Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

[Andy Lutomirski]

On Tue, Jan 23, 2018 at 4:47 PM, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:
> On 01/23/2018 03:14 PM, Woodhouse, David wrote:
>> On Tue, 2018-01-23 at 14:49 -0800, Andi Kleen wrote:
>>>> 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.
>>>
>>> I agree. A reasonable strategy would be to only enable it for
>>> processes that have dumpable disabled. This should be already set for
>>> high value processes like GPG, and allows others to opt-in if
>>> they need to.
>>
>> That seems to make sense, and I think was the solution we were
>> approaching for IBPB on context switch too, right?
>>
>> Are we generally agreed on dumpable as the criterion for both of those?
>>
>
> It is a reasonable approach.  Let a process who needs max security
> opt in with disabled dumpable. It can have a flush with IBPB clear before
> starting to run, and have STIBP set while running.
>

Do we maybe want a separate opt in?  I can easily imagine things like
web browsers that *don't* want to be non-dumpable but do want this
opt-in.

Also, what's the performance hit of STIBP?