RE: [PATCH v5 07/19] vfio: Block device access via device fd until device is opened

Liu, Yi L
2023-03-01T09:22:20Z

> From: Liu, Yi L <yi.l.liu@intel.com>
> Sent: Monday, February 27, 2023 7:11 PM
>
> 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/group.c | 6 ++++++
> drivers/vfio/vfio.h | 1 +
> drivers/vfio/vfio_main.c | 16 ++++++++++++++++
> 3 files changed, 23 insertions(+)
>
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index 960b1bcb606b..d8771d585cb1 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -197,6 +197,12 @@ static int vfio_device_group_open(struct
> vfio_device_file *df)
> if (device->open_count == 0)
> vfio_device_put_kvm(device);
>
> + /*
> + * Paired with smp_load_acquire() in vfio_device_fops::ioctl/
> + * read/write/mmap
> + */
> + smp_store_release(&df->access_granted, true);
> +

A bug. If ret is false, it should not set df->access_granted. Would
be fixed in the next version.

Regards,
Yi Liu
> mutex_unlock(&device->dev_set->lock);
>
> out_unlock:
> diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> index 7c1ea870d8f3..2e3cb284711d 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 609700748082..d16ac573e290 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -1106,6 +1106,10 @@ static long vfio_device_fops_unl_ioctl(struct file
> *filep,
> struct vfio_device *device = df->device;
> int ret;
>
> + /* Paired with smp_store_release() in vfio_device_group_open()
> */
> + if (!smp_load_acquire(&df->access_granted))
> + return -EINVAL;
> +
> ret = vfio_device_pm_runtime_get(device);
> if (ret)
> return ret;
> @@ -1133,6 +1137,10 @@ 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() in vfio_device_group_open()
> */
> + if (!smp_load_acquire(&df->access_granted))
> + return -EINVAL;
> +
> if (unlikely(!device->ops->read))
> return -EINVAL;
>
> @@ -1146,6 +1154,10 @@ 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() in vfio_device_group_open()
> */
> + if (!smp_load_acquire(&df->access_granted))
> + return -EINVAL;
> +
> if (unlikely(!device->ops->write))
> return -EINVAL;
>
> @@ -1157,6 +1169,10 @@ 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() in vfio_device_group_open()
> */
> + if (!smp_load_acquire(&df->access_granted))
> + return -EINVAL;
> +
> if (unlikely(!device->ops->mmap))
> return -EINVAL;
>
> --
> 2.34.1