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Fri, 26 Mar 2021 13:25:14 +0000 (GMT) Received: from b01ledav004.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 5FA78112065; Fri, 26 Mar 2021 13:25:14 +0000 (GMT) Received: from cpe-66-24-58-13.stny.res.rr.com (unknown [9.85.149.97]) by b01ledav004.gho.pok.ibm.com (Postfix) with ESMTP; Fri, 26 Mar 2021 13:25:14 +0000 (GMT) Subject: Re: [PATCH v5 1/1] s390/vfio-ap: fix circular lockdep when setting/clearing crypto masks To: Halil Pasic Cc: linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, stable@vger.kernel.org, borntraeger@de.ibm.com, cohuck@redhat.com, kwankhede@nvidia.com, pbonzini@redhat.com, alex.williamson@redhat.com, pasic@linux.vnet.ibm.com References: <20210325124640.23995-1-akrowiak@linux.ibm.com> <20210325124640.23995-2-akrowiak@linux.ibm.com> <20210325213210.62cb11b9.pasic@linux.ibm.com> From: Tony Krowiak Message-ID: <4973feca-4eee-7139-26e6-1b926c017263@linux.ibm.com> Date: Fri, 26 Mar 2021 09:25:14 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <20210325213210.62cb11b9.pasic@linux.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US X-TM-AS-GCONF: 00 X-Proofpoint-GUID: CDWOdLx8MJgpB0FOURAFBtbTSM4jko47 X-Proofpoint-ORIG-GUID: MANubsScoTQwJ0YObvjP5J89Ayk0dy7Q X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.369,18.0.761 definitions=2021-03-26_06:2021-03-26,2021-03-26 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 mlxscore=0 suspectscore=0 adultscore=0 priorityscore=1501 malwarescore=0 mlxlogscore=999 lowpriorityscore=0 phishscore=0 bulkscore=0 clxscore=1011 spamscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2103250000 definitions=main-2103260098 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 3/25/21 4:32 PM, Halil Pasic wrote: > On Thu, 25 Mar 2021 08:46:40 -0400 > Tony Krowiak wrote: > >> This patch fixes a lockdep splat introduced by commit f21916ec4826 >> ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated"). >> The lockdep splat only occurs when starting a Secure Execution guest. >> Crypto virtualization (vfio_ap) is not yet supported for SE guests; >> however, in order to avoid this problem when support becomes available, >> this fix is being provided. >> >> The circular locking dependency was introduced when the setting of the >> masks in the guest's APCB was executed while holding the matrix_dev->lock. >> While the lock is definitely needed to protect the setting/unsetting of the >> matrix_mdev->kvm pointer, it is not necessarily critical for setting the >> masks; so, the matrix_dev->lock will be released while the masks are being >> set or cleared. >> >> Keep in mind, however, that another process that takes the matrix_dev->lock >> can get control while the masks in the guest's APCB are being set or >> cleared as a result of the driver being notified that the KVM pointer >> has been set or unset. This could result in invalid access to the >> matrix_mdev->kvm pointer by the intervening process. To avoid this >> scenario, two new fields are being added to the ap_matrix_mdev struct: >> >> struct ap_matrix_mdev { >> ... >> bool kvm_busy; >> wait_queue_head_t wait_for_kvm; >> ... >> }; >> >> The functions that handle notification that the KVM pointer value has >> been set or cleared will set the kvm_busy flag to true until they are done >> processing at which time they will set it to false and wake up the tasks on >> the matrix_mdev->wait_for_kvm wait queue. Functions that require >> access to matrix_mdev->kvm will sleep on the wait queue until they are >> awakened at which time they can safely access the matrix_mdev->kvm >> field. >> >> Fixes: f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated") >> Cc: stable@vger.kernel.org >> Signed-off-by: Tony Krowiak > Reviewed-by: Halil Pasic > > I intend to give a couple of work-days to others, and if nobody objects > merge this. (I will wait till Tuesday.) Thanks Halil. > > I've tested it and it does silence the lockdep splat. > > Regards, > Halil