[Intel-gfx] [PATCH v3 07/15] vfio: Block device access via device fd until device is opened

[Liu, Yi L]

> From: Alex Williamson <alex.williamson at redhat.com>
> Sent: Wednesday, February 15, 2023 6:47 AM
> 
> On Mon, 13 Feb 2023 07:13:40 -0800
> Yi Liu <yi.l.liu at 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 at nvidia.com>
> > Signed-off-by: Yi Liu <yi.l.liu at intel.com>
> > Reviewed-by: Kevin Tian <kevin.tian at intel.com>
> > ---
> >  drivers/vfio/vfio.h      |  1 +
> >  drivers/vfio/vfio_main.c | 34 +++++++++++++++++++++++++++++++++-
> >  2 files changed, 34 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> > index 11e56fe079a1..d56cdb114024 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 c517252aba19..2267057240bd 100644
> > --- a/drivers/vfio/vfio_main.c
> > +++ b/drivers/vfio/vfio_main.c
> > @@ -476,7 +476,15 @@ int vfio_device_open(struct vfio_device_file *df)
> >  			device->open_count--;
> >  	}
> >
> > -	return ret;
> > +	if (ret)
> > +		return ret;
> > +
> > +	/*
> > +	 * Paired with smp_load_acquire() in vfio_device_fops::ioctl/
> > +	 * read/write/mmap
> > +	 */
> > +	smp_store_release(&df->access_granted, true);
> > +	return 0;
> >  }
> >
> >  void vfio_device_close(struct vfio_device_file *df)
> > @@ -1104,8 +1112,14 @@ static long vfio_device_fops_unl_ioctl(struct
> file *filep,
> >  {
> >  	struct vfio_device_file *df = filep->private_data;
> >  	struct vfio_device *device = df->device;
> > +	bool access;
> >  	int ret;
> >
> > +	/* Paired with smp_store_release() in vfio_device_open() */
> > +	access = smp_load_acquire(&df->access_granted);
> > +	if (!access)
> > +		return -EINVAL;
> > +
> 
> Nit,
> 
> 	if (!smp_load_acquire(&df->access_granted))
> 		...

Got it. also updated other similar lines.

> Thanks,
> Alex
> 
> >  	ret = vfio_device_pm_runtime_get(device);
> >  	if (ret)
> >  		return ret;
> > @@ -1132,6 +1146,12 @@ 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;
> > +	bool access;
> > +
> > +	/* Paired with smp_store_release() in vfio_device_open() */
> > +	access = smp_load_acquire(&df->access_granted);
> > +	if (!access)
> > +		return -EINVAL;
> >
> >  	if (unlikely(!device->ops->read))
> >  		return -EINVAL;
> > @@ -1145,6 +1165,12 @@ static ssize_t vfio_device_fops_write(struct file
> *filep,
> >  {
> >  	struct vfio_device_file *df = filep->private_data;
> >  	struct vfio_device *device = df->device;
> > +	bool access;
> > +
> > +	/* Paired with smp_store_release() in vfio_device_open() */
> > +	access = smp_load_acquire(&df->access_granted);
> > +	if (!access)
> > +		return -EINVAL;
> >
> >  	if (unlikely(!device->ops->write))
> >  		return -EINVAL;
> > @@ -1156,6 +1182,12 @@ 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;
> > +	bool access;
> > +
> > +	/* Paired with smp_store_release() in vfio_device_open() */
> > +	access = smp_load_acquire(&df->access_granted);
> > +	if (!access)
> > +		return -EINVAL;
> >
> >  	if (unlikely(!device->ops->mmap))
> >  		return -EINVAL;

Regards,
Yi Liu