KVM: x86: PMU Event Filter

Some events can provide a guest with information about other guests or the
host (e.g. L3 cache stats); providing the capability to restrict access
to a "safe" set of events would limit the potential for the PMU to be used
in any side channel attacks. This change introduces a new VM ioctl that
sets an event filter. If the guest attempts to program a counter for
any blacklisted or non-whitelisted event, the kernel counter won't be
created, so any RDPMC/RDMSR will show 0 instances of that event.
Signed-off-by: Eric Hankland <ehankland@google.com>
[Lots of changes. All remaining bugs are probably mine. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -4065,6 +4065,32 @@ KVM_ARM_VCPU_FINALIZE call.
 See KVM_ARM_VCPU_INIT for details of vcpu features that require finalization
 using this ioctl.
 
+4.120 KVM_SET_PMU_EVENT_FILTER
+
+Capability: KVM_CAP_PMU_EVENT_FILTER
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_pmu_event_filter (in)
+Returns: 0 on success, -1 on error
+
+struct kvm_pmu_event_filter {
+       __u32 action;
+       __u32 nevents;
+       __u64 events[0];
+};
+
+This ioctl restricts the set of PMU events that the guest can program.
+The argument holds a list of events which will be allowed or denied.
+The eventsel+umask of each event the guest attempts to program is compared
+against the events field to determine whether the guest should have access.
+This only affects general purpose counters; fixed purpose counters can
+be disabled by changing the perfmon CPUID leaf.
+
+Valid values for 'action':
+#define KVM_PMU_EVENT_ALLOW 0
+#define KVM_PMU_EVENT_DENY 1
+
+
 5. The kvm_run structure
 ------------------------
 

arch/x86/include/asm/kvm_host.h

@@ -933,6 +933,8 @@ struct kvm_arch {
 
 	bool guest_can_read_msr_platform_info;
 	bool exception_payload_enabled;
+
+	struct kvm_pmu_event_filter *pmu_event_filter;
 };
 
 struct kvm_vm_stat {

arch/x86/include/uapi/asm/kvm.h

@@ -422,4 +422,14 @@ struct kvm_nested_state {
 	__u8 data[0];
 };
 
+/* for KVM_CAP_PMU_EVENT_FILTER */
+struct kvm_pmu_event_filter {
+       __u32 action;
+       __u32 nevents;
+       __u64 events[0];
+};
+
+#define KVM_PMU_EVENT_ALLOW 0
+#define KVM_PMU_EVENT_DENY 1
+
 #endif /* _ASM_X86_KVM_H */

arch/x86/kvm/pmu.c

@@ -22,6 +22,9 @@
 #include "lapic.h"
 #include "pmu.h"
 
+/* This keeps the total size of the filter under 4k. */
+#define KVM_PMU_EVENT_FILTER_MAX_EVENTS 63
+
 /* NOTE:
  * - Each perf counter is defined as "struct kvm_pmc";
  * - There are two types of perf counters: general purpose (gp) and fixed.
@@ -144,6 +147,10 @@ void reprogram_gp_counter(struct kvm_pmc *pmc, u64 eventsel)
 {
 	unsigned config, type = PERF_TYPE_RAW;
 	u8 event_select, unit_mask;
+	struct kvm *kvm = pmc->vcpu->kvm;
+	struct kvm_pmu_event_filter *filter;
+	int i;
+	bool allow_event = true;
 
 	if (eventsel & ARCH_PERFMON_EVENTSEL_PIN_CONTROL)
 		printk_once("kvm pmu: pin control bit is ignored\n");
@@ -155,6 +162,22 @@ void reprogram_gp_counter(struct kvm_pmc *pmc, u64 eventsel)
 	if (!(eventsel & ARCH_PERFMON_EVENTSEL_ENABLE) || !pmc_is_enabled(pmc))
 		return;
 
+	filter = srcu_dereference(kvm->arch.pmu_event_filter, &kvm->srcu);
+	if (filter) {
+		for (i = 0; i < filter->nevents; i++)
+			if (filter->events[i] ==
+			    (eventsel & AMD64_RAW_EVENT_MASK_NB))
+				break;
+		if (filter->action == KVM_PMU_EVENT_ALLOW &&
+		    i == filter->nevents)
+			allow_event = false;
+		if (filter->action == KVM_PMU_EVENT_DENY &&
+		    i < filter->nevents)
+			allow_event = false;
+	}
+	if (!allow_event)
+		return;
+
 	event_select = eventsel & ARCH_PERFMON_EVENTSEL_EVENT;
 	unit_mask = (eventsel & ARCH_PERFMON_EVENTSEL_UMASK) >> 8;
 
