linux-kernel - Re: [PATCH v4 05/12] KVM: x86/pmu: Error when user sets the GLOBAL

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Date:   Fri, 7 Apr 2023 08:43:58 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 05/12] KVM: x86/pmu: Error when user sets the
 GLOBAL_STATUS reserved bits

On Fri, Apr 07, 2023, Like Xu wrote:
> On 7/4/2023 7:45 am, Sean Christopherson wrote:
> > On Tue, Feb 14, 2023, Like Xu wrote:
> > > From: Like Xu <likexu@...cent.com>
> > > 
> > > If the user space sets reserved bits when restoring the MSR_CORE_
> > > PERF_GLOBAL_STATUS register, these bits will be accidentally returned
> > > when the guest runs a read access to this register, and cannot be cleared
> > > up inside the guest, which makes the guest's PMI handler very confused.
> > 
> > The changelog needs to state what the patch actually does.
> > 
> > > Signed-off-by: Like Xu <likexu@...cent.com>
> > > ---
> > >   arch/x86/kvm/vmx/pmu_intel.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> > > index 904f832fc55d..aaea25d2cae8 100644
> > > --- a/arch/x86/kvm/vmx/pmu_intel.c
> > > +++ b/arch/x86/kvm/vmx/pmu_intel.c
> > > @@ -397,7 +397,7 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > >   			reprogram_fixed_counters(pmu, data);
> > >   		break;
> > >   	case MSR_CORE_PERF_GLOBAL_STATUS:
> > > -		if (!msr_info->host_initiated)
> > > +		if (!msr_info->host_initiated || (data & pmu->global_ovf_ctrl_mask))
> > 
> > This is wrong.  Bits 60:58 are reserved in IA32_PERF_GLOBAL_OVF_CTRL, but are
> > ASCI, CTR_FREEZE, and LBR_FREEZE respectively in MSR_CORE_PERF_GLOBAL_STATUS.
> 
> CTR_FREEZE and LBR_FREEZE are only required for the guest CPUID.0AH: EAX[7:0]>3.
> PMU support (ASCI bit) for guest SGX isn't supported either.
> 
> So for now, reusing pmu->global_ovf_ctrl_mask here is effective enough.

And "good enough for now" is exactly how we end up with bugs, especially when
"good enough" relies on assumptions that aren't well documented.