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Subject: Re: [PATCH] KVM: nSVM: vmentry ignores EFER.LMA and possibly
 RFLAGS.VM
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Jim Mattson <jmattson@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm list <kvm@vger.kernel.org>
Date:   Fri, 10 Jul 2020 11:37:13 +0300
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On Thu, 2020-07-09 at 20:41 +0200, Paolo Bonzini wrote:
> On 09/07/20 20:40, Jim Mattson wrote:
> > On Thu, Jul 9, 2020 at 11:31 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
> > > On 09/07/20 20:28, Jim Mattson wrote:
> > > > > That said, the VMCB here is guest memory and it can change under our
> > > > > feet between nested_vmcb_checks and nested_prepare_vmcb_save.  Copying
> > > > > the whole save area is overkill, but we probably should copy at least
> > > > > EFER/CR0/CR3/CR4 in a struct at the beginning of nested_svm_vmrun; this
> > > > > way there'd be no TOC/TOU issues between nested_vmcb_checks and
> > > > > nested_svm_vmrun.  This would also make it easier to reuse the checks in
> > > > > svm_set_nested_state.  Maybe Maxim can look at it while I'm on vacation,
> > > > > as he's eager to do more nSVM stuff. :D
> > > > 
> > > > I fear that nested SVM is rife with TOCTTOU issues.
> > > 
> > > I am pretty sure about that, actually. :)
> > > 
> > > Another possibility to stomp them in a more efficient manner could be to
> > > rely on the dirty flags, and use them to set up an in-memory copy of the
> > > VMCB.
> > 
> > That sounds like a great idea! Is Maxim going to look into that?
> > 
> 
> Now he is!

Yep :-)

Best regards,
	Maxim Levitsky

> 
> Paolo
>