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

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

On Mon, 13 Feb 2023 07:13:40 -0800
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>
> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> Reviewed-by: Kevin Tian <kevin.tian@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))
		...

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;