[PATCH v4 1/6] KVM: arm64: Do not change the PMU event filter after a VCPU has run

[Alexandru Elisei]

From: Marc Zyngier <maz at kernel.org>

Userspace can specify which events a guest is allowed to use with the
KVM_ARM_VCPU_PMU_V3_FILTER attribute. The list of allowed events can be
identified by a guest from reading the PMCEID{0,1}_EL0 registers.

Changing the PMU event filter after a VCPU has run can cause reads of the
registers performed before the filter is changed to return different values
than reads performed with the new event filter in place. The architecture
defines the two registers as read-only, and this behaviour contradicts
that.

Keep track when the first VCPU has run and deny changes to the PMU event
filter to prevent this from happening.

Signed-off-by: Marc Zyngier <maz at kernel.org>
[ Alexandru E: Added commit message, updated ioctl documentation ]
Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
---
Tested with a hacked version of kvmtool [1], which sets the event filter
for VCPU 1 only after VCPU 0 has already run.

[1] https://gitlab.arm.com/linux-arm/kvmtool-ae/-/tree/test-pmu-filter-after-vcpu-has-run-v1

 Documentation/virt/kvm/devices/vcpu.rst |  2 +-
 arch/arm64/include/asm/kvm_host.h       |  1 +
 arch/arm64/kvm/arm.c                    |  4 +++
 arch/arm64/kvm/pmu-emul.c               | 33 +++++++++++++++----------
 4 files changed, 26 insertions(+), 14 deletions(-)

diff --git a/Documentation/virt/kvm/devices/vcpu.rst b/Documentation/virt/kvm/devices/vcpu.rst
index 60a29972d3f1..d063aaee5bb7 100644
--- a/Documentation/virt/kvm/devices/vcpu.rst
+++ b/Documentation/virt/kvm/devices/vcpu.rst
@@ -70,7 +70,7 @@ irqchip.
 	 -ENODEV  PMUv3 not supported or GIC not initialized
 	 -ENXIO   PMUv3 not properly configured or in-kernel irqchip not
 	 	  configured as required prior to calling this attribute
-	 -EBUSY   PMUv3 already initialized
+	 -EBUSY   PMUv3 already initialized or a VCPU has already run
 	 -EINVAL  Invalid filter range
 	 =======  ======================================================
 
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 5bc01e62c08a..2869259e10c0 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -136,6 +136,7 @@ struct kvm_arch {
 
 	/* Memory Tagging Extension enabled for the guest */
 	bool mte_enabled;
+	bool ran_once;
 };
 
 struct kvm_vcpu_fault_info {
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index a4a0063df456..f4e7e38a19e9 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -634,6 +634,10 @@ int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
 	if (kvm_vm_is_protected(kvm))
 		kvm_call_hyp_nvhe(__pkvm_vcpu_init_traps, vcpu);
 
+	mutex_lock(&kvm->lock);
+	kvm->arch.ran_once = true;
+	mutex_unlock(&kvm->lock);
+
 	return ret;
 }
 
diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index fbcfd4ec6f92..bc771bc1a041 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -924,6 +924,8 @@ static bool pmu_irq_is_valid(struct kvm *kvm, int irq)
 
 int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
 {
+	struct kvm *kvm = vcpu->kvm;
+
 	if (!kvm_vcpu_has_pmu(vcpu))
 		return -ENODEV;
 
@@ -941,7 +943,7 @@ int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
 		int __user *uaddr = (int __user *)(long)attr->addr;
 		int irq;
 
-		if (!irqchip_in_kernel(vcpu->kvm))
+		if (!irqchip_in_kernel(kvm))
 			return -EINVAL;
 
 		if (get_user(irq, uaddr))
@@ -951,7 +953,7 @@ int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
 		if (!(irq_is_ppi(irq) || irq_is_spi(irq)))
 			return -EINVAL;
 
-		if (!pmu_irq_is_valid(vcpu->kvm, irq))
+		if (!pmu_irq_is_valid(kvm, irq))
 			return -EINVAL;
 
 		if (kvm_arm_pmu_irq_initialized(vcpu))
@@ -966,7 +968,7 @@ int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
 		struct kvm_pmu_event_filter filter;
 		int nr_events;
 
-		nr_events = kvm_pmu_event_mask(vcpu->kvm) + 1;
+		nr_events = kvm_pmu_event_mask(kvm) + 1;
 
 		uaddr = (struct kvm_pmu_event_filter __user *)(long)attr->addr;
 
@@ -978,12 +980,17 @@ int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
 		     filter.action != KVM_PMU_EVENT_DENY))
 			return -EINVAL;
 
-		mutex_lock(&vcpu->kvm->lock);
+		mutex_lock(&kvm->lock);
+
+		if (kvm->arch.ran_once) {
+			mutex_unlock(&kvm->lock);
+			return -EBUSY;
+		}
 
-		if (!vcpu->kvm->arch.pmu_filter) {
-			vcpu->kvm->arch.pmu_filter = bitmap_alloc(nr_events, GFP_KERNEL_ACCOUNT);
-			if (!vcpu->kvm->arch.pmu_filter) {
-				mutex_unlock(&vcpu->kvm->lock);
+		if (!kvm->arch.pmu_filter) {
+			kvm->arch.pmu_filter = bitmap_alloc(nr_events, GFP_KERNEL_ACCOUNT);
+			if (!kvm->arch.pmu_filter) {
+				mutex_unlock(&kvm->lock);
 				return -ENOMEM;
 			}
 
@@ -994,17 +1001,17 @@ int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
 			 * events, the default is to allow.
 			 */
 			if (filter.action == KVM_PMU_EVENT_ALLOW)
-				bitmap_zero(vcpu->kvm->arch.pmu_filter, nr_events);
+				bitmap_zero(kvm->arch.pmu_filter, nr_events);
 			else
-				bitmap_fill(vcpu->kvm->arch.pmu_filter, nr_events);
+				bitmap_fill(kvm->arch.pmu_filter, nr_events);
 		}
 
 		if (filter.action == KVM_PMU_EVENT_ALLOW)
-			bitmap_set(vcpu->kvm->arch.pmu_filter, filter.base_event, filter.nevents);
+			bitmap_set(kvm->arch.pmu_filter, filter.base_event, filter.nevents);
 		else
-			bitmap_clear(vcpu->kvm->arch.pmu_filter, filter.base_event, filter.nevents);
+			bitmap_clear(kvm->arch.pmu_filter, filter.base_event, filter.nevents);
 
-		mutex_unlock(&vcpu->kvm->lock);
+		mutex_unlock(&kvm->lock);
 
 		return 0;
 	}
-- 
2.34.1