Re: [PATCH 6/7] x86/svm: Set IBPB when running a different VCPU

[Konrad Rzeszutek Wilk]

On Tue, Jan 09, 2018 at 05:49:08PM +0100, Paolo Bonzini wrote:
> On 09/01/2018 17:23, Arjan van de Ven wrote:
> > On 1/9/2018 8:17 AM, Paolo Bonzini wrote:
> >> On 09/01/2018 16:19, Arjan van de Ven wrote:
> >>> On 1/9/2018 7:00 AM, Liran Alon wrote:
> >>>>
> >>>> ----- arjan@xxxxxxxxxxxxxxx wrote:
> >>>>
> >>>>> On 1/9/2018 3:41 AM, Paolo Bonzini wrote:
> >>>>>> The above ("IBRS simply disables the indirect branch predictor")
> >>>>>> was my
> >>>>>> take-away message from private discussion with Intel.  My guess is
> >>>>>> that
> >>>>>> the vendors are just handwaving a spec that doesn't match what
> >>>>>> they have
> >>>>>> implemented, because honestly a microcode update is unlikely to do
> >>>>>> much
> >>>>>> more than an old-fashioned chicken bit.  Maybe on Skylake it does
> >>>>>> though, since the performance characteristics of IBRS are so
> >>>>>> different
> >>>>>> from previous processors.  Let's ask Arjan who might have more
> >>>>>> information about it, and hope he actually can disclose it...
> >>>>>
> >>>>> IBRS will ensure that, when set after the ring transition, no earlier
> >>>>> branch prediction data is used for indirect branches while IBRS is
> >>>>> set
> >>
> >> Let me ask you my questions, which are independent of L0/L1/L2
> >> terminology.
> >>
> >> 1) Is vmentry/vmexit considered a ring transition, even if the guest is
> >> running in ring 0?  If IBRS=1 in the guest and the host is using IBRS,
> >> the host will not do a wrmsr on exit.  Is this safe for the host kernel?
> > 
> > I think the CPU folks would want us to write the msr again.
> 
> Want us, or need us---and if we don't do that, what happens?  And if we
> have to do it, how is IBRS=1 different from an IBPB?...

Arjan says 'ring transition' but I am pretty sure it is more of 'prediction
mode change'. And from what I have gathered so far moving from lower (guest)
to higher (hypervisor) has no bearing on the branch predicator. Meaning
the guest ring0 can attack us if we don't touch this MSR.

We have to WRMSR 0x48 to 1 to flush out lower prediction. Aka this is a
'reset' button and at every 'prediction mode' you have to hit this.


<sigh> Can we have a discussion on making an kvm-security mailing list
where we can figure all this out during embargo and not have these
misunderstandings.

> 
> Since I am at it, what happens on *current generation* CPUs if you
> always leave IBRS=1?  Slow and safe, or fast and unsafe?
> 
> >> 2) How will the future processors work where IBRS should always be =1?
> > 
> > IBRS=1 should be "fire and forget this ever happened".
> > This is the only time anyone should use IBRS in practice
> 
> And IBPB too I hope?  But besides that, I need to know exactly how that
> is implemented to ensure that it's doing the right thing.
> 
> > (and then the host turns it on and makes sure to not expose it to the
> > guests I hope)
> 
> That's not that easy, because guests might have support for SPEC_CTRL
> but not for IA32_ARCH_CAPABILITIES.
> 
> You could disable the SPEC_CTRL bit, but then the guest might think it
> is not secure.  It might also actually *be* insecure, if you migrated to
> an older CPU where IBRS is not fire-and-forget.
> 
> Paolo