From: Tony Krowiak <akrowiak@linux.ibm.com>
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>
Subject: Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback
Date: Tue, 18 May 2021 14:40:50 -0400 [thread overview]
Message-ID: <d413938b-3fcf-836d-adab-92340ff63eff@linux.ibm.com> (raw)
In-Reply-To: <d426bb7e-39d8-6eaf-047e-05eb70cdaeb7@de.ibm.com>
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.
>>
next prev parent reply other threads:[~2021-05-18 18:40 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-10 21:48 [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback Tony Krowiak
2021-05-10 21:56 ` Tony Krowiak
2021-05-12 10:35 ` Cornelia Huck
2021-05-12 12:41 ` Jason Gunthorpe
2021-05-12 15:32 ` Christian Borntraeger
2021-05-12 16:50 ` Jason Gunthorpe
2021-05-13 14:19 ` Tony Krowiak
2021-05-13 14:18 ` Tony Krowiak
2021-05-13 17:25 ` Jason Gunthorpe
2021-05-13 17:32 ` Halil Pasic
2021-05-13 17:34 ` Jason Gunthorpe
2021-05-12 16:49 ` Christian Borntraeger
2021-05-12 18:35 ` Halil Pasic
2021-05-13 14:35 ` Tony Krowiak
2021-05-13 17:45 ` Halil Pasic
2021-05-13 19:23 ` Tony Krowiak
2021-05-14 0:15 ` Halil Pasic
2021-05-17 13:37 ` Tony Krowiak
2021-05-17 19:10 ` Halil Pasic
2021-05-18 9:30 ` Christian Borntraeger
2021-05-18 13:42 ` Tony Krowiak
2021-05-18 13:59 ` Christian Borntraeger
2021-05-18 15:33 ` Halil Pasic
2021-05-18 17:01 ` Christian Borntraeger
2021-05-18 23:27 ` Halil Pasic
2021-05-19 8:17 ` Christian Borntraeger
2021-05-19 11:22 ` Christian Borntraeger
2021-05-19 12:59 ` Halil Pasic
2021-05-19 13:02 ` Jason Gunthorpe
2021-05-19 11:25 ` Halil Pasic
2021-05-18 18:14 ` Tony Krowiak
2021-05-18 18:22 ` Christian Borntraeger
2021-05-18 18:40 ` Tony Krowiak [this message]
2021-05-18 13:41 ` Tony Krowiak
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