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

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Date:   Tue, 18 May 2021 11:30:27 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>,
        Tony Krowiak <akrowiak@...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 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'm not sure I understood correctly what kind of a scenario are
> you asking for. PQAP AQIC and mdev remove are independent
> events originated in userspace, so AFAIK we may not assume
> that the execution of two won't overlap, nor are we allowed
> to make assumptions on how does the execution of these two
> overlap (except for the things we explicitly ensure -- e.g.
> some parts are made mutually exclusive using the matrix_dev->lock
> lock).
> 
> Regards,
> Halil
>