On 5/13/21 8:15 PM, Halil Pasic wrote:
On Thu, 13 May 2021 15:23:27 -0400 Tony Krowiak akrowiak@linux.ibm.com wrote:
On 5/13/21 1:45 PM, Halil Pasic wrote:
On Thu, 13 May 2021 10:35:05 -0400 Tony Krowiak akrowiak@linux.ibm.com wrote:
On 5/12/21 2:35 PM, Halil Pasic wrote:
On Mon, 10 May 2021 17:48:37 -0400 Tony Krowiak akrowiak@linux.ibm.com wrote:
The mdev remove callback for the vfio_ap device driver bails out with -EBUSY if the mdev is in use by a KVM guest. The intended purpose was to prevent the mdev from being removed while in use; however, returning a non-zero rc does not prevent removal. This could result in a memory leak of the resources allocated when the mdev was created. In addition, the KVM guest will still have access to the AP devices assigned to the mdev even though the mdev no longer exists.
To prevent this scenario, cleanup will be done - including unplugging the AP adapters, domains and control domains - regardless of whether the mdev is in use by a KVM guest or not.
Fixes: 258287c994de ("s390: vfio-ap: implement mediated device open callback") Cc: stable@vger.kernel.org Signed-off-by: Tony Krowiak akrowiak@stny.rr.com
drivers/s390/crypto/vfio_ap_ops.c | 13 ++----------- 1 file changed, 2 insertions(+), 11 deletions(-)diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c index b2c7e10dfdcd..f90c9103dac2 100644 --- a/drivers/s390/crypto/vfio_ap_ops.c +++ b/drivers/s390/crypto/vfio_ap_ops.c @@ -26,6 +26,7 @@
static int vfio_ap_mdev_reset_queues(struct mdev_device *mdev); static struct vfio_ap_queue *vfio_ap_find_queue(int apqn);+static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev);
static int match_apqn(struct device *dev, const void *data) {@@ -366,17 +367,7 @@ static int vfio_ap_mdev_remove(struct mdev_device *mdev) struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
mutex_lock(&matrix_dev->lock);
- /*
* If the KVM pointer is in flux or the guest is running, disallow* un-assignment of control domain.*/- if (matrix_mdev->kvm_busy || matrix_mdev->kvm) {
mutex_unlock(&matrix_dev->lock);return -EBUSY;- }
- vfio_ap_mdev_reset_queues(mdev);
- vfio_ap_mdev_unset_kvm(matrix_mdev); list_del(&matrix_mdev->node); kfree(matrix_mdev);
Are we at risk of handle_pqap() in arch/s390/kvm/priv.c using an already freed pqap_hook (which is a member of the matrix_mdev pointee that is freed just above my comment).
I'm aware of the fact that vfio_ap_mdev_unset_kvm() does a matrix_mdev->kvm->arch.crypto.pqap_hook = NULL but that is AFRICT not done under any lock relevant for handle_pqap(). I guess the idea is, I guess, the check cited below
static int handle_pqap(struct kvm_vcpu *vcpu) [..] /* * Verify that the hook callback is registered, lock the owner * and call the hook. */ if (vcpu->kvm->arch.crypto.pqap_hook) { if (!try_module_get(vcpu->kvm->arch.crypto.pqap_hook->owner)) return -EOPNOTSUPP; ret = vcpu->kvm->arch.crypto.pqap_hook->hook(vcpu); module_put(vcpu->kvm->arch.crypto.pqap_hook->owner); if (!ret && vcpu->run->s.regs.gprs[1] & 0x00ff0000) kvm_s390_set_psw_cc(vcpu, 3); return ret; }
is going to catch it, but I'm not sure it is guaranteed to catch it. Opinions?
The hook itself - handle_pqap() function in vfio_ap_ops.c - also checks to see if the reference to the hook is set and terminates with an error if it is not. If the hook is invoked subsequent to the remove callback above, all should be fine since the check is also done under the matrix_dev->lock.
I don't quite understand your logic. Let us assume matrix_mdev was freed, but vcpu->kvm->arch.crypto.pqap_hook still points to what used to be (*matrix_mdev).pqap_hook. In that case the function pointer vcpu->kvm->arch.crypto.pqap_hook->hook is used after it was freed, and may not point to the handle_pqap() function in vfio_ap_ops.c, thus the check you are referring to ain't necessarily relevant. Than is if you mean the check in the handle_pqap() function in vfio_ap_ops.c; if you mean the check in handle_pqap() in arch/s390/kvm/priv.c, that one is not done under the matrix_dev->lock. Or do I have a hole somewhere in my reasoning?
What I am saying is the vcpu->kvm->arch.crypto.pqap_hook will either be NULL or point to the handle_pqap() function in the vfio_ap driver.
Please read the code again. In my reading of the code vcpu->kvm->arch.crypto.pqap_hook is never supposed to point to >(or does point to) the handle_pqap() function defined in vfio_ap_ops.c. It points to the pqap_hook member of struct ap_matrix_mdev (the type of the member is struct kvm_s390_module_hook, which in turn has a function pointer member called hook, which is supposed to hold the address of handle_pqap() function defined in vfio_ap_ops.c, and thus point to it).
You are correct, we are looking at the same code.
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?
Furthermore I believe we first need to get to common ground on this one before proceeding any further. If you happen to preserve your opinion after checking again, I think we should try to discuss this offline, as one of us is likely looking at the wrong code.
Regards, Halil
In the latter case, the handler in the driver will get called and try to acquire the matrix_dev->lock. The function that sets the vcpu->kvm->arch.crypto.pqap_hook to NULL also takes that lock. If the pointer is still active, then the handler will do its thing. If not, then the handler will return without enabling or disabling IRQs. That should not be a problem since the unset_kvm function resets the queues which will disable the IRQs.
I don't see how the vcpu->kvm->arch.crypto.pqap_hook can point to anything other than the handler or be NULL unless KVM is gone. Based on my observations of the behavior, unless there is some other way for the remove callback to be invoked other than in response to a request from userspace via the sysfs remove attribute, it will not get called until the file descriptor is closed in which case the release callback will also unset_kvm. I think you are worrying about something that will likely never happen.
Regards, Halil