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Violators will be prosecuted for from ; Wed, 26 Jun 2019 22:13:59 +0100 Subject: Re: [PATCH v4 3/7] s390: zcrypt: driver callback to indicate resource in use From: Tony Krowiak 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> Date: Wed, 26 Jun 2019 17:13:50 -0400 MIME-Version: 1.0 In-Reply-To: <2366c6b6-fd9e-0c32-0e9d-018cd601a0ad@linux.ibm.com> Content-Type: text/plain; charset="utf-8"; format="flowed" Content-Language: en-US Content-Transfer-Encoding: 8bit Message-Id: <44f13e89-2fb4-bf8c-7849-641aae8d08cc@linux.ibm.com> Sender: linux-s390-owner@vger.kernel.org List-ID: To: Cornelia Huck 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 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? Tony K > >> >