[v3,2/3] KVM: x86: Control guest reads of MSR_PLATFORM_INFO

Commit Message

Drew Schmitt Aug. 20, 2018, 5:32 p.m. UTC

Add KVM_CAP_MSR_PLATFORM_INFO so that userspace can disable guest access
to reads of MSR_PLATFORM_INFO.

Disabling access to reads of this MSR gives userspace the control to "expose"
this platform-dependent information to guests in a clear way. As it exists
today, guests that read this MSR would get unpopulated information if userspace
hadn't already set it (and prior to this patch series, only the CPUID faulting
information could have been populated). This existing interface could be
confusing if guests don't handle the potential for incorrect/incomplete
information gracefully (e.g. zero reported for base frequency).

Signed-off-by: Drew Schmitt <[e-mail]>
---
v2 -> v3: added more to commit message
---
 Documentation/virtual/kvm/api.txt |  9 +++++++++
 arch/x86/include/asm/kvm_host.h   |  2 ++
 arch/x86/kvm/x86.c                | 10 ++++++++++
 include/uapi/linux/kvm.h          |  1 +
 4 files changed, 22 insertions(+)

Patch

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index d10944e619d3d..29b54ed0978c2 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -4391,6 +4391,15 @@  all such vmexits.
 
 Do not enable KVM_FEATURE_PV_UNHALT if you disable HLT exits.
 
+7.14 KVM_CAP_MSR_PLATFORM_INFO
+
+Architectures: x86
+Parameters: args[0] whether feature should be enabled or not
+
+With this capability, a guest may read the MSR_PLATFORM_INFO MSR. Otherwise,
+a #GP would be raised when the guest tries to access. Currently, this
+capability does not enable write permissions of this MSR for the guest.
+
 8. Other capabilities.
 ----------------------
 
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c13cd28d9d1be..c0497c7a4869f 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -853,6 +853,8 @@  struct kvm_arch {
 
 	bool x2apic_format;
 	bool x2apic_broadcast_quirk_disabled;
+
+	bool guest_can_read_msr_platform_info;
 };
 
 struct kvm_vm_stat {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0278cde1893ca..b27d88fc6eb8f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2743,6 +2743,9 @@  int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		msr_info->data = vcpu->arch.osvw.status;
 		break;
 	case MSR_PLATFORM_INFO:
+		if (!msr_info->host_initiated &&
+		    !vcpu->kvm->arch.guest_can_read_msr_platform_info)
+			return 1;
 		msr_info->data = vcpu->arch.msr_platform_info;
 		break;
 	case MSR_MISC_FEATURES_ENABLES:
@@ -2890,6 +2893,7 @@  int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
  	case KVM_CAP_SPLIT_IRQCHIP:
 	case KVM_CAP_IMMEDIATE_EXIT:
 	case KVM_CAP_GET_MSR_FEATURES:
+	case KVM_CAP_MSR_PLATFORM_INFO:
 		r = 1;
 		break;
 	case KVM_CAP_SYNC_REGS:
@@ -4259,6 +4263,10 @@  static int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 			kvm->arch.pause_in_guest = true;
 		r = 0;
 		break;
+	case KVM_CAP_MSR_PLATFORM_INFO:
+		kvm->arch.guest_can_read_msr_platform_info = cap->args[0];
+		r = 0;
+		break;
 	default:
 		r = -EINVAL;
 		break;
@@ -8724,6 +8732,8 @@  int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	kvm->arch.kvmclock_offset = -ktime_get_boot_ns();
 	pvclock_update_vm_gtod_copy(kvm);
 
+	kvm->arch.guest_can_read_msr_platform_info = true;
+
 	INIT_DELAYED_WORK(&kvm->arch.kvmclock_update_work, kvmclock_update_fn);
 	INIT_DELAYED_WORK(&kvm->arch.kvmclock_sync_work, kvmclock_sync_fn);
 
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index b6270a3b38e9f..f3367041b609b 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -949,6 +949,7 @@  struct kvm_ppc_resize_hpt {
 #define KVM_CAP_GET_MSR_FEATURES 153
 #define KVM_CAP_HYPERV_EVENTFD 154
 #define KVM_CAP_HYPERV_TLBFLUSH 155
+#define KVM_CAP_MSR_PLATFORM_INFO 156
 
 #ifdef KVM_CAP_IRQ_ROUTING