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@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)
>>>
>
>
>
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