Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback

Tony Krowiak
2021-05-18T13:42:12Z

On 5/18/21 5:30 AM, Christian Borntraeger wrote:
>
>
> On 17.05.21 21:10, Halil Pasic wrote:
>> On Mon, 17 May 2021 09:37:42 -0400
>> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>>
>>>>
>>>> Because of this, I don't think the rest of your argument is valid.
>>>
>>> Okay, so your concern is that between the point in time the
>>> vcpu->kvm->arch.crypto.pqap_hook pointer is checked in
>>> priv.c and the point in time the handle_pqap() function
>>> in vfio_ap_ops.c is called, the memory allocated for the
>>> matrix_mdev containing the struct kvm_s390_module_hook
>>> may get freed, thus rendering the function pointer invalid.
>>> While not impossible, that seems extremely unlikely to
>>> happen. Can you articulate a scenario where that could
>>> even occur?
>>
>> Malicious userspace. We tend to do the pqap aqic just once
>> in the guest right after the queue is detected. I do agree
>> it ain't very likely to happen during normal operation. But why are
>> you asking?
>
> Would it help, if the code in priv.c would read the hook once
> and then only work on the copy? We could protect that with rcu
> and do a synchronize rcu in vfio_ap_mdev_unset_kvm after
> unsetting the pointer?

I'll look into this.

>>
>> I'm not sure I understood correctly what kind of a scenario are
>> you asking for. PQAP AQIC and mdev remove are independent
>> events originated in userspace, so AFAIK we may not assume
>> that the execution of two won't overlap, nor are we allowed
>> to make assumptions on how does the execution of these two
>> overlap (except for the things we explicitly ensure -- e.g.
>> some parts are made mutually exclusive using the matrix_dev->lock
>> lock).
>>
>> Regards,
>> Halil
>>