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

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Date:   Tue, 9 Jan 2018 16:11:12 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Jim Mattson <jmattson@...gle.com>, jun.nakajima@...el.com
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Liran Alon <liran.alon@...cle.com>, dwmw@...zon.co.uk,
        bp@...en8.de, aliguori@...zon.com,
        Tom Lendacky <thomas.lendacky@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH 6/7] x86/svm: Set IBPB when running a different VCPU

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@...r.us.oracle.com') from the email. Adding Jun.
> 
> On Tue, Jan 9, 2018 at 12:47 PM, Konrad Rzeszutek Wilk
> <konrad.wilk@...cle.com> 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@...ux.intel.com 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