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

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Message-ID: <a341f7ca-4739-db84-74e0-27859bba9eeb@redhat.com>
Date:   Thu, 9 Jul 2020 20:41:29 +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 20:40, Jim Mattson wrote:
> 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?
> 

Now he is!

Paolo