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

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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.

Regards

On Wed, Nov 2, 2016 at 10:25 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> 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@xxxxxxxxxx>
>
> 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)
>>



-- 
Ignacio Alvarado
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