From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:52370 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726054AbfF0HZ2 (ORCPT ); Thu, 27 Jun 2019 03:25:28 -0400 Date: Thu, 27 Jun 2019 09:25:18 +0200 From: Cornelia Huck Subject: Re: [PATCH v4 3/7] s390: zcrypt: driver callback to indicate resource in use Message-ID: <20190627092518.1f8d7d48.cohuck@redhat.com> In-Reply-To: <44f13e89-2fb4-bf8c-7849-641aae8d08cc@linux.ibm.com> References: <1560454780-20359-1-git-send-email-akrowiak@linux.ibm.com> <1560454780-20359-4-git-send-email-akrowiak@linux.ibm.com> <20190618182558.7d7e025a.cohuck@redhat.com> <2366c6b6-fd9e-0c32-0e9d-018cd601a0ad@linux.ibm.com> <44f13e89-2fb4-bf8c-7849-641aae8d08cc@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Sender: linux-s390-owner@vger.kernel.org List-ID: To: Tony Krowiak Cc: linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, freude@linux.ibm.com, borntraeger@de.ibm.com, frankja@linux.ibm.com, david@redhat.com, mjrosato@linux.ibm.com, schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com, pmorel@linux.ibm.com, pasic@linux.ibm.com, alex.williamson@redhat.com, kwankhede@nvidia.com On Wed, 26 Jun 2019 17:13:50 -0400 Tony Krowiak wrote: > On 6/19/19 9:04 AM, Tony Krowiak wrote: > > On 6/18/19 12:25 PM, Cornelia Huck wrote: > >> On Thu, 13 Jun 2019 15:39:36 -0400 > >> Tony Krowiak wrote: > >> > >>> Introduces a new driver callback to prevent a root user from unbinding > >>> an AP queue from its device driver if the queue is in use. This prevents > >>> a root user from inadvertently taking a queue away from a guest and > >>> giving it to the host, or vice versa. The callback will be invoked > >>> whenever a change to the AP bus's apmask or aqmask sysfs interfaces may > >>> result in one or more AP queues being removed from its driver. If the > >>> callback responds in the affirmative for any driver queried, the change > >>> to the apmask or aqmask will be rejected with a device in use error. > >>> > >>> For this patch, only non-default drivers will be queried. Currently, > >>> there is only one non-default driver, the vfio_ap device driver. The > >>> vfio_ap device driver manages AP queues passed through to one or more > >>> guests and we don't want to unexpectedly take AP resources away from > >>> guests which are most likely independently administered. > >>> > >>> Signed-off-by: Tony Krowiak > >>> --- > >>>   drivers/s390/crypto/ap_bus.c | 138 > >>> +++++++++++++++++++++++++++++++++++++++++-- > >>>   drivers/s390/crypto/ap_bus.h |   3 + > >>>   2 files changed, 135 insertions(+), 6 deletions(-) > >> > >> Hm... I recall objecting to this patch before, fearing that it makes it > >> possible for a bad actor to hog resources that can't be removed by > >> root, even forcefully. (I have not had time to look at the intervening > >> versions, so I might be missing something.) > >> > >> Is there a way for root to forcefully override this? > > > > You recall correctly; however, after many internal crypto team > > discussions, it was decided that this feature was important > > and should be kept. > > > > Allow me to first address your fear that a bad actor can hog > > resources that can't be removed by root. With this enhancement, > > there is nothing preventing a root user from taking resources > > from a matrix mdev, it simply forces him/her to follow the > > proper procedure. The resources to be removed must first be > > unassigned from the matrix mdev to which they are assigned. > > The AP bus's /sys/bus/ap/apmask and /sys/bus/ap/aqmask > > sysfs attributes can then be edited to transfer ownership > > of the resources to zcrypt. > > > > The rationale for keeping this feature is: > > > > * It is a bad idea to steal an adapter in use from a guest. In the worst > >   case, the guest could end up without access to any crypto adapters > >   without knowing why. This could lead to performance issues on guests > >   that rely heavily on crypto such as guests used for blockchain > >   transactions. > > > > * There are plenty of examples in linux of the kernel preventing a root > >   user from performing a task. For example, a module can't be removed > >   if references are still held for it. Another example would be trying > >   to bind a CEX4 adapter to a device driver not registered for CEX4; > >   this action will also be rejected. > > > > * The semantics are much cleaner and the logic is far less complicated. > > > > * It forces the use of the proper procedure to change ownership of AP > >   queues. > > > > Any feedback on this? Had not yet time to look at this, sorry. > > Tony K > > > > >> > > >