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

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Date:   Tue, 18 May 2021 19:01:42 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Tony Krowiak <akrowiak@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, cohuck@...hat.com,
        pasic@...ux.vnet.ibm.com, jjherne@...ux.ibm.com, jgg@...dia.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        stable@...r.kernel.org, Tony Krowiak <akrowiak@...y.rr.com>
Subject: Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback



On 18.05.21 17:33, Halil Pasic wrote:
> On Tue, 18 May 2021 15:59:36 +0200
> Christian Borntraeger <borntraeger@...ibm.com> 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@...ux.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?
> 
> Unfortunately just "the hook" is ambiguous in this context. We
> have kvm->arch.crypto.pqap_hook that is supposed to point to
> a struct kvm_s390_module_hook member of struct ap_matrix_mdev
> which is also called pqap_hook. And struct kvm_s390_module_hook
> has function pointer member named "hook".

I was referring to the full struct.
> 
>>>
>>> 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.
> 
> In my opinion, we should make the accesses to the
> kvm->arch.crypto.pqap_hook pointer properly synchronized. I'm
> not sure if that is what you are proposing. How do we usually
> do synchronisation on the stuff that lives in kvm->arch?
> 

RCU is a method of synchronization. We  make sure that structure
pqap_hook is still valid as long as we are inside the rcu read
lock. So the idea is: clear pointer, wait until all old readers
have finished and the proceed with getting rid of the structure.