Re: [PATCH v7 3/7] KVM: VMX: Pass through CET related MSRs to Guest

[Jim Mattson]

On Thu, Sep 26, 2019 at 7:17 PM Yang Weijiang <weijiang.yang@xxxxxxxxx> wrote:
>
> CET MSRs pass through Guest directly to enhance performance.
> CET runtime control settings are stored in MSR_IA32_{U,S}_CET,
> Shadow Stack Pointer(SSP) are stored in MSR_IA32_PL{0,1,2,3}_SSP,
> SSP table base address is stored in MSR_IA32_INT_SSP_TAB,
> these MSRs are defined in kernel and re-used here.

All of these new guest MSRs will have to be enumerated by
KVM_GET_MSR_INDEX_LIST.

> MSR_IA32_U_CET and MSR_IA32_PL3_SSP are used for user mode protection,
> the contents could differ from process to process, therefore,
> kernel needs to save/restore them during context switch, it makes
> sense to pass through them so that the guest kernel can
> use xsaves/xrstors to operate them efficiently. Other MSRs are used
> for non-user mode protection. See CET spec for detailed info.

I assume that XSAVES & XRSTORS bypass the MSR permission bitmap, like
other instructions that manipulate MSRs (e.g. SWAPGS, RDTSCP, etc.).
Is the guest OS likely to use RDMSR/WRMSR to access these MSRs?

> The difference between CET VMCS state fields and xsave components is that,
> the former used for CET state storage during VMEnter/VMExit,
> whereas the latter used for state retention between Guest task/process
> switch.
>
> Co-developed-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c   |  1 +
>  arch/x86/kvm/cpuid.h   |  2 ++
>  arch/x86/kvm/vmx/vmx.c | 39 +++++++++++++++++++++++++++++++++++++++
>  3 files changed, 42 insertions(+)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 1aa86b87b6ab..0a47b9e565be 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -66,6 +66,7 @@ u64 kvm_supported_xss(void)
>  {
>         return KVM_SUPPORTED_XSS & kvm_x86_ops->supported_xss();
>  }
> +EXPORT_SYMBOL_GPL(kvm_supported_xss);
>
>  #define F(x) bit(X86_FEATURE_##x)
>
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index d78a61408243..1d77b880084d 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -27,6 +27,8 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>
>  int cpuid_query_maxphyaddr(struct kvm_vcpu *vcpu);
>
> +u64 kvm_supported_xss(void);
> +
>  static inline int cpuid_maxphyaddr(struct kvm_vcpu *vcpu)
>  {
>         return vcpu->arch.maxphyaddr;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index a84198cff397..f720baa7a9ba 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7001,6 +7001,43 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
>                 vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
>  }
>
> +static void vmx_intercept_cet_msrs(struct kvm_vcpu *vcpu)

Nit: It seems like this function adjusts the MSR permission bitmap so
as *not* to intercept the CET MSRs.

> +{
> +       struct vcpu_vmx *vmx = to_vmx(vcpu);
> +       unsigned long *msr_bitmap;
> +       u64 kvm_xss;
> +       bool cet_en;
> +
> +       msr_bitmap = vmx->vmcs01.msr_bitmap;

What about nested guests? (i.e. vmcs02).

> +       kvm_xss = kvm_supported_xss();
> +       cet_en = guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) ||
> +                guest_cpuid_has(vcpu, X86_FEATURE_IBT);
> +       /*
> +        * U_CET is a must for USER CET, per CET spec., U_CET and PL3_SPP are
> +        * a bundle for USER CET xsaves.
> +        */
> +       if (cet_en && (kvm_xss & XFEATURE_MASK_CET_USER)) {
> +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_U_CET, MSR_TYPE_RW);
> +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PL3_SSP, MSR_TYPE_RW);
> +       }

Since this is called from vmx_cpuid_update, what happens if cet_en was
previously true and now it's false?

> +       /*
> +        * S_CET is a must for KERNEL CET, PL0_SSP ... PL2_SSP are a bundle
> +        * for CET KERNEL xsaves.
> +        */
> +       if (cet_en && (kvm_xss & XFEATURE_MASK_CET_KERNEL)) {
> +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_S_CET, MSR_TYPE_RW);
> +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PL0_SSP, MSR_TYPE_RW);
> +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PL1_SSP, MSR_TYPE_RW);
> +               vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PL2_SSP, MSR_TYPE_RW);
> +
> +               /* SSP_TAB only available for KERNEL SHSTK.*/
> +               if (guest_cpuid_has(vcpu, X86_FEATURE_SHSTK))
> +                       vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_INT_SSP_TAB,
> +                                                     MSR_TYPE_RW);
> +       }
> +}
> +
>  static void vmx_cpuid_update(struct kvm_vcpu *vcpu)
>  {
>         struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -7025,6 +7062,8 @@ static void vmx_cpuid_update(struct kvm_vcpu *vcpu)
>         if (boot_cpu_has(X86_FEATURE_INTEL_PT) &&
>                         guest_cpuid_has(vcpu, X86_FEATURE_INTEL_PT))
>                 update_intel_pt_cfg(vcpu);
> +
> +       vmx_intercept_cet_msrs(vcpu);
>  }
>
>  static void vmx_set_supported_cpuid(u32 func, struct kvm_cpuid_entry2 *entry)
> --
> 2.17.2
>