Re: [PATCH v3 14/21] KVM:VMX: Add a synthetic MSR to allow userspace to access GUEST_SSP

Binbin Wu
2023-05-23T08:57:36Z

On 5/11/2023 12:08 PM, Yang Weijiang wrote:
> Introduce a host-only synthetic MSR, MSR_KVM_GUEST_SSP, so that the VMM
> can read/write the guest's SSP, e.g. to migrate CET state. Use a synthetic
> MSR, e.g. as opposed to a VCPU_REG_, as GUEST_SSP is subject to the same
> consistency checks as the PL*_SSP MSRs, i.e. can share code.
>
> Co-developed-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> ---
> arch/x86/include/uapi/asm/kvm_para.h | 1 +
> arch/x86/kvm/vmx/vmx.c | 15 ++++++++++++---
> arch/x86/kvm/x86.c | 4 ++++
> 3 files changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> index 6e64b27b2c1e..7af465e4e0bd 100644
> --- a/arch/x86/include/uapi/asm/kvm_para.h
> +++ b/arch/x86/include/uapi/asm/kvm_para.h
> @@ -58,6 +58,7 @@
> #define MSR_KVM_ASYNC_PF_INT 0x4b564d06
> #define MSR_KVM_ASYNC_PF_ACK 0x4b564d07
> #define MSR_KVM_MIGRATION_CONTROL 0x4b564d08
> +#define MSR_KVM_GUEST_SSP 0x4b564d09
>
> struct kvm_steal_time {
> __u64 steal;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 0ccaa467d7d3..72149156bbd3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2095,9 +2095,13 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> break;
> case MSR_IA32_U_CET:
> case MSR_IA32_PL3_SSP:
> + case MSR_KVM_GUEST_SSP:
> if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
> return 1;
> - kvm_get_xsave_msr(msr_info);
> + if (msr_info->index == MSR_KVM_GUEST_SSP)
> + msr_info->data = vmcs_readl(GUEST_SSP);
According to the change of the kvm_cet_is_msr_accessible() below,
kvm_cet_is_msr_accessible() will return false for MSR_KVM_GUEST_SSP,
then this code is unreachable?

> + else
> + kvm_get_xsave_msr(msr_info);
> break;
> case MSR_IA32_DEBUGCTLMSR:
> msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
> @@ -2413,15 +2417,20 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> break;
> case MSR_IA32_U_CET:
> case MSR_IA32_PL3_SSP:
> + case MSR_KVM_GUEST_SSP:
> if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
> return 1;
> if (is_noncanonical_address(data, vcpu))
> return 1;
> if (msr_index == MSR_IA32_U_CET && (data & GENMASK(9, 6)))
> return 1;
> - if (msr_index == MSR_IA32_PL3_SSP && (data & GENMASK(2, 0)))
> + if ((msr_index == MSR_IA32_PL3_SSP ||
> + msr_index == MSR_KVM_GUEST_SSP) && (data & GENMASK(2, 0)))
> return 1;
> - kvm_set_xsave_msr(msr_info);
> + if (msr_index == MSR_KVM_GUEST_SSP)
> + vmcs_writel(GUEST_SSP, data);
> + else
> + kvm_set_xsave_msr(msr_info);
> break;
> case MSR_IA32_PERF_CAPABILITIES:
> if (data && !vcpu_to_pmu(vcpu)->version)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2e3a39c9297c..baac6acebd40 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -13642,6 +13642,10 @@ bool kvm_cet_is_msr_accessible(struct kvm_vcpu *vcpu, struct msr_data *msr)
> !guest_cpuid_has(vcpu, X86_FEATURE_IBT))
> return false;
>
> + /* The synthetic MSR is for userspace access only. */
> + if (msr->index == MSR_KVM_GUEST_SSP)
> + return false;
> +
> if (msr->index == MSR_IA32_PL3_SSP &&
> !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK))
> return false;