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Subject: Re: [PATCH 03/15] KVM: SVM: Inject #UD on RDTSCP when it should be
 disabled in the guest
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Sean Christopherson <seanjc@google.com>
Cc:     Jim Mattson <jmattson@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Xiaoyao Li <xiaoyao.li@intel.com>,
        Reiji Watanabe <reijiw@google.com>
Date:   Tue, 11 May 2021 15:34:00 +0300
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On Mon, 2021-05-10 at 16:56 +0000, 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@redhat.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.

I vote even stronger for this!

I recently created what Paulo called an 'evil' test to stress KVM related
bugs in nested migration by simulating a nested migration with a vCPU reset,
followed by reload of all of its state including nested state.

It is ugly since as you say I have to  manually load the reset state, and thus 
using 'KVM_VCPU_RESET' ioctl instead would make this test much more 
cleaner and even more 'evil'.

Best regards,
	Maxim Levitsky


> 
> 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().
>