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

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Message-ID: <717a1b5d-1bf3-5f72-147a-faccd4611b87@redhat.com>
Date:   Thu, 9 Jul 2020 20:31:36 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.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 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.

Paolo