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Tue, 18 May 2021 17:01:43 +0000 (GMT) Received: from d06av24.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B32484204D; Tue, 18 May 2021 17:01:42 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.171.73.129]) by d06av24.portsmouth.uk.ibm.com (Postfix) with ESMTP; Tue, 18 May 2021 17:01:42 +0000 (GMT) Subject: Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback To: Halil Pasic Cc: Tony Krowiak , 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 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> <20210518173351.39646b45.pasic@linux.ibm.com> From: Christian Borntraeger Message-ID: Date: Tue, 18 May 2021 19:01:42 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.0 MIME-Version: 1.0 In-Reply-To: <20210518173351.39646b45.pasic@linux.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: 1J4hzD5DvedK2mPLyLxEDiDgbc_Sc2PC X-Proofpoint-GUID: f9looOVwZanR89q8a1ZH-nBY4czJPeag X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.391,18.0.761 definitions=2021-05-18_08:2021-05-18,2021-05-18 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 impostorscore=0 clxscore=1015 priorityscore=1501 malwarescore=0 adultscore=0 bulkscore=0 mlxscore=0 suspectscore=0 mlxlogscore=999 phishscore=0 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2104190000 definitions=main-2105180113 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18.05.21 17:33, Halil Pasic wrote: > On Tue, 18 May 2021 15:59:36 +0200 > 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 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? > > Unfortunately just "the hook" is ambiguous in this context. We > have kvm->arch.crypto.pqap_hook that is supposed to point to > a struct kvm_s390_module_hook member of struct ap_matrix_mdev > which is also called pqap_hook. And struct kvm_s390_module_hook > has function pointer member named "hook". I was referring to the full struct. > >>> >>> 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. > > In my opinion, we should make the accesses to the > kvm->arch.crypto.pqap_hook pointer properly synchronized. I'm > not sure if that is what you are proposing. How do we usually > do synchronisation on the stuff that lives in kvm->arch? > RCU is a method of synchronization. We make sure that structure pqap_hook is still valid as long as we are inside the rcu read lock. So the idea is: clear pointer, wait until all old readers have finished and the proceed with getting rid of the structure.