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SubjectRe: [PATCH 1/2] KVM: vmx, pmu: accept 0 for absent MSRs when host-initiated
On Tue, May 31, 2022, Paolo Bonzini wrote:
> Whenever an MSR is part of KVM_GET_MSR_INDEX_LIST, as is the case for
> MSR_IA32_DS_AREA, MSR_ARCH_LBR_DEPTH or MSR_ARCH_LBR_CTL, it has to be
> always settable with KVM_SET_MSR. Accept a zero value for these MSRs
> to obey the contract.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/vmx/pmu_intel.c | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 3e04d0407605..66496cb41494 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -367,8 +367,9 @@ static bool arch_lbr_depth_is_valid(struct kvm_vcpu *vcpu, u64 depth)
> {
> struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>
> - if (!kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR))
> - return false;
> + if (!kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR) ||
> + !guest_cpuid_has(vcpu, X86_FEATURE_ARCH_LBR))
> + return depth == 0;
>
> return (depth == pmu->kvm_arch_lbr_depth);
> }
> @@ -378,7 +379,7 @@ static bool arch_lbr_ctl_is_valid(struct kvm_vcpu *vcpu, u64 ctl)
> struct kvm_cpuid_entry2 *entry;
>
> if (!kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR))
> - return false;
> + return ctl == 0;
>
> if (ctl & ~KVM_ARCH_LBR_CTL_MASK)
> goto warn;
> @@ -510,6 +511,8 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> }
> break;
> case MSR_IA32_DS_AREA:
> + if (msr_info->host_initiated && data && !guest_cpuid_has(vcpu, X86_FEATURE_DS))
> + return 1;
> if (is_noncanonical_address(data, vcpu))
> return 1;
> pmu->ds_area = data;
> @@ -525,7 +528,11 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case MSR_ARCH_LBR_DEPTH:
> if (!arch_lbr_depth_is_valid(vcpu, data))
> return 1;
> +
> lbr_desc->records.nr = data;
> + if (!guest_cpuid_has(vcpu, X86_FEATURE_ARCH_LBR))
> + return 0;

This is wrong, it will allow an unchecked wrmsrl() to MSR_ARCH_LBR_DEPTH if
X86_FEATURE_ARCH_LBR is not supported by hardware but userspace forces it in
guest CPUID.

This the only user of arch_lbr_depth_is_valid(), just open code the logic.

> +
> /*
> * Writing depth MSR from guest could either setting the
> * MSR or resetting the LBR records with the side-effect.
> @@ -535,6 +542,8 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case MSR_ARCH_LBR_CTL:
> if (!arch_lbr_ctl_is_valid(vcpu, data))
> break;
> + if (!guest_cpuid_has(vcpu, X86_FEATURE_ARCH_LBR))
> + return 0;

Similar bug here.

Can we just punt this out of kvm/queue until its been properly reviewed? At the
barest of glances, there are multiple flaws that should block this from being
merged. Based on the number of checks against X86_FEATURE_ARCH_LBR in KVM, and
my vague recollection of the passthrough behavior, this is a _massive_ feature.

The pr_warn_ratelimited() shouldn't exist; it's better than a non-ratelimited warn,
but it's ultimately useless.

This should check kvm_cpu_has() to ensure the field exists, e.g. if the feature
is supported in hardware but cpu_has_vmx_arch_lbr() returns false for whatever
reason.

if (!init_event) {
if (static_cpu_has(X86_FEATURE_ARCH_LBR))
vmcs_write64(GUEST_IA32_LBR_CTL, 0);

intel_pmu_lbr_is_enabled() is going to be a performance problem, e.g. _should_ be
gated by static_cpu_has() to avoid overhead on CPUs without arch LBRs, and is
going to incur a _guest_ CPUID lookup on X86_FEATURE_PDCM for every VM-Entry if
arch LBRs are exposed to the guest (at least, I think that's what it does).

>
> vmcs_write64(GUEST_IA32_LBR_CTL, data);
>
> --
> 2.31.1
>
>

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