Re: [RFC PATCH 1/1] vfio-pci/iommu: Detach iommu group on remove path

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 2015-07-22 at 10:54 -0600, Alex Williamson wrote:
> On Tue, 2015-07-21 at 19:44 +0200, Gerald Schaefer wrote:
> > When a user completes the VFIO_SET_IOMMU ioctl and the vfio-pci device is
> > removed thereafter (before any other ioctl like VFIO_GROUP_GET_DEVICE_FD),
> > then the detach_dev callback of the underlying IOMMU API is never called.
> > 
> > This patch adds a call to vfio_group_try_dissolve_container() to the remove
> > path, which will trigger the missing detach_dev callback in this scenario.
> > 
> > Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
> > ---
> >  drivers/vfio/vfio.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> > index 2fb29df..9c5c784 100644
> > --- a/drivers/vfio/vfio.c
> > +++ b/drivers/vfio/vfio.c
> > @@ -711,6 +711,8 @@ static bool vfio_dev_present(struct vfio_group *group, struct device *dev)
> >  	return true;
> >  }
> >  
> > +static void vfio_group_try_dissolve_container(struct vfio_group *group);
> > +
> >  /*
> >   * Decrement the device reference count and wait for the device to be
> >   * removed.  Open file descriptors for the device... */
> > @@ -785,6 +787,7 @@ void *vfio_del_group_dev(struct device *dev)
> >  		}
> >  	} while (ret <= 0);
> >  
> > +	vfio_group_try_dissolve_container(group);
> >  	vfio_group_put(group);
> >  
> >  	return device_data;
> 
> 
> This won't work, vfio_group_try_dissolve_container() decrements
> container_users, which an unused device is not.  Imagine if we had more
> than one device in the iommu group, one device is removed and the
> container is dissolved despite the user holding a reference and other
> viable devices remaining.  Additionally, from an isolation perspective,
> an unbind from vfio-pci should not pull the device out of the iommu
> domain, it's part of the domain because it's not isolated and that
> continues even after unbind.
> 
> I think what you want to do is detach a device from the iommu domain
> only when it's being removed from iommu group, such as through
> iommu_group_remove_device().  We already have a bit of an asymmetry
> there as iommu_group_add_device() will add devices to the currently
> active iommu domain for the group, but iommu_group_remove_device() does
> not appear to do the reverse.  Thanks,

BTW, VT-d on x86 avoids a leak using its own notifier_block,
drivers/iommu/intel-iommu.c:device_notifier() catches
BUS_NOTIFY_REMOVED_DEVICE and removes the device from the domain (the
domain_exit() there is only used for non-IOMMU-API domains).  It's
possible that's the only IOMMU driver that avoids a leak due to the
scenario you describe.  Thanks,

Alex