[PATCH v5 4/8] KVM: VMX: Ignore guest CPUID for host userspace writes to DEBUGCTL

From: Sean Christopherson
Date: Wed Oct 05 2022 - 20:03:48 EST

Ignore guest CPUID for host userspace writes to the DEBUGCTL MSR, KVM's
ABI is that setting CPUID vs. state can be done in any order, i.e. KVM
allows userspace to stuff MSRs prior to setting the guest's CPUID that
makes the new MSR "legal".

Keep the vmx_get_perf_capabilities() check for guest writes, even though
it's technically unnecessary since the vCPU's PERF_CAPABILITIES is
consulted when refreshing LBR support. A future patch will clean up
vmx_get_perf_capabilities() to avoid the RDMSR on every call, at which
point the paranoia will incur no meaningful overhead.

Note, prior to vmx_get_perf_capabilities() checking that the host fully
supports LBRs via x86_perf_get_lbr(), KVM effectively relied on
intel_pmu_lbr_is_enabled() to guard against host userspace enabling LBRs
on platforms without full support.

Fixes: c646236344e9 ("KVM: vmx/pmu: Add PMU_CAP_LBR_FMT check when guest LBR is enabled")
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
arch/x86/kvm/vmx/vmx.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 97fc873c37fa..e70ac91cd2fb 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2021,16 +2021,16 @@ static u64 nested_vmx_truncate_sysenter_addr(struct kvm_vcpu *vcpu,
return (unsigned long)data;

-static u64 vcpu_supported_debugctl(struct kvm_vcpu *vcpu)
+static u64 vmx_get_supported_debugctl(struct kvm_vcpu *vcpu, bool host_initiated)
u64 debugctl = 0;

if (boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT) &&
- guest_cpuid_has(vcpu, X86_FEATURE_BUS_LOCK_DETECT))
+ (host_initiated || guest_cpuid_has(vcpu, X86_FEATURE_BUS_LOCK_DETECT)))

if ((vmx_get_perf_capabilities() & PMU_CAP_LBR_FMT) &&
- intel_pmu_lbr_is_enabled(vcpu))
+ (host_initiated || intel_pmu_lbr_is_enabled(vcpu)))

return debugctl;
@@ -2105,7 +2105,9 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
vmcs_writel(GUEST_SYSENTER_ESP, data);
- u64 invalid = data & ~vcpu_supported_debugctl(vcpu);
+ u64 invalid;
+ invalid = data & ~vmx_get_supported_debugctl(vcpu, msr_info->host_initiated);
if (report_ignored_msrs)
vcpu_unimpl(vcpu, "%s: BTF|LBR in IA32_DEBUGCTLMSR 0x%llx, nop\n",