On Mon, 27 Mar 2023 02:40:31 -0700
Yi Liu <yi.l.liu@xxxxxxxxx> 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@xxxxxxxxxx>
> Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Tested-by: Terrence Xu <terrence.xu@xxxxxxxxx>
> Tested-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Tested-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> Signed-off-by: Yi Liu <yi.l.liu@xxxxxxxxx>
> ---
> 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,
Alex
> +}
> +
> /**
> * 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);
>
