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Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation
To:     "Woodhouse, David" <dwmw@amazon.co.uk>,
        Andy Lutomirski <luto@amacapital.net>,
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From:   Tom Lendacky <thomas.lendacky@amd.com>
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Date:   Tue, 23 Jan 2018 16:37:35 -0600
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On 1/23/2018 10:20 AM, Woodhouse, David wrote:
> On Tue, 2018-01-23 at 10:12 -0600, Tom Lendacky wrote:
>>
>>>> +.macro UNRESTRICT_IB_SPEC
>>>> +    ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
>>>> +    PUSH_MSR_REGS
>>>> +    WRMSR_ASM $MSR_IA32_SPEC_CTRL, $0, $0
>>>  
>> I think you should be writing 2, not 0, since I'm reasonably
>> confident that we want STIBP on.  Can you explain why you're writing
>> 0?
>>
>> Do we want to talk about STIBP in general?  Should it be (yet another)
>> boot option to enable or disable?  If there is STIBP support without
>> IBRS support, it could be a set and forget at boot time.
> 
> We haven't got patches which enable STIBP in general. The kernel itself
> is safe either way with retpoline, or because IBRS implies STIBP too
> (that is, there's no difference between writing 1 and 3).
> 
> So STIBP is purely about protecting userspace processes from one
> another, and VM guests from one another, when they run on HT siblings.
> 
> There's an argument that there are so many other information leaks
> between HT siblings that we might not care. Especially as it's hard to
> *tell* when you're scheduling, whether you trust all the processes (or
> guests) on your HT siblings right now... let alone later when
> scheduling another process if you need to *now* set STIBP on a sibling
> which is no longer save from this process now running.
> 
> I'm not sure we want to set STIBP *unconditionally* either because of
> the performance implications.
> 
> For IBRS we had an answer and it was just ugly. For STIBP we don't
> actually have an answer for "how do we use this?". Do we?

Not sure.  Maybe to start, the answer might be to allow it to be set for
the ultra-paranoid, but in general don't enable it by default.  Having it
enabled would be an alternative to someone deciding to disable SMT, since
that would have even more of a performance impact.

Thanks,
Tom

> 
>