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

[Jim Mattson]

If my documentation is up-to-date, writing IBRS does not clear the RSB
(except for parts which contain an RSB that is not filled by 32
CALLs).

On Tue, Jan 9, 2018 at 1:11 PM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> On Tue, Jan 09, 2018 at 12:57:38PM -0800, Jim Mattson wrote:
>> Before VM-entry, don't we need to flush the BHB and the RSB to avoid
>> revealing KASLR information to the guest? (Thanks to Liran for
>> pointing this out.)
>
> Exactly.
>
> Or is is touching with any value good enough?
>
> (Removing 't@xxxxxxxxxxxxxxxxxx') from the email. Adding Jun.
>>
>> On Tue, Jan 9, 2018 at 12:47 PM, Konrad Rzeszutek Wilk
>> <konrad.wilk@xxxxxxxxxx> wrote:
>> > On Tue, Jan 09, 2018 at 03:39:09PM -0500, Konrad Rzeszutek Wilk wrote:
>> >> 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.
>> >
>> > I suppose means that when we VMENTER the original fix (where we
>> > compare the host to guest) can stay - as we entering an lower prediction
>> > mode. I wonder then what does writting 0 do to it? A nop?
>> >
>> >>
>> >>
>> >> <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