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In-Reply-To: <9240ada8-8e18-d2b2-006e-41ededb89efb@redhat.com>
From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Thu, 1 Aug 2019 11:35:17 +0800
Message-ID: <CANRm+CwJyShOHCanUNmeq8Rr3OWJc1iw0vq5ZAF_WLD-0mSEHA@mail.gmail.com>
Subject: Re: [PATCH v2 3/3] KVM: Fix leak vCPU's VMCS value into other pCPU
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        "# v3 . 10+" <stable@vger.kernel.org>,
        Marc Zyngier <Marc.Zyngier@arm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>
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On Wed, 31 Jul 2019 at 20:55, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 31/07/19 13:39, Wanpeng Li wrote:
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index ed061d8..12f2c91 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2506,7 +2506,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
> >                               continue;
> >                       if (vcpu == me)
> >                               continue;
> > -                     if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
> > +                     if (READ_ONCE(vcpu->preempted) && swait_active(&vcpu->wq))
> >                               continue;
> >                       if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
> >                               !kvm_arch_vcpu_in_kernel(vcpu))
> >
>
> This cannot work.  swait_active means you are waiting, so you cannot be
> involuntarily preempted.
>
> The problem here is simply that kvm_vcpu_has_events is being called
> without holding the lock.  So kvm_arch_vcpu_runnable is okay, it's the
> implementation that's wrong.
>
> Just rename the existing function to just vcpu_runnable and make a new
> arch callback kvm_arch_dy_runnable.   kvm_arch_dy_runnable can be
> conservative and only returns true for a subset of events, in particular
> for x86 it can check:
>
> - vcpu->arch.pv.pv_unhalted
>
> - KVM_REQ_NMI or KVM_REQ_SMI or KVM_REQ_EVENT
>
> - PIR.ON if APICv is set
>
> Ultimately, all variables accessed in kvm_arch_dy_runnable should be
> accessed with READ_ONCE or atomic_read.
>
> And for all architectures, kvm_vcpu_on_spin should check
> list_empty_careful(&vcpu->async_pf.done)
>
> It's okay if your patch renames the function in non-x86 architectures,
> leaving the fix to maintainers.  So, let's CC Marc and Christian since
> ARM and s390 have pretty complex kvm_arch_vcpu_runnable as well.

Ok, just sent patch to do this.

Regards,
Wanpeng Li