linux-kernel - Re: [PATCH v6 5/7] KVM: x86: VMX: Prevent MSR passthrough when MSR access is denied

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Message-ID: <e61772b5-4c64-8407-2b52-dbba2d488f90@amazon.com>
Date:   Wed, 16 Sep 2020 22:36:08 +0200
From:   Alexander Graf <graf@...zon.com>
To:     Aaron Lewis <aaronlewis@...gle.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        KarimAllah Raslan <karahmed@...zon.de>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        kvm list <kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 5/7] KVM: x86: VMX: Prevent MSR passthrough when MSR
 access is denied



On 16.09.20 22:13, Aaron Lewis wrote:
> 
>>>
>>>> +
>>>>    /*
>>>>     * These 2 parameters are used to config the controls for Pause-Loop Exiting:
>>>>     * ple_gap:    upper bound on the amount of time between two successive
>>>> @@ -622,6 +642,41 @@ static inline bool report_flexpriority(void)
>>>>           return flexpriority_enabled;
>>>>    }
>>>
>>> One thing that seems to be missing is removing MSRs from the
>>> permission bitmap or resetting the permission bitmap to its original
>>> state before adding changes on top of it.  This would be needed on
>>> subsequent calls to kvm_vm_ioctl_set_msr_filter().  When that happens
>>> the original changes made by KVM_REQ_MSR_FILTER_CHANGED need to be
>>> backed out before applying the new set.
>>
>> I'm not sure I follow. Subsequent calls to set_msr_filter() will invoke
>> the "please reset the whole MSR passthrough bitmap to a consistent
>> state" which will then reapply the in-kvm desired state through the
>> bitmap and filter state on top on each of those.
>>
> 
> Yes, you're correct.  I discovered this after the fact by adding a
> test to the selftest I wrote for the deny list system which I revamped
> to work for your filter system.  It proved the permission bitmaps are
> in fact set as expected on subsequent calls.  You can disregard this
> comment.
> 
> As a side note, I'm happy to share the test if you'd like. I also used
> it to uncover an issue in the first commit of this series.

I really enjoy the tests that you submitted and would love to see you 
add your test coverage to the filtering patch set :)


Thanks!

Alex




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