KVM: arm64: Destroy mpidr_data for 'late' vCPU creation

A particularly annoying userspace could create a vCPU after KVM has
computed mpidr_data for the VM, either by racing against VGIC
initialization or having a userspace irqchip.

In any case, this means mpidr_data no longer fully describes the VM, and
attempts to find the new vCPU with kvm_mpidr_to_vcpu() will fail. The
fix is to discard mpidr_data altogether, as it is only a performance
optimization and not required for correctness. In all likelihood KVM
will recompute the mappings when KVM_RUN is called on the new vCPU.

Note that reads of mpidr_data are not guarded by a lock; promote to RCU
to cope with the possibility of mpidr_data being invalidated at runtime.

Fixes: 54a8006d0b ("KVM: arm64: Fast-track kvm_mpidr_to_vcpu() when mpidr_data is available")
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240508071952.2035422-1-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/arm.c

@@ -195,6 +195,23 @@ void kvm_arch_create_vm_debugfs(struct kvm *kvm)
 	kvm_sys_regs_create_debugfs(kvm);
 }
 
+static void kvm_destroy_mpidr_data(struct kvm *kvm)
+{
+	struct kvm_mpidr_data *data;
+
+	mutex_lock(&kvm->arch.config_lock);
+
+	data = rcu_dereference_protected(kvm->arch.mpidr_data,
+					 lockdep_is_held(&kvm->arch.config_lock));
+	if (data) {
+		rcu_assign_pointer(kvm->arch.mpidr_data, NULL);
+		synchronize_rcu();
+		kfree(data);
+	}
+
+	mutex_unlock(&kvm->arch.config_lock);
+}
+
 /**
  * kvm_arch_destroy_vm - destroy the VM data structure
  * @kvm:	pointer to the KVM struct
@@ -209,7 +226,8 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
 	if (is_protected_kvm_enabled())
 		pkvm_destroy_hyp_vm(kvm);
 
-	kfree(kvm->arch.mpidr_data);
+	kvm_destroy_mpidr_data(kvm);
+
 	kfree(kvm->arch.sysreg_masks);
 	kvm_destroy_vcpus(kvm);
 
@@ -395,6 +413,13 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.hw_mmu = &vcpu->kvm->arch.mmu;
 
+	/*
+	 * This vCPU may have been created after mpidr_data was initialized.
+	 * Throw out the pre-computed mappings if that is the case which forces
+	 * KVM to fall back to iteratively searching the vCPUs.
+	 */
+	kvm_destroy_mpidr_data(vcpu->kvm);
+
 	err = kvm_vgic_vcpu_init(vcpu);
 	if (err)
 		return err;
@@ -594,7 +619,8 @@ static void kvm_init_mpidr_data(struct kvm *kvm)
 
 	mutex_lock(&kvm->arch.config_lock);
 
-	if (kvm->arch.mpidr_data || atomic_read(&kvm->online_vcpus) == 1)
+	if (rcu_access_pointer(kvm->arch.mpidr_data) ||
+	    atomic_read(&kvm->online_vcpus) == 1)
 		goto out;
 
 	kvm_for_each_vcpu(c, vcpu, kvm) {
@@ -631,7 +657,7 @@ static void kvm_init_mpidr_data(struct kvm *kvm)
 		data->cmpidr_to_idx[index] = c;
 	}
 
-	kvm->arch.mpidr_data = data;
+	rcu_assign_pointer(kvm->arch.mpidr_data, data);
 out:
 	mutex_unlock(&kvm->arch.config_lock);
 }
@@ -2470,22 +2496,28 @@ out_err:
 
 struct kvm_vcpu *kvm_mpidr_to_vcpu(struct kvm *kvm, unsigned long mpidr)
 {
-	struct kvm_vcpu *vcpu;
+	struct kvm_vcpu *vcpu = NULL;
+	struct kvm_mpidr_data *data;
 	unsigned long i;
 
 	mpidr &= MPIDR_HWID_BITMASK;
 
-	if (kvm->arch.mpidr_data) {
+	rcu_read_lock();
-		u16 idx = kvm_mpidr_index(kvm->arch.mpidr_data, mpidr);
+	data = rcu_dereference(kvm->arch.mpidr_data);
 
-		vcpu = kvm_get_vcpu(kvm,
+	if (data) {
-				    kvm->arch.mpidr_data->cmpidr_to_idx[idx]);
+		u16 idx = kvm_mpidr_index(data, mpidr);
+
+		vcpu = kvm_get_vcpu(kvm, data->cmpidr_to_idx[idx]);
 		if (mpidr != kvm_vcpu_get_mpidr_aff(vcpu))
 			vcpu = NULL;
-
-		return vcpu;
 	}
 
+	rcu_read_unlock();
+
+	if (vcpu)
+		return vcpu;
+
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		if (mpidr == kvm_vcpu_get_mpidr_aff(vcpu))
 			return vcpu;