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

Tony Krowiak
2021-05-18T18:14:09Z

On 5/18/21 9:59 AM, Christian Borntraeger wrote:
>
>
> On 18.05.21 15:42, Tony Krowiak wrote:
>>
>>
>> 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 think it could work. in priv.c use rcu_readlock, save the
> pointer, do the check and call, call rcu_read_unlock.
> In vfio_ap use rcu_assign_pointer to set the pointer and
> after setting it to zero call sychronize_rcu.
>
> Halil, I think we can do this as an addon patch as it makes
> sense to have this callback pointer protected independent of
> this patch. Agree?

I agree that this is a viable option; however, this does not
guarantee that the matrix_mdev is not freed thus rendering
the function pointer to the interception handler invalid unless
that is also included within the rcu_readlock/rcu_read_unlock.
That is not possible given the matrix_mdev is freed within
the remove callback and the pointer to the structure that
contains the interception handler function pointer is cleared
in the vfio_ap_mdev_unset_kvm() function. I am working on
a patch and should be able to post it before EOD or first thing
tomorrow.