linux-kernel - Re: [PATCH 03/15] KVM: SVM: Inject #UD on RDTSCP when it should be disabled in the guest

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Message-ID: <YKQUZF8Ejxh7Eytg@google.com>
Date:   Tue, 18 May 2021 19:24:20 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Reiji Watanabe <reijiw@...gle.com>
Subject: Re: [PATCH 03/15] KVM: SVM: Inject #UD on RDTSCP when it should be
 disabled in the guest

On Tue, May 18, 2021, Paolo Bonzini wrote:
> On 10/05/21 18:56, Sean Christopherson wrote:
> > On Mon, May 10, 2021, Maxim Levitsky wrote:
> > > On Tue, 2021-05-04 at 14:58 -0700, Jim Mattson wrote:
> > > > On Tue, May 4, 2021 at 2:57 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
> > > > > On 04/05/21 23:53, Sean Christopherson wrote:
> > > > > > > Does the right thing happen here if the vCPU is in guest mode when
> > > > > > > userspace decides to toggle the CPUID.80000001H:EDX.RDTSCP bit on or
> > > > > > > off?
> > > > > > I hate our terminology.  By "guest mode", do you mean running the vCPU, or do
> > > > > > you specifically mean running in L2?
> > > > > > 
> > > > > 
> > > > > Guest mode should mean L2.
> > > > > 
> > > > > (I wonder if we should have a capability that says "KVM_SET_CPUID2 can
> > > > > only be called prior to KVM_RUN").
> > > > 
> > > > It would certainly make it easier to reason about potential security issues.
> > > > 
> > > I vote too for this.
> > 
> > Alternatively, what about adding KVM_VCPU_RESET to let userspace explicitly
> > pull RESET#, and defining that ioctl() to freeze the vCPU model?  I.e. after
> > userspace resets the vCPU, KVM_SET_CPUID (and any other relevant ioctls() is
> > disallowed.
> > 
> > Lack of proper RESET emulation is annoying, e.g. userspace has to manually stuff
> > EDX after vCPU creation to get the right value at RESET.  A dedicated ioctl()
> > would kill two birds with one stone, without having to add yet another "2"
> > ioctl().
> 
> That has a disadvantage of opting into the more secure behavior, but we can
> do both (forbidding KVM_SET_CPUID2 after both KVM_RUN and KVM_RESET).

Doesn't changing KVM_SET_CPUID2 need to be opt-in as well, e.g. if the strict
behavior is activated via a capability?