Re: [PATCH 8/8] vfio/mdev: Improve the create/remove sequence

[Kirti Wankhede]

On 3/23/2019 4:50 AM, Parav Pandit wrote:
> There are five problems with current code structure.
> 1. mdev device is placed on the mdev bus before it is created in the
> vendor driver. Once a device is placed on the mdev bus without creating
> its supporting underlying vendor device, an open() can get triggered by
> userspace on partially initialized device.
> Below ladder diagram highlight it.
> 
>       cpu-0                                       cpu-1
>       -----                                       -----
>    create_store()
>      mdev_create_device()
>        device_register()
>           ...
>          vfio_mdev_probe()
>          ...creates char device
>                                         vfio_mdev_open()
>                                           parent->ops->open(mdev)
>                                             vfio_ap_mdev_open()
>                                               matrix_mdev = NULL
>         [...]
>         parent->ops->create()
>           vfio_ap_mdev_create()
>             mdev_set_drvdata(mdev, matrix_mdev);
>             /* Valid pointer set above */
> 

VFIO interface uses sysfs path of device or PCI device's BDF where it
checks sysfs file for that device exist.
In case of VFIO mdev device, above situation will never happen as open
will only get called if sysfs entry for that device exist.

If you don't use VFIO interface then this situation can arise. In that
case probe() can be used for very basic initialization then create
actual char device from create().


> 2. Current creation sequence is,
>    parent->ops_create()
>    groups_register()
> 
> Remove sequence is,
>    parent->ops->remove()
>    groups_unregister()
> However, remove sequence should be exact mirror of creation sequence.
> Once this is achieved, all users of the mdev will be terminated first
> before removing underlying vendor device.
> (Follow standard linux driver model).
> At that point vendor's remove() ops shouldn't failed because device is
> taken off the bus that should terminate the users.
> 

If VMM or user space application is using mdev device,
parent->ops->remove() can return failure. In that case sysfs files
shouldn't be removed. Hence above sequence is followed for remove.

Standard linux driver model doesn't allow remove() to fail, but in
of mdev framework, interface is defined to handle such error case.


> 3. Additionally any new mdev driver that wants to work on mdev device
> during probe() routine registered using mdev_register_driver() needs to
> get stable mdev structure.
> 

Things that you are trying to handle with mdev structure from probe(),
couldn't that be moved to create()?


> 4. In following sequence, child devices created while removing mdev parent
> device can be left out, or it may lead to race of removing half
> initialized child mdev devices.
> 
> issue-1:
> --------
>        cpu-0                         cpu-1
>        -----                         -----
>                                   mdev_unregister_device()
>                                      device_for_each_child()
>                                         mdev_device_remove_cb()
>                                             mdev_device_remove()
> create_store()
>   mdev_device_create()                   [...]
>        device_register()
>                                   parent_remove_sysfs_files()
>                                   /* BUG: device added by cpu-0
>                                    * whose parent is getting removed.
>                                    */
> 
> issue-2:
> --------
>        cpu-0                         cpu-1
>        -----                         -----
> create_store()
>   mdev_device_create()                   [...]
>        device_register()
> 
>        [...]                      mdev_unregister_device()
>                                      device_for_each_child()
>                                         mdev_device_remove_cb()
>                                             mdev_device_remove()
> 
>        mdev_create_sysfs_files()
>        /* BUG: create is adding
>         * sysfs files for a device
>         * which is undergoing removal.
>         */
>                                  parent_remove_sysfs_files()
> 
> 5. Below crash is observed when user initiated remove is in progress
> and mdev_unregister_driver() completes parent unregistration.
> 
>        cpu-0                         cpu-1
>        -----                         -----
> remove_store()
>    mdev_device_remove()
>    active = false;
>                                   mdev_unregister_device()
>                                     remove type
>    [...]
>    mdev_remove_ops() crashes.
> 
> This is similar race like create() racing with mdev_unregister_device().
> 
> mtty mtty: MDEV: Registered
> iommu: Adding device 83b8f4f2-509f-382f-3c1e-e6bfe0fa1001 to group 57
> vfio_mdev 83b8f4f2-509f-382f-3c1e-e6bfe0fa1001: MDEV: group_id = 57
> mdev_device_remove sleep started
> mtty mtty: MDEV: Unregistering
> mtty_dev: Unloaded!
> BUG: unable to handle kernel paging request at ffffffffc027d668
> PGD af9818067 P4D af9818067 PUD af981a067 PMD 8583c3067 PTE 0
> Oops: 0000 [#1] SMP PTI
> CPU: 15 PID: 3517 Comm: bash Kdump: loaded Not tainted 5.0.0-rc7-vdevbus+ #2
> Hardware name: Supermicro SYS-6028U-TR4+/X10DRU-i+, BIOS 2.0b 08/09/2016
> RIP: 0010:mdev_device_remove_ops+0x1a/0x50 [mdev]
> Call Trace:
>  mdev_device_remove+0xef/0x130 [mdev]
>  remove_store+0x77/0xa0 [mdev]
>  kernfs_fop_write+0x113/0x1a0
>  __vfs_write+0x33/0x1b0
>  ? rcu_read_lock_sched_held+0x64/0x70
>  ? rcu_sync_lockdep_assert+0x2a/0x50
>  ? __sb_start_write+0x121/0x1b0
>  ? vfs_write+0x17c/0x1b0
>  vfs_write+0xad/0x1b0
>  ? trace_hardirqs_on_thunk+0x1a/0x1c
>  ksys_write+0x55/0xc0
>  do_syscall_64+0x5a/0x210
> 
> Therefore, mdev core is improved in following ways to overcome above
> issues.
> 
> 1. Before placing mdev devices on the bus, perform vendor drivers
> creation which supports the mdev creation.
> This ensures that mdev specific all necessary fields are initialized
> before a given mdev can be accessed by bus driver.
> 
> 2. During remove flow, first remove the device from the bus. This
> ensures that any bus specific devices and data is cleared.
> Once device is taken of the mdev bus, perform remove() of mdev from the
> vendor driver.
>

