Re: [syzbot] WARNING in static_key_slow_try_dec (2)

[Tetsuo Handa]

Hello.

Commit 4547700a4d190ac4 ("KVM: x86: Consolidate APIC base RESET initialization code")
changed the location to set MSR_IA32_APICBASE_ENABLE flag. And syzbot is reporting
underflow bug due to too late initialization of vcpu->arch.apic_base member. We need to
make sure that vcpu->arch.apic_base is initialized before kvm_free_lapic() is called.

kvm_vm_ioctl() {
  kvm_vm_ioctl_create_vcpu() {
    kvm_arch_vcpu_create() {
      if (something_went_wrong)
        goto fail_free_lapic;
      /* vcpu->arch.apic_base is initialized when something_went_wrong is false. */
      kvm_vcpu_reset() {
        kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event) {
          vcpu->arch.apic_base = APIC_DEFAULT_PHYS_BASE | MSR_IA32_APICBASE_ENABLE;
        }
      }
      return 0;
    fail_free_lapic:
      kvm_free_lapic() {
        /* vcpu->arch.apic_base is not yet initialized when something_went_wrong is true. */
        if (!(vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE))
          static_branch_slow_dec_deferred(&apic_hw_disabled); // <= underflow bug.
      }
      return r;
    }
  }
}