@@ -351,3 +374,43 @@ void kvm_pmu_destroy(struct kvm_vcpu *vcpu)
 {
 	kvm_pmu_reset(vcpu);
 }
+
+int kvm_vm_ioctl_set_pmu_event_filter(struct kvm *kvm, void __user *argp)
+{
+	struct kvm_pmu_event_filter tmp, *filter;
+	size_t size;
+	int r;
+
+	if (copy_from_user(&tmp, argp, sizeof(tmp)))
+		return -EFAULT;
+
+	if (tmp.action != KVM_PMU_EVENT_ALLOW &&
+	    tmp.action != KVM_PMU_EVENT_DENY)
+		return -EINVAL;
+
+	if (tmp.nevents > KVM_PMU_EVENT_FILTER_MAX_EVENTS)
+		return -E2BIG;
+
+	size = struct_size(filter, events, tmp.nevents);
+	filter = kmalloc(size, GFP_KERNEL_ACCOUNT);
+	if (!filter)
+		return -ENOMEM;
+
+	r = -EFAULT;
+	if (copy_from_user(filter, argp, size))
+		goto cleanup;
+
+	/* Ensure nevents can't be changed between the user copies. */
+	*filter = tmp;
+
+	mutex_lock(&kvm->lock);
+	rcu_swap_protected(kvm->arch.pmu_event_filter, filter,
+			   mutex_is_locked(&kvm->lock));
+	mutex_unlock(&kvm->lock);
+
+	synchronize_srcu_expedited(&kvm->srcu);
+ 	r = 0;
+cleanup:
+	kfree(filter);
+ 	return r;
+}

arch/x86/kvm/pmu.h

@@ -118,6 +118,7 @@ void kvm_pmu_refresh(struct kvm_vcpu *vcpu);
 void kvm_pmu_reset(struct kvm_vcpu *vcpu);
 void kvm_pmu_init(struct kvm_vcpu *vcpu);
 void kvm_pmu_destroy(struct kvm_vcpu *vcpu);
+int kvm_vm_ioctl_set_pmu_event_filter(struct kvm *kvm, void __user *argp);
 
 bool is_vmware_backdoor_pmc(u32 pmc_idx);
 

arch/x86/kvm/x86.c

@@ -3132,6 +3132,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_SET_BOOT_CPU_ID:
  	case KVM_CAP_SPLIT_IRQCHIP:
 	case KVM_CAP_IMMEDIATE_EXIT:
+	case KVM_CAP_PMU_EVENT_FILTER:
 	case KVM_CAP_GET_MSR_FEATURES:
 	case KVM_CAP_MSR_PLATFORM_INFO:
 	case KVM_CAP_EXCEPTION_PAYLOAD:
@@ -4978,6 +4979,9 @@ long kvm_arch_vm_ioctl(struct file *filp,
 		r = kvm_vm_ioctl_hv_eventfd(kvm, &hvevfd);
 		break;
 	}
+	case KVM_SET_PMU_EVENT_FILTER:
+		r = kvm_vm_ioctl_set_pmu_event_filter(kvm, argp);
+		break;
 	default:
 		r = -ENOTTY;
 	}
@@ -9428,6 +9432,7 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
 	kvm_ioapic_destroy(kvm);
 	kvm_free_vcpus(kvm);
 	kvfree(rcu_dereference_check(kvm->arch.apic_map, 1));
+	kfree(srcu_dereference_check(kvm->arch.pmu_event_filter, &kvm->srcu, 1));
 	kvm_mmu_uninit_vm(kvm);
 	kvm_page_track_cleanup(kvm);
 	kvm_hv_destroy_vm(kvm);

include/uapi/linux/kvm.h

@@ -995,6 +995,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_ARM_SVE 170
 #define KVM_CAP_ARM_PTRAUTH_ADDRESS 171
 #define KVM_CAP_ARM_PTRAUTH_GENERIC 172
+#define KVM_CAP_PMU_EVENT_FILTER 173
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
@@ -1329,6 +1330,8 @@ struct kvm_s390_ucas_mapping {
 #define KVM_PPC_GET_RMMU_INFO	  _IOW(KVMIO,  0xb0, struct kvm_ppc_rmmu_info)
 /* Available with KVM_CAP_PPC_GET_CPU_CHAR */
 #define KVM_PPC_GET_CPU_CHAR	  _IOR(KVMIO,  0xb1, struct kvm_ppc_cpu_char)
+/* Available with KVM_CAP_PMU_EVENT_FILTER */
+#define KVM_SET_PMU_EVENT_FILTER  _IOW(KVMIO,  0xb2, struct kvm_pmu_event_filter)
 
 /* ioctl for vm fd */
 #define KVM_CREATE_DEVICE	  _IOWR(KVMIO,  0xe0, struct kvm_create_device)