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

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Date:   Tue, 18 May 2021 15:59:36 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>,
        Halil Pasic <pasic@...ux.ibm.com>
Cc:     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 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?
> 
> 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?