Re: [PATCH] KVM: x86/pmu: Don't expose guest LBR if the LBR_SELECT is shared per physical core

[Like Xu <like.xu.linux@gmail.com>]

On Mon, Aug 9, 2021 at 10:12 PM Liang, Kan <kan.liang@linux.intel.com> wrote:
>
>
>
> On 8/9/2021 3:48 AM, Like Xu wrote:
> > From: Like Xu <likexu@tencent.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@tencent.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 ?

>
> 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
> >