linux-kernel - Re: [PATCH] KVM: x86/vPMU: Forbid writing to MSR_F15H_PERF MSRs when guest doesn't have X86_FEATURE_PERFCTR

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Date:   Thu, 25 Mar 2021 16:33:08 +0800
From:   Haiwei Li <lihaiwei.kernel@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Wei Huang <wei.huang2@....com>, Joerg Roedel <joro@...tes.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/vPMU: Forbid writing to MSR_F15H_PERF MSRs when
 guest doesn't have X86_FEATURE_PERFCTR_CORE

On Thu, Mar 25, 2021 at 4:10 PM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Haiwei Li <lihaiwei.kernel@...il.com> writes:
>
> > On Tue, Mar 23, 2021 at 4:48 PM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> >>
> >> MSR_F15H_PERF_CTL0-5, MSR_F15H_PERF_CTR0-5 MSRs are only available when
> >> X86_FEATURE_PERFCTR_CORE CPUID bit was exposed to the guest. KVM, however,
> >> allows these MSRs unconditionally because kvm_pmu_is_valid_msr() ->
> >> amd_msr_idx_to_pmc() check always passes and because kvm_pmu_set_msr() ->
> >> amd_pmu_set_msr() doesn't fail.
> >>
> >> In case of a counter (CTRn), no big harm is done as we only increase
> >> internal PMC's value but in case of an eventsel (CTLn), we go deep into
> >> perf internals with a non-existing counter.
> >>
> >> Note, kvm_get_msr_common() just returns '0' when these MSRs don't exist
> >> and this also seems to contradict architectural behavior which is #GP
> >> (I did check one old Opteron host) but changing this status quo is a bit
> >> scarier.
> >
> > When msr doesn't exist, kvm_get_msr_common() returns KVM_MSR_RET_INVALID
> > in `default:` and kvm_complete_insn_gp() will inject #GP to guest.
> >
>
> I'm looking at the following in kvm_get_msr_common():
>
>         switch (msr_info->index) {
>         ...
>         case MSR_F15H_PERF_CTL0 ... MSR_F15H_PERF_CTR5:
>         ...
>                 if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
>                         return kvm_pmu_get_msr(vcpu, msr_info);
>                 msr_info->data = 0;
>                 break;
>         ...
>         }
>         return 0;
>
> so it's kind of 'always exists' or am I wrong?

I am sorry. You are right. You were talking about `MSR_F15H_PERF_CTL0
... MSR_F15H_PERF_CTR5`.
I thought you were talking about the registers not catched in
kvm_get_msr_common().

>
> > Also i have wrote a kvm-unit-test, tested both on amd EPYC and intel
> > CascadeLake. A #GP error was printed.
> > Just like:
> >
> > Unhandled exception 13 #GP at ip 0000000000400420
> > error_code=0000      rflags=00010006      cs=00000008
> > rax=0000000000000000 rcx=0000000000000620 rdx=00000000006164a0
> > rbx=0000000000009500
> > rbp=0000000000517490 rsi=0000000000616ae0 rdi=0000000000000001
> >  r8=0000000000000001  r9=00000000000003f8 r10=000000000000000d
> > r11=0000000000000000
> > r12=0000000000000000 r13=0000000000000000 r14=0000000000000000
> > r15=0000000000000000
> > cr0=0000000080000011 cr2=0000000000000000 cr3=000000000040b000
> > cr4=0000000000000020
> > cr8=0000000000000000
> > STACK: @400420 400338
>
> Did this happen on read or write? The later is expected, the former is
> not. Could you maybe drop your code here, I'd like to see what's going
> on.

I did a bad test. The msr tested is not catched in
kvm_get_msr_common(). As said, I misunderstood
what you meant.

I have tested your patch and replied. If you have any to test, I'm
glad to help. :)

--
Haiwei Li