've been able to learn, the only way to avoid security
>> vulnerabilities due to Spectre variant 1 (CVE-2017-5753, “bounds check
>> bypass”) is to insert fences to control the relevant speculative reads. I'm
>> interested in doing this because I work on a numerical modelling library
>> that is used in many applications, which are used to handle valuable
>> information. There's been at least one piece of malware that specifically
>> targeted one of those applications, so I work at a moderate level of
>> paranoia.
>>
>> I've found information about __builtin_load_no_speculate, but inserting
>> those by hand into ten million lines of branchy C code that's under active
>> development is not an attractive prospect.
>>
>> MSVC has recently gained a /QSpectre option that tries to do this for you
>> (
>> *https://blogs.msdn.microsoft.com/vcblog/2018/01/15/spectre-mitigations-in-msvc/*
>> <https://blogs.msdn.microsoft.com/vcblog/2018/01/15/spectre-mitigations-in-msvc/>).
>> It only handles a very limited range of cases at present (
>> *https://www.paulkocher.com/doc/MicrosoftCompilerSpectreMitigation.html*
>> <https://www.paulkocher.com/doc/MicrosoftCompilerSpectreMitigation.html>),
>> but Microsoft are working on improving that. Red Hat tell me that there is
>> work underway to add something similar to GCC, although it's probably a
>> year away.
>>
>> While such a capability can't be completely fool-proof, I can well
>> believe that it's possible to do as good a job as bored humans, and it will
>> be much cheaper.
>>
>> Are there any plans to add something equivalent to Clang?
>>
>> Thanks,
>>
>> --
>> John Dallman
>> DF PL TO OT PC PDE
>> Technology & Innovation
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