Re: [PATCH] KVM: Disable irq while unregistering user notifier

[Paolo Bonzini <pbonzini@redhat.com>]

On 03/11/2016 23:26, Ignacio Alvarado wrote:
> The risk would still be present since fire_user_return_notifiers could
> potentially run at any time and kvm_on_user return could be called
> from drop_user_return_notifiers either from the kvm_put_kvm path or
> the kvm_arch_exit path.

So, do the unregistration

        locals->registered = false;
        user_return_notifier_unregister(urn);

first in kvm_on_user_return?  Nothing else needs to run with IRQs
disabled, and doing it first prevents reentering the code from
kvm_arch_hardware_disable.

Paolo

> Regards
> 
> On Wed, Nov 2, 2016 at 10:25 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>>
>> On 22/10/2016 00:08, Ignacio Alvarado wrote:
>>> Function user_notifier_unregister should be called only once for each
>>> registered user notifier.
>>>
>>> Function kvm_arch_hardware_disable can be executed from an IPI context
>>> which could cause a race condition with a VCPU returning to user mode
>>> and attempting to unregister the notifier.
>>>
>>> Signed-off-by: Ignacio Alvarado <ikalvarado@google.com>
>>
>> Should drop_user_return_notifiers instead be moved to kvm_arch_exit?
>>
>> Thanks,
>>
>> Paolo
>>
>>> ---
>>>  arch/x86/kvm/x86.c | 10 ++++++++++
>>>  1 file changed, 10 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index e375235..51f87f0 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -210,7 +210,15 @@ static void kvm_on_user_return(struct user_return_notifier *urn)
>>>       struct kvm_shared_msrs *locals
>>>               = container_of(urn, struct kvm_shared_msrs, urn);
>>>       struct kvm_shared_msr_values *values;
>>> +     unsigned long flags;
>>>
>>> +     /*
>>> +      * Disabling irqs at this point since the following code could be
>>> +      * interrupted and executed through kvm_arch_hardware_disable()
>>> +      */
>>> +     local_irq_save(flags);
>>> +     if (!locals->registered)
>>> +             goto out;
>>>       for (slot = 0; slot < shared_msrs_global.nr; ++slot) {
>>>               values = &locals->values[slot];
>>>               if (values->host != values->curr) {
>>> @@ -220,6 +228,8 @@ static void kvm_on_user_return(struct user_return_notifier *urn)
>>>       }
>>>       locals->registered = false;
>>>       user_return_notifier_unregister(urn);
>>> +out:
>>> +     local_irq_restore(flags);
>>>  }
>>>
>>>  static void shared_msr_update(unsigned slot, u32 msr)
>>>
> 
> 
>