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Subject: Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback
To:     Tony Krowiak <akrowiak@linux.ibm.com>,
        Halil Pasic <pasic@linux.ibm.com>
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        cohuck@redhat.com, pasic@linux.vnet.ibm.com, jjherne@linux.ibm.com,
        jgg@nvidia.com, alex.williamson@redhat.com, kwankhede@nvidia.com,
        stable@vger.kernel.org, Tony Krowiak <akrowiak@stny.rr.com>
References: <20210510214837.359717-1-akrowiak@linux.ibm.com>
 <20210512203536.4209c29c.pasic@linux.ibm.com>
 <4c156ab8-da49-4867-f29c-9712c2628d44@linux.ibm.com>
 <20210513194541.58d1628a.pasic@linux.ibm.com>
 <243086e2-08a0-71ed-eb7e-618a62b007e4@linux.ibm.com>
 <20210514021500.60ad2a22.pasic@linux.ibm.com>
 <594374f6-8cf6-4c22-0bac-3b224c55bbb6@linux.ibm.com>
 <20210517211030.368ca64b.pasic@linux.ibm.com>
 <966a60ad-bdde-68d0-ae2f-06121c6ad970@de.ibm.com>
 <9ebd5fd8-b093-e5bc-e680-88fa7a9b085c@linux.ibm.com>
 <494af62b-dc9a-ef2c-1869-d8f5ed239504@de.ibm.com>
 <00c86767-ce8b-7050-f665-75f33fabe118@linux.ibm.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <d426bb7e-39d8-6eaf-047e-05eb70cdaeb7@de.ibm.com>
Date:   Tue, 18 May 2021 20:22:34 +0200
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On 18.05.21 20:14, Tony Krowiak wrote:
> 
> 
> 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.

The trick should be the sychronize_rcu. This will put the deleting
code (vfio_ap_mdev_unset_kvm) to sleep until the rcu read section
has finished. So if you first set the pointer to zero, then call
synchronize_rcu the code will only progress until all users of
the old poiner have finished.

> 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.
>