KVM: arm64: vgic: Hold config_lock while tearing down a CPU interface

Tearing down a vcpu CPU interface involves freeing the private interrupt
array. If we don't hold the lock, we may race against another thread
trying to configure it. Yeah, fuzzers do wonderful things...

Taking the lock early solves this particular problem.

Fixes: 03b3d00a70 ("KVM: arm64: vgic: Allocate private interrupts on demand")
Reported-by: Alexander Potapenko <glider@google.com>
Tested-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240808091546.3262111-1-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/vgic/vgic-init.c

@@ -438,14 +438,13 @@ void kvm_vgic_destroy(struct kvm *kvm)
 	unsigned long i;
 
 	mutex_lock(&kvm->slots_lock);
+	mutex_lock(&kvm->arch.config_lock);
 
 	vgic_debug_destroy(kvm);
 
 	kvm_for_each_vcpu(i, vcpu, kvm)
 		__kvm_vgic_vcpu_destroy(vcpu);
 
-	mutex_lock(&kvm->arch.config_lock);
-
 	kvm_vgic_dist_destroy(kvm);
 
 	mutex_unlock(&kvm->arch.config_lock);