linux-kernel - Re: [PATCH v3 7/9] KVM: VMX: Add guest physical address check in EPT violation and misconfig

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Date:   Fri, 23 Oct 2020 09:59:03 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Mohammed Gamal <mgamal@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3 7/9] KVM: VMX: Add guest physical address check in EPT
 violation and misconfig

On Fri, Oct 23, 2020 at 2:22 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 23/10/20 05:14, Sean Christopherson wrote:
> >>>> +
> >>>> +       /*
> >>>> +        * Check that the GPA doesn't exceed physical memory limits, as that is
> >>>> +        * a guest page fault.  We have to emulate the instruction here, because
> >>>> +        * if the illegal address is that of a paging structure, then
> >>>> +        * EPT_VIOLATION_ACC_WRITE bit is set.  Alternatively, if supported we
> >>>> +        * would also use advanced VM-exit information for EPT violations to
> >>>> +        * reconstruct the page fault error code.
> >>>> +        */
> >>>> +       if (unlikely(kvm_mmu_is_illegal_gpa(vcpu, gpa)))
> >>>> +               return kvm_emulate_instruction(vcpu, 0);
> >>>> +
> >>> Is kvm's in-kernel emulator up to the task? What if the instruction in
> >>> question is AVX-512, or one of the myriad instructions that the
> >>> in-kernel emulator can't handle? Ice Lake must support the advanced
> >>> VM-exit information for EPT violations, so that would seem like a
> >>> better choice.
> >>>
> >> Anyone?
> >
> > Using "advanced info" if it's supported seems like the way to go.  Outright
> > requiring it is probably overkill; if userspace wants to risk having to kill a
> > (likely broken) guest, so be it.
>
> Yeah, the instruction is expected to page-fault here.  However the
> comment is incorrect and advanced information does not help here.
>
> The problem is that page fault error code bits cannot be reconstructed
> from bits 0..2 of the EPT violation exit qualification, if bit 8 is
> clear in the exit qualification (that is, if the access causing the EPT
> violation is to a paging-structure entry).  In that case bits 0..2 refer
> to the paging-structure access rather than to the final access.  In fact
> advanced information is not available at all for paging-structure access
> EPT violations.

True, but the in-kernel emulator can only handle a very small subset
of the available instructions.

If bit 8 is set in the exit qualification, we should use the advanced
VM-exit information. If it's clear, we should just do a software page
walk of the guest's x86 page tables. The in-kernel emulator should
only be used as a last resort on hardware that doesn't support the
advanced VM-exit information for EPT violations.