Re: Avoid speculative indirect calls in kernel

[Jon Masters <jcm@redhat.com>]

On 01/04/2018 01:33 PM, Linus Torvalds wrote:
> On Thu, Jan 4, 2018 at 3:26 AM, Pavel Machek <pavel@ucw.cz> wrote:
>> On Wed 2018-01-03 15:51:35, Linus Torvalds wrote:
>>>
>>> A *competent* CPU engineer would fix this by making sure speculation
>>> doesn't happen across protection domains. Maybe even a L1 I$ that is
>>> keyed by CPL.
>>
>> Would that be enough?
> 
> No, you'd need to add the CPL to the branch target buffer itself, not the I$ L1.
> 
> And as somebody pointed out, that only helps the user space messing
> with the kernel. It doesn't help the "one user context fools another
> user context to mispredict". (Where the user contexts might be a
> JIT'ed JS vs the rest of the web browser).
> 
> So you really would want to just make sure the full address is used to
> index (or at least verify) the BTB lookup, and even then you'd then
> need to invalidate the BTB on context switches so that one context
> can't fill in data for another context.

IMO the correct hardware fix is to index the BTB using the full VA
including the ASID/PCID. And guarantee (as is the case) that there is
not a live conflict between address space identifiers with entries.

The sad thing is that even the latest academic courses recommend
"optimizing" branch predictors with a few low order bits (e.g. 31 in
Intel's case, various others for different vendors). The fix for variant
3 is similarly not that difficult in new hardware: don't allow the
speculated load to happen by enforcing the permission check at the right
time. The last several editions of Computer Architecture spell this out
in Appendix B (page 37 or thereabouts).

Jon.


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