Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
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>
From:   Tony Krowiak <akrowiak@linux.ibm.com>
Message-ID: <9ebd5fd8-b093-e5bc-e680-88fa7a9b085c@linux.ibm.com>
Date:   Tue, 18 May 2021 09:42:05 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.8.1
MIME-Version: 1.0
In-Reply-To: <966a60ad-bdde-68d0-ae2f-06121c6ad970@de.ibm.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Precedence: bulk


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'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
>>