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: 54a8006 ("KVM: arm64: Fast-track kvm_mpidr_to_vcpu() when mpidr_data is available")
Signed-off-by: Oliver Upton <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Marc Zyngier <[email protected]>

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);
 }
@@ -2470,21 +2496,27 @@ static int __init init_hyp_mode(void)
 
 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) {
-  u16 idx = kvm_mpidr_index(kvm->arch.mpidr_data, mpidr);
+ rcu_read_lock();
+ data = rcu_dereference(kvm->arch.mpidr_data);
 
- vcpu = kvm_get_vcpu(kvm,
- kvm->arch.mpidr_data->cmpidr_to_idx[idx]);
+ if (data) {
+ 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;
+ }
 
+ rcu_read_unlock();
+
+ if (vcpu)
  return vcpu;
- }
 
  kvm_for_each_vcpu(i, vcpu, kvm) {
  if (mpidr == kvm_vcpu_get_mpidr_aff(vcpu))