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Subject: Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback
To:     Christian Borntraeger <borntraeger@de.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>
 <d426bb7e-39d8-6eaf-047e-05eb70cdaeb7@de.ibm.com>
From:   Tony Krowiak <akrowiak@linux.ibm.com>
Message-ID: <d413938b-3fcf-836d-adab-92340ff63eff@linux.ibm.com>
Date:   Tue, 18 May 2021 14:40:50 -0400
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On 5/18/21 2:22 PM, Christian Borntraeger wrote:
>
>
> 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.

Yes, that is my understanding too.

>
>> 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.
>>