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Wed, 10 Feb 2021 20:34:26 +0000 (GMT) Received: from b03ledav001.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 15CA76E04E; Wed, 10 Feb 2021 20:34:24 +0000 (GMT) Received: from cpe-66-24-58-13.stny.res.rr.com (unknown [9.85.203.235]) by b03ledav001.gho.boulder.ibm.com (Postfix) with ESMTP; Wed, 10 Feb 2021 20:34:24 +0000 (GMT) Subject: Re: [PATCH 1/1] s390/vfio-ap: fix circular lockdep when setting/clearing crypto masks To: Cornelia Huck Cc: linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, stable@vger.kernel.org, borntraeger@de.ibm.com, kwankhede@nvidia.com, pbonzini@redhat.com, alex.williamson@redhat.com, pasic@linux.vnet.ibm.com References: <20210209194830.20271-1-akrowiak@linux.ibm.com> <20210209194830.20271-2-akrowiak@linux.ibm.com> <20210210115334.46635966.cohuck@redhat.com> From: Tony Krowiak Message-ID: <6e2842e4-334d-6592-a781-5b85ec0ed13c@linux.ibm.com> Date: Wed, 10 Feb 2021 15:34:24 -0500 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: <20210210115334.46635966.cohuck@redhat.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-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.369,18.0.737 definitions=2021-02-10_10:2021-02-10,2021-02-10 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 bulkscore=0 spamscore=0 adultscore=0 priorityscore=1501 malwarescore=0 phishscore=0 impostorscore=0 mlxlogscore=999 lowpriorityscore=0 mlxscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2102100183 Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On 2/10/21 5:53 AM, Cornelia Huck wrote: > On Tue, 9 Feb 2021 14:48:30 -0500 > Tony Krowiak wrote: > >> This patch fixes a circular locking dependency in the CI introduced by >> commit f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM >> pointer invalidated"). The lockdep only occurs when starting a Secure >> Execution guest. Crypto virtualization (vfio_ap) is not yet supported for >> SE guests; however, in order to avoid CI errors, this fix is being >> provided. >> >> The circular lockdep was introduced when the masks in the guest's APCB >> were taken under the matrix_dev->lock. While the lock is definitely >> needed to protect the setting/unsetting of the KVM pointer, it is not >> necessarily critical for setting the masks, so this will not be done under >> protection of the matrix_dev->lock. >> >> Fixes: f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated") >> Cc: stable@vger.kernel.org >> Signed-off-by: Tony Krowiak >> --- >> drivers/s390/crypto/vfio_ap_ops.c | 75 ++++++++++++++++++------------- >> 1 file changed, 45 insertions(+), 30 deletions(-) >> >> static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev) >> { >> - kvm_arch_crypto_clear_masks(matrix_mdev->kvm); >> - matrix_mdev->kvm->arch.crypto.pqap_hook = NULL; >> - vfio_ap_mdev_reset_queues(matrix_mdev->mdev); >> - kvm_put_kvm(matrix_mdev->kvm); >> - matrix_mdev->kvm = NULL; >> + if (matrix_mdev->kvm) { > If you're doing setting/unsetting under matrix_dev->lock, is it > possible that matrix_mdev->kvm gets unset between here and the next > line, as you don't hold the lock? That is highly unlikely because the only place the matrix_mdev->kvm pointer is cleared is in this function which is called from only two places: the notifier that handles the VFIO_GROUP_NOTIFY_SET_KVM notification when the KVM pointer is cleared; the vfio_ap_mdev_release() function which is called when the mdev fd is closed (i.e., when the guest is shut down). The fact is, with the only end-to-end implementation currently available, the notifier callback is never invoked to clear the KVM pointer because the vfio_ap_mdev_release callback is invoked first and it unregisters the notifier callback. Having said that, I suppose there is no guarantee that there will not be different userspace clients in the future that do things in a different order. At the very least, it wouldn't hurt to protect against that as you suggest below. > > Maybe you could > - grab a reference to kvm while holding the lock > - call the mask handling functions with that kvm reference > - lock again, drop the reference, and do the rest of the processing? > >> + kvm_arch_crypto_clear_masks(matrix_mdev->kvm); >> + mutex_lock(&matrix_dev->lock); >> + matrix_mdev->kvm->arch.crypto.pqap_hook = NULL; >> + vfio_ap_mdev_reset_queues(matrix_mdev->mdev); >> + kvm_put_kvm(matrix_mdev->kvm); >> + matrix_mdev->kvm = NULL; >> + mutex_unlock(&matrix_dev->lock); >> + } >> }