[Intel-gfx] [PATCH v9 10/25] vfio: Make vfio_device_open() single open for device cdev path

[Liu, Yi L]

Hi Eric,

> From: Eric Auger <eric.auger at redhat.com>
> Sent: Friday, April 7, 2023 5:48 PM
> 
> Hi Yi,
> 
> On 4/1/23 17:18, Yi Liu wrote:
> > VFIO group has historically allowed multi-open of the device FD. This
> > was made secure because the "open" was executed via an ioctl to the
> > group FD which is itself only single open.
> >
> > However, no known use of multiple device FDs today. It is kind of a
> > strange thing to do because new device FDs can naturally be created
> > via dup().
> >
> > When we implement the new device uAPI (only used in cdev path) there is
> > no natural way to allow the device itself from being multi-opened in a
> > secure manner. Without the group FD we cannot prove the security context
> > of the opener.
> >
> > Thus, when moving to the new uAPI we block the ability of opening
> > a device multiple times. Given old group path still allows it we store
> > a vfio_group pointer in struct vfio_device_file to differentiate.
> >
> > Reviewed-by: Kevin Tian <kevin.tian at intel.com>
> > Reviewed-by: Jason Gunthorpe <jgg at nvidia.com>
> > Tested-by: Terrence Xu <terrence.xu at intel.com>
> > Tested-by: Nicolin Chen <nicolinc at nvidia.com>
> > Tested-by: Yanting Jiang <yanting.jiang at intel.com>
> > Signed-off-by: Yi Liu <yi.l.liu at intel.com>
> > ---
> >  drivers/vfio/group.c     | 2 ++
> >  drivers/vfio/vfio.h      | 2 ++
> >  drivers/vfio/vfio_main.c | 7 +++++++
> >  3 files changed, 11 insertions(+)
> >
> > diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> > index d55ce3ca44b7..1af4b9e012a7 100644
> > --- a/drivers/vfio/group.c
> > +++ b/drivers/vfio/group.c
> > @@ -245,6 +245,8 @@ static struct file *vfio_device_open_file(struct vfio_device
> *device)
> >  		goto err_out;
> >  	}
> >
> > +	df->group = device->group;
> > +
> in previous patches df fields were protected with various locks. I refer
> to vfio_device_group_open() implementation. No need here?

yes, no need for group. It should be static in the lifecircle of df.

> 
> By the way since the group is set here, wrt [PATCH v9 06/25] kvm/vfio:
> Accept vfio device file from userspace you have a way to determine if a
> device was opened in the legacy way, no?

yes, by this we can tell if a device file is opened by legacy or cdev.
But I guess the problem in patch 06/25 is we need to know if the order
between set_kvm and open_device is needed. is it? that order requirement
is due to that the kvm pointer is needed by open_device() callback. e.g.
kvmgt. For other vfio users, this order is not needed or even the
KVM_DEV_VFIO_FILE is not needed if vfio is not used to do device passthrough.

> >  	ret = vfio_device_group_open(df);
> >  	if (ret)
> >  		goto err_free;
> > diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> > index b2f20b78a707..f1a448f9d067 100644
> > --- a/drivers/vfio/vfio.h
> > +++ b/drivers/vfio/vfio.h
> > @@ -18,6 +18,8 @@ struct vfio_container;
> >
> >  struct vfio_device_file {
> >  	struct vfio_device *device;
> > +	struct vfio_group *group;
> > +
> >  	bool access_granted;
> >  	spinlock_t kvm_ref_lock; /* protect kvm field */
> >  	struct kvm *kvm;
> > diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> > index 6d5d3c2180c8..c8721d5d05fa 100644
> > --- a/drivers/vfio/vfio_main.c
> > +++ b/drivers/vfio/vfio_main.c
> > @@ -477,6 +477,13 @@ int vfio_device_open(struct vfio_device_file *df)
> >
> >  	lockdep_assert_held(&device->dev_set->lock);
> >
> > +	/*
> > +	 * Only the group path allows the device opened multiple times.
> allows the device to be opened multiple times

got it.

Thanks,
Yi Liu

> > +	 * The device cdev path doesn't have a secure way for it.
> > +	 */
> > +	if (device->open_count != 0 && !df->group)
> > +		return -EINVAL;
> > +
> >  	device->open_count++;
> >  	if (device->open_count == 1) {
> >  		ret = vfio_device_first_open(df);
> Thanks
> 
> Eric