linux-kernel - Re: [PATCH v3 00/17] KVM: x86/pmu: Add support to enable Guest PEBS via DS

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Date:   Mon, 25 Jan 2021 20:07:06 +0800
From:   "Xu, Like" <like.xu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Andi Kleen <andi@...stfloor.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, eranian@...gle.com,
        kvm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, wei.w.wang@...el.com,
        luwei.kang@...el.com, linux-kernel@...r.kernel.org,
        Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v3 00/17] KVM: x86/pmu: Add support to enable Guest PEBS
 via DS

On 2021/1/25 19:13, Peter Zijlstra wrote:
> On Mon, Jan 25, 2021 at 04:08:22PM +0800, Like Xu wrote:
>> Hi Peter,
>>
>> On 2021/1/22 17:56, Peter Zijlstra wrote:
>>> On Fri, Jan 15, 2021 at 10:51:38AM -0800, Sean Christopherson wrote:
>>>> On Fri, Jan 15, 2021, Andi Kleen wrote:
>>>>>> I'm asking about ucode/hardare.  Is the "guest pebs buffer write -> PEBS PMI"
>>>>>> guaranteed to be atomic?
>>>>> Of course not.
>>>> So there's still a window where the guest could observe the bad counter index,
>>>> correct?
>>> Guest could do a hypercall to fix up the DS area before it tries to read
>>> it I suppose. Or the HV could expose the index mapping and have the
>>> guest fix up it.
>> A weird (malicious) guest would read unmodified PEBS records in the
>> guest PEBS buffer from other vCPUs without the need for hypercall or
>> index mapping from HV.
>>
>> Do you see any security issues on this host index leak window?
>>
>>> Adding a little virt crud on top shouldn't be too hard.
>>>
>> The patches 13-17 in this version has modified the guest PEBS buffer
>> to correct the index mapping information in the guest PEBS records.
> Right, but given there is no atomicity between writing the DS area and
> triggering the PMI (as already established earlier in this thread), a
> malicious guest can already access this information, no?
>

So under the premise that counter cross-mapping is allowed,
how can hypercall help fix it ?

Personally,  I think it is acceptable at the moment, and
no security issues based on this have been defined and found.