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

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Message-ID: <47eb1a4a-d015-b573-d773-e34e578ad753@amazon.de>
Date:   Tue, 20 Oct 2020 12:52:20 +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 12:34, Paolo Bonzini wrote:
> 
> On 20/10/20 11:48, Alexander Graf wrote:
>>
>>      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.
> 
> Yes, but the idea is that x2apic registers are always allowed, even
> overriding default_allow, and therefore it makes no sense to have them
> in a range.  The patch is only making things fail early for userspace,
> the policy is defined by Sean's patch.

I don't think we should fail on the following:

{
     default_allow: false,
     ranges: [
         {
             flags: KVM_MSR_FILTER_READ,
             nmsrs: 4096,
             base: 0,
             bitmap: { 1, 1, 1, 1, [...] },
         },
         {
             flags: KVM_MSR_FILTER_READ,
             nmsrs: 4096,
             base: 0xc0000000,
             bitmap: { 1, 1, 1, 1, [...] },
         },
     ],
}

as a way to say "everything in normal ranges is allowed, the rest please 
deflect". Or even just to set default policies with less ranges.

Or to say it differently: Why can't we just check explicitly after 
setting up all filter lists whether x2apic MSRs are *denied*? If so, 
clear the filter and return -EINVAL.

That still leaves the case where x2apic is not handled in-kernel, but 
I'm perfectly happy to ignore that one as "user space should not care" :).


Alex



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