RE: [PATCH v8 08/24] vfio: Block device access via device fd until device is opened

["Liu, Yi L" <yi.l.liu@intel.com>]

> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Wednesday, March 29, 2023 5:34 AM
> 
> On Mon, 27 Mar 2023 02:40:31 -0700
> Yi Liu <yi.l.liu@intel.com> wrote:
> 
> > Allow the vfio_device file to be in a state where the device FD is
> > opened but the device cannot be used by userspace (i.e. its .open_device()
> > hasn't been called). This inbetween state is not used when the device
> > FD is spawned from the group FD, however when we create the device FD
> > directly by opening a cdev it will be opened in the blocked state.
> >
> > The reason for the inbetween state is that userspace only gets a FD but
> > doesn't gain access permission until binding the FD to an iommufd. So in
> > the blocked state, only the bind operation is allowed. Completing bind
> > will allow user to further access the device.
> >
> > This is implemented by adding a flag in struct vfio_device_file to mark
> > the blocked state and using a simple smp_load_acquire() to obtain the
> > flag value and serialize all the device setup with the thread accessing
> > this device.
> >
> > Following this lockless scheme, it can safely handle the device FD
> > unbound->bound but it cannot handle bound->unbound. To allow this
> we'd
> > need to add a lock on all the vfio ioctls which seems costly. So once
> > device FD is bound, it remains bound until the FD is closed.
> >
> > Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> > Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> > Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> > Tested-by: Terrence Xu <terrence.xu@intel.com>
> > Tested-by: Nicolin Chen <nicolinc@nvidia.com>
> > Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
> > Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> > ---
> >  drivers/vfio/group.c     | 11 ++++++++++-
> >  drivers/vfio/vfio.h      |  1 +
> >  drivers/vfio/vfio_main.c | 41 ++++++++++++++++++++++++++++++++++---
> ---
> >  3 files changed, 46 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> > index 9a7b2765eef6..4f267ae7bebc 100644
> > --- a/drivers/vfio/group.c
> > +++ b/drivers/vfio/group.c
> > @@ -194,9 +194,18 @@ static int vfio_device_group_open(struct
> vfio_device_file *df)
> >  	df->iommufd = device->group->iommufd;
> >
> >  	ret = vfio_device_open(df);
> > -	if (ret)
> > +	if (ret) {
> >  		df->iommufd = NULL;
> > +		goto out_put_kvm;
> > +	}
> > +
> > +	/*
> > +	 * Paired with smp_load_acquire() in vfio_device_fops::ioctl/
> > +	 * read/write/mmap
> > +	 */
> > +	smp_store_release(&df->access_granted, true);
> >
> > +out_put_kvm:
> >  	if (device->open_count == 0)
> >  		vfio_device_put_kvm(device);
> >
> > diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> > index cffc08f5a6f1..854f2c97cb9a 100644
> > --- a/drivers/vfio/vfio.h
> > +++ b/drivers/vfio/vfio.h
> > @@ -18,6 +18,7 @@ struct vfio_container;
> >
> >  struct vfio_device_file {
> >  	struct vfio_device *device;
> > +	bool access_granted;
> >  	spinlock_t kvm_ref_lock; /* protect kvm field */
> >  	struct kvm *kvm;
> >  	struct iommufd_ctx *iommufd; /* protected by struct
> vfio_device_set::lock */
> > diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> > index 2ea6cb6d03c7..b515bbda4c74 100644
> > --- a/drivers/vfio/vfio_main.c
> > +++ b/drivers/vfio/vfio_main.c
> > @@ -1114,6 +1114,10 @@ static long vfio_device_fops_unl_ioctl(struct
> file *filep,
> >  	struct vfio_device *device = df->device;
> >  	int ret;
> >
> > +	/* Paired with smp_store_release() following vfio_device_open() */
> > +	if (!smp_load_acquire(&df->access_granted))
> > +		return -EINVAL;
> > +
> >  	ret = vfio_device_pm_runtime_get(device);
> >  	if (ret)
> >  		return ret;
> > @@ -1141,6 +1145,10 @@ static ssize_t vfio_device_fops_read(struct file
> *filep, char __user *buf,
> >  	struct vfio_device_file *df = filep->private_data;
> >  	struct vfio_device *device = df->device;
> >
> > +	/* Paired with smp_store_release() following vfio_device_open() */
> > +	if (!smp_load_acquire(&df->access_granted))
> > +		return -EINVAL;
> > +
> >  	if (unlikely(!device->ops->read))
> >  		return -EINVAL;
> >
> > @@ -1154,6 +1162,10 @@ static ssize_t vfio_device_fops_write(struct
> file *filep,
> >  	struct vfio_device_file *df = filep->private_data;
> >  	struct vfio_device *device = df->device;
> >
> > +	/* Paired with smp_store_release() following vfio_device_open() */
> > +	if (!smp_load_acquire(&df->access_granted))
> > +		return -EINVAL;
> > +
> >  	if (unlikely(!device->ops->write))
> >  		return -EINVAL;
> >
> > @@ -1165,6 +1177,10 @@ static int vfio_device_fops_mmap(struct file
> *filep, struct vm_area_struct *vma)
> >  	struct vfio_device_file *df = filep->private_data;
> >  	struct vfio_device *device = df->device;
> >
> > +	/* Paired with smp_store_release() following vfio_device_open() */
> > +	if (!smp_load_acquire(&df->access_granted))
> > +		return -EINVAL;
> > +
> >  	if (unlikely(!device->ops->mmap))
> >  		return -EINVAL;
> >
> > @@ -1201,6 +1217,24 @@ bool vfio_file_is_valid(struct file *file)
> >  }
> >  EXPORT_SYMBOL_GPL(vfio_file_is_valid);
> >
> > +/*
> > + * Return true if the input file is a vfio device file and has opened
> > + * the input device. Otherwise, return false.
> > + */
> > +static bool vfio_file_has_device_access(struct file *file,
> > +					struct vfio_device *device)
> > +{
> > +	struct vfio_device *vdev = vfio_device_from_file(file);
> > +	struct vfio_device_file *df;
> > +
> > +	if (!vdev || vdev != device)
> > +		return false;
> > +
> > +	df = file->private_data;
> > +
> > +	return READ_ONCE(df->access_granted);
> 
> Why did we change from smp_load_acquire() to READ_ONCE() here?
> Thanks,

It should still use smp_load_acquire(). When this is added, its major usage
is under the dev_set->lock, so it is functionally ok since the writer of the
df->access_granted is also under the dev_set->lock. However, as an helper
in vfio_core, it should use smp_load_acquire() otherwise needs to add
lockdep to check if dev_set->lock is held.

Thanks,
Yi Liu

> 
> > +}
> > +
> >  /**
> >   * vfio_file_has_dev - True if the VFIO file is a handle for device
> >   * @file: VFIO file to check
> > @@ -1211,17 +1245,12 @@ EXPORT_SYMBOL_GPL(vfio_file_is_valid);
> >  bool vfio_file_has_dev(struct file *file, struct vfio_device *device)
> >  {
> >  	struct vfio_group *group;
> > -	struct vfio_device *vdev;
> >
> >  	group = vfio_group_from_file(file);
> >  	if (group)
> >  		return vfio_group_has_dev(group, device);
> >
> > -	vdev = vfio_device_from_file(file);
> > -	if (vdev)
> > -		return vdev == device;
> > -
> > -	return false;
> > +	return vfio_file_has_device_access(file, device);
> >  }
> >  EXPORT_SYMBOL_GPL(vfio_file_has_dev);
> >