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Subject: Re: [PATCH] KVM: VMX: Forbid userspace MSR filters for x2APIC
To:     "Graf (AWS), Alexander" <graf@amazon.de>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Aaron Lewis <aaronlewis@google.com>,
        Peter Xu <peterx@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>
References: <20201019170519.1855564-1-pbonzini@redhat.com>
 <618E2129-7AB5-4F0D-A6C9-E782937FE935@amazon.de>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <c9dd6726-2783-2dfd-14d1-5cec6f69f051@redhat.com>
Date:   Tue, 20 Oct 2020 11:27:23 +0200
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On 19/10/20 19:45, Graf (AWS), Alexander wrote:
>> +        * In principle it would be possible to trap x2apic ranges
>> +        * if !lapic_in_kernel.  This however would be complicated
>> +        * because KVM_X86_SET_MSR_FILTER can be called before
>> +        * KVM_CREATE_IRQCHIP or KVM_ENABLE_CAP.
>> +        */
>> +       for (i = 0; i < ARRAY_SIZE(filter.ranges); i++)
>> +               if (range_overlaps_x2apic(&filter.ranges[i]))
>> +                       return -EINVAL;
>
> What if the default action of the filter is to "deny"? Then only an
> MSR filter to allow access to x2apic MSRs would make the full
> filtering logic adhere to the constraints, no?

Right; or more precisely, that is handled by Sean's patch that he had
posted earlier.  This patch only makes it impossible to set up such a
filter.

(That said, this is a quick patch that I posted yesterday just before
dinner.  I'll send out the real deal with docs fixes and better commit
message).

Paolo

> Also, this really deserves a comment in the API documentation :).
> 
> In fact, I think a pure comment in documentation is enough. Just make
> x2apic && filter on them "undefined behavior".