linux-kernel - Re: [PATCH] KVM: x86/pmu: Don't expose guest LBR if the LBR

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Date:   Mon, 9 Aug 2021 14:03:29 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/pmu: Don't expose guest LBR if the LBR_SELECT is
 shared per physical core



On 8/9/2021 11:08 AM, Like Xu wrote:
> On Mon, Aug 9, 2021 at 10:12 PM Liang, Kan <kan.liang@...ux.intel.com> wrote:
>>
>>
>>
>> On 8/9/2021 3:48 AM, Like Xu wrote:
>>> From: Like Xu <likexu@...cent.com>
>>>
>>> According to Intel SDM, the Last Branch Record Filtering Select Register
>>> (R/W) is defined as shared per physical core rather than per logical core
>>> on some older Intel platforms: Silvermont, Airmont, Goldmont and Nehalem.
>>>
>>> To avoid LBR attacks or accidental data leakage, on these specific
>>> platforms, KVM should not expose guest LBR capability even if HT is
>>> disabled on the host, considering that the HT state can be dynamically
>>> changed, yet the KVM capabilities are initialized at module initialisation.
>>>
>>> Fixes: be635e34c284 ("KVM: vmx/pmu: Expose LBR_FMT in the MSR_IA32_PERF_CAPABILITIES")
>>> Signed-off-by: Like Xu <likexu@...cent.com>
>>> ---
>>>    arch/x86/include/asm/intel-family.h |  1 +
>>>    arch/x86/kvm/vmx/capabilities.h     | 19 ++++++++++++++++++-
>>>    2 files changed, 19 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/include/asm/intel-family.h b/arch/x86/include/asm/intel-family.h
>>> index 27158436f322..f35c915566e3 100644
>>> --- a/arch/x86/include/asm/intel-family.h
>>> +++ b/arch/x86/include/asm/intel-family.h
>>> @@ -119,6 +119,7 @@
>>>
>>>    #define INTEL_FAM6_ATOM_SILVERMONT  0x37 /* Bay Trail, Valleyview */
>>>    #define INTEL_FAM6_ATOM_SILVERMONT_D        0x4D /* Avaton, Rangely */
>>> +#define INTEL_FAM6_ATOM_SILVERMONT_X3        0x5D /* X3-C3000 based on Silvermont */
>>
>>
>> Please submit a separate patch if you want to add a new CPU ID. Also,
>> the comments should be platform code name, not the model.
>>
>> AFAIK, Atom X3 should be SoFIA which is for mobile phone. It's an old
>> product. I don't think I enabled it in perf. I have no idea why you want
>> to add it here for KVM. If you have a product and want to enable it, I
>> guess you may want to enable it for perf first.
> 
> Thanks for your clarification about SoFIA. I'll drop 0x5D check
> for V2 since we doesn't have host support as you said.
> 
> Do the other models here and the idea of banning guest LBR make sense to you ?
> 

For the Atom after Silvermont, I don't think hyper-threading is 
supported. That's why it's per physical core. I don't think we should 
disable LBR because of it.

For Nehalem, it seems possible that the MSR_LBR_SELECT can be overridden 
if the other logical core has a different configure. But I'm not sure 
whether it brings any severe problems. Logical core A may miss some LBRs 
or get extra LBRs, but I don't think LBRs can be leaked to Logical core 
B. Also, Nehalem is a 13+ year old machine. Not sure how many people 
still use it.

LBR format 0 is also a valid format version, LBR_FORMAT_32. It seems 
this patch just forces the format to LBR_FORMAT_32 for these machines. 
It doesn't sound correct.

Thanks,
Kan
>>
>>>    #define INTEL_FAM6_ATOM_SILVERMONT_MID      0x4A /* Merriefield */
>>>
>>>    #define INTEL_FAM6_ATOM_AIRMONT             0x4C /* Cherry Trail, Braswell */
>>> diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
>>> index 4705ad55abb5..ff9596d7112d 100644
>>> --- a/arch/x86/kvm/vmx/capabilities.h
>>> +++ b/arch/x86/kvm/vmx/capabilities.h
>>> @@ -3,6 +3,7 @@
>>>    #define __KVM_X86_VMX_CAPS_H
>>>
>>>    #include <asm/vmx.h>
>>> +#include <asm/cpu_device_id.h>
>>>
>>>    #include "lapic.h"
>>>
>>> @@ -376,6 +377,21 @@ static inline bool vmx_pt_mode_is_host_guest(void)
>>>        return pt_mode == PT_MODE_HOST_GUEST;
>>>    }
>>>
>>> +static const struct x86_cpu_id lbr_select_shared_cpu[] = {
>>> +     X86_MATCH_INTEL_FAM6_MODEL(ATOM_SILVERMONT, NULL),
>>> +     X86_MATCH_INTEL_FAM6_MODEL(ATOM_SILVERMONT_MID, NULL),
>>> +     X86_MATCH_INTEL_FAM6_MODEL(ATOM_SILVERMONT_D, NULL),
>>> +     X86_MATCH_INTEL_FAM6_MODEL(ATOM_SILVERMONT_X3, NULL),
>>> +     X86_MATCH_INTEL_FAM6_MODEL(ATOM_AIRMONT_MID, NULL),
>>> +     X86_MATCH_INTEL_FAM6_MODEL(ATOM_GOLDMONT, NULL),
>>> +     X86_MATCH_INTEL_FAM6_MODEL(ATOM_GOLDMONT_PLUS, NULL),
>>> +     X86_MATCH_INTEL_FAM6_MODEL(NEHALEM_EP, NULL),
>>> +     X86_MATCH_INTEL_FAM6_MODEL(NEHALEM, NULL),
>>> +     X86_MATCH_INTEL_FAM6_MODEL(NEHALEM_G, NULL),
>>> +     X86_MATCH_INTEL_FAM6_MODEL(NEHALEM_EX, NULL),
>>> +     {}
>>> +};
>>> +
>>>    static inline u64 vmx_get_perf_capabilities(void)
>>>    {
>>>        u64 perf_cap = 0;
>>> @@ -383,7 +399,8 @@ static inline u64 vmx_get_perf_capabilities(void)
>>>        if (boot_cpu_has(X86_FEATURE_PDCM))
>>>                rdmsrl(MSR_IA32_PERF_CAPABILITIES, perf_cap);
>>>
>>> -     perf_cap &= PMU_CAP_LBR_FMT;
>>> +     if (!x86_match_cpu(lbr_select_shared_cpu))
>>> +             perf_cap &= PMU_CAP_LBR_FMT;
>>>
>>>        /*
>>>         * Since counters are virtualized, KVM would support full
>>>