linux-kernel - Re: [PATCH] KVM: VMX: Forbid userspace MSR filters for x2APIC

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Message-ID: <bce2aee1-bfac-0640-066b-068fa5f12cf8@amazon.de>
Date:   Tue, 20 Oct 2020 11:48:07 +0200
From:   Alexander Graf <graf@...zon.de>
To:     Paolo Bonzini <pbonzini@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Aaron Lewis <aaronlewis@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH] KVM: VMX: Forbid userspace MSR filters for x2APIC

On 20.10.20 11:27, Paolo Bonzini wrote:
> 
> 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.

What I'm saying is that a "filter rule" can either mean "allow" or 
"deny". Here you're only checking if there is a rule. What you want to 
filter for is whether there is a denying rule (including default 
fallback) for x2apic after all rules are in place.

Imagine you add the following filter:

{
     count: 1,
     default_allow: false,
     ranges: [
         {
             flags: KVM_MSR_FILTER_READ,
             nmsrs: 1,
             base: MSR_EFER,
             bitmap: { 1 },
         },
     ],
}

That filter would set all x2apic registers to "deny", but would not be 
caught by the code above. Conversely, a range that explicitly allows 
x2apic ranges with default_allow=0 would be rejected by this patch.

Alex

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