linux-kernel - Re: [PATCH] KVM: nSVM: vmentry ignores EFER.LMA and possibly RFLAGS.VM

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Date:   Thu, 9 Jul 2020 11:40:53 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH] KVM: nSVM: vmentry ignores EFER.LMA and possibly RFLAGS.VM

On Thu, Jul 9, 2020 at 11:31 AM Paolo Bonzini <pbonzini@...hat.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?