Re: Avoid speculative indirect calls in kernel

[Jon Masters]

+ Jeff Law, Nick Clifton

On 01/04/2018 03:20 AM, Woodhouse, David wrote:
> On Thu, 2018-01-04 at 03:11 +0100, Paolo Bonzini wrote:
>> On 04/01/2018 02:59, Alan Cox wrote:
>>>> But then, exactly because the retpoline approach adds quite some cruft
>>>> and leaves something to be desired, why even bother?
>>>
>>> Performance
>>
>> Dunno.  If I care about mitigating this threat, I wouldn't stop at
>> retpolines even if the full solution has pretty bad performance (it's
>> roughly in the same ballpark as PTI).  But if I don't care, I wouldn't
>> want retpolines either, since they do introduce a small slowdown (10-20
>> cycles per indirect branch, meaning that after a thousand such papercuts
>> they become slower than the full solution).
>>
>> A couple manually written asm retpolines may be good as mitigation to
>> block the simplest PoCs (Linus may disagree), but patching the compiler,
>> getting alternatives right, etc. will take a while.  The only redeeming
>> grace of retpolines is that they don't require a microcode update, but
>> the microcode will be out there long before these patches are included
>> and trickle down to distros...  I just don't see the point in starting
>> from retpolines or drawing the line there.
> 
> No, really. The full mitigation with the microcode update and IBRS
> support is *slow*. Horribly slow.

It is horribly slow, though the story changes with CPU generation as
others noted (and what needs disabling in the microcode). We did various
analysis of the retpoline patches, including benchmarks, and we decided
that the fastest and safest approach for Tue^W yesterday was to use the
new MSRs. Especially in light of the corner cases we would need to
address for an empty RSB, etc. I'm adding Jeff Law because he and the
tools team have done analysis on this and he may have thoughts.

There's also a cross-architecture concern here in that different
solutions are needed across architectures. Retpolines are not endorsed
or recommended by every architecture vendor at this time. It's important
to make sure the necessary cross-vendor discussion happens now that it
can happen in the open.

Longer term, it'll be good to see BTBs tagged using the full address
space (including any address space IDs...) in future silicon.

Jon.

-- 
Computer Architect | Sent from my Fedora powered laptop