Re: [9/8] KVM: x86: limit MSR_IA32_SPEC_CTRL access based on CPUID availability

[Jim Mattson]

It's really hard to tell which patches are being proposed for which
repositories, but assuming that everything else is correct, I don't
think your condition is adequate. What if the physical CPU and the
virtual CPU both have CPUID.(EAX=7H,ECX=0):EDX[26], but only the
physical CPU has CPUID.(EAX=7H,ECX=0):EDX[27]? If the guest has write
access to MSR_IA32_SPEC_CTRL, it can set MSR_IA32_SPEC_CTRL[1]
(STIBP), even though setting that bit in the guest should raise #GP.

On Tue, Jan 30, 2018 at 8:43 AM, Mihai Carabas <mihai.carabas@xxxxxxxxxx> wrote:
> On 30.01.2018 18:33, Jim Mattson wrote:
>>
>> All MSR intercepts are enabled by default, so I don't think this patch
>> does anything at all, unless I'm missing some context.
>>
>
> Currently on upstream some MSR are intercepted:
> https://github.com/torvalds/linux/blob/master/arch/x86/kvm/vmx.c#L6838
>
> In particular to this patch, the MSR_IA32_SPEC_CTRL intercept is disabled in
> 3/8: https://patchwork.kernel.org/patch/10151889/
>
>
>
>> On Tue, Jan 30, 2018 at 5:21 AM, Mihai Carabas <mihai.carabas@xxxxxxxxxx>
>> wrote:
>>>
>>> Hello Paolo,
>>>
>>> I've back ported this patch on 4.1, after adding the per-vcpu MSR bitmap.
>>> Also enabled the SPEC_CTRL_MSR intercept if qemu instructed so [1].
>>>
>>> Reviewed-by: Mihai Carabas <mihai.carabas@xxxxxxxxxx>
>>>
>>> [1]
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -8391,6 +8391,16 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm
>>> *kvm, unsigned int id)
>>>          vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PRED_CMD,
>>> MSR_TYPE_R | MSR_TYPE_W);
>>>          vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_BNDCFGS,
>>> MSR_TYPE_R | MSR_TYPE_W);
>>>
>>> +       /*
>>> +        * If the physical CPU or the vCPU of this VM doesn't
>>> +        * support SPEC_CTRL feature, catch each access to it.
>>> +        */
>>> +       if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
>>> +            !guest_cpuid_has_spec_ctrl(&vmx->vcpu))
>>> +               vmx_enable_intercept_for_msr(
>>> +                       msr_bitmap,
>>> +                       MSR_IA32_SPEC_CTRL,
>>> +                       MSR_TYPE_R | MSR_TYPE_W);
>>>
>>>          /*
>>>           * If PML is turned on, failure on enabling PML just results in
>>> failure
>>>
>>>
>>>
>>> On 09.01.2018 14:03, Paolo Bonzini wrote:
>>>>
>>>>
>>>> MSR_IA32_SPEC_CTRL is not available unless CPU[7,0].EDX[26] is 1.
>>>> Check that against host CPUID or guest CPUID, respectively for
>>>> host-initiated and guest-initiated accesses.
>>>>
>>>> Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
>>>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>>>> ---
>>>>          This is for after X86_FEATURE_SPEC_CTRL is added to Linux, but
>>>>          I still wanted to ack Jim's improvement.
>>>>
>>>>    arch/x86/kvm/svm.c | 8 ++++++++
>>>>    arch/x86/kvm/vmx.c | 8 ++++++++
>>>>    2 files changed, 16 insertions(+)
>>>>
>>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>>>> index 97126c2bd663..3a646580d7c5 100644
>>>> --- a/arch/x86/kvm/svm.c
>>>> +++ b/arch/x86/kvm/svm.c
>>>> @@ -3648,6 +3648,10 @@ static int svm_get_msr(struct kvm_vcpu *vcpu,
>>>> struct msr_data *msr_info)
>>>>                  msr_info->data = svm->nested.vm_cr_msr;
>>>>                  break;
>>>>          case MSR_IA32_SPEC_CTRL:
>>>> +               if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
>>>> +                   (!msr_info->host_initiated &&
>>>> +                            !guest_cpuid_has(vcpu,
>>>> X86_FEATURE_SPEC_CTRL)))
>>>> +                       return 1;
>>>>                  msr_info->data = svm->spec_ctrl;
>>>>                  break;
>>>>          case MSR_IA32_UCODE_REV:
>>>> @@ -3806,6 +3810,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu,
>>>> struct msr_data *msr)
>>>>                  vcpu_unimpl(vcpu, "unimplemented wrmsr: 0x%x data
>>>> 0x%llx\n", ecx, data);
>>>>                  break;
>>>>          case MSR_IA32_SPEC_CTRL:
>>>> +               if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
>>>> +                   (!msr_info->host_initiated &&
>>>> +                            !guest_cpuid_has(vcpu,
>>>> X86_FEATURE_SPEC_CTRL)))
>>>> +                       return 1;
>>>>                  svm->spec_ctrl = data;
>>>>                  break;
>>>>          case MSR_IA32_APICBASE:
>>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>>> index 49b4a2d61603..42bc7ee293e4 100644
>>>> --- a/arch/x86/kvm/vmx.c
>>>> +++ b/arch/x86/kvm/vmx.c
>>>> @@ -3368,6 +3368,10 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu,
>>>> struct msr_data *msr_info)
>>>>                  msr_info->data = guest_read_tsc(vcpu);
>>>>                  break;
>>>>          case MSR_IA32_SPEC_CTRL:
>>>> +               if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
>>>> +                   (!msr_info->host_initiated &&
>>>> +                    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
>>>> +                       return 1;
>>>>                  msr_info->data = to_vmx(vcpu)->spec_ctrl;
>>>>                  break;
>>>>          case MSR_IA32_SYSENTER_CS:
>>>> @@ -3510,6 +3514,10 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu,
>>>> struct msr_data *msr_info)
>>>>                  kvm_write_tsc(vcpu, msr_info);
>>>>                  break;
>>>>          case MSR_IA32_SPEC_CTRL:
>>>> +               if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
>>>> +                   (!msr_info->host_initiated &&
>>>> +                    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
>>>> +                       return 1;
>>>>                  to_vmx(vcpu)->spec_ctrl = data;
>>>>                  break;
>>>>          case MSR_IA32_CR_PAT:
>>>>
>>>
>