If user space application is using the device and someone underneath
remove the device from bus, how would use space application know that
device is being removed?
If DMA is setup, user space application is accessing that memory and
device is removed from bus - how will you restrict to not to remove that
device? If remove() is not restricted then host might crash.
I know Linux kernel device core model doesn't allow remove() to fail,
but we had tackled that problem for mdev devices in this framework. I
prefer not to change this behavior. This will regress existing working
drivers.


> 3. Linux core device model provides way to register and auto unregister
> the device sysfs attribute groups at dev->groups.
> Make use of this groups to let core create the groups and simplify code
> to avoid explicit groups creation and removal.
> 
> 4. Wait for any ongoing mdev create() and remove() to finish before
> unregistering parent device using srcu. This continues to allow multiple
> create and remove to progress in parallel. At the same time guard parent
> removal while parent is being access by create() and remove callbacks.
> 

Agreed with this.
Alex already mentioned, it would be better to have separate patch for
this fix.

Thanks,
Kirti

> Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> Signed-off-by: Parav Pandit <parav@xxxxxxxxxxxx>
> ---
>  drivers/vfio/mdev/mdev_core.c    | 142 +++++++++++++++++++++------------------
>  drivers/vfio/mdev/mdev_private.h |   7 +-
>  drivers/vfio/mdev/mdev_sysfs.c   |   6 +-
>  3 files changed, 84 insertions(+), 71 deletions(-)
> 
> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> index 944a058..8fe0ed1 100644
> --- a/drivers/vfio/mdev/mdev_core.c
> +++ b/drivers/vfio/mdev/mdev_core.c
> @@ -84,6 +84,7 @@ static void mdev_release_parent(struct kref *kref)
>  						  ref);
>  	struct device *dev = parent->dev;
>  
> +	cleanup_srcu_struct(&parent->unreg_srcu);
>  	kfree(parent);
>  	put_device(dev);
>  }
> @@ -103,56 +104,30 @@ static inline void mdev_put_parent(struct mdev_parent *parent)
>  		kref_put(&parent->ref, mdev_release_parent);
>  }
>  
> -static int mdev_device_create_ops(struct kobject *kobj,
> -				  struct mdev_device *mdev)
> +static int mdev_device_must_remove(struct mdev_device *mdev)
>  {
> -	struct mdev_parent *parent = mdev->parent;
> +	struct mdev_parent *parent;
> +	struct mdev_type *type;
>  	int ret;
>  
> -	ret = parent->ops->create(kobj, mdev);
> -	if (ret)
> -		return ret;
> +	type = to_mdev_type(mdev->type_kobj);
>  
> -	ret = sysfs_create_groups(&mdev->dev.kobj,
> -				  parent->ops->mdev_attr_groups);
> +	mdev_remove_sysfs_files(&mdev->dev, type);
> +	device_del(&mdev->dev);
> +	parent = mdev->parent;
> +	ret = parent->ops->remove(mdev);
>  	if (ret)
> -		parent->ops->remove(mdev);
> +		dev_err(&mdev->dev, "Remove failed: err=%d\n", ret);
>  
> +	/* Balances with device_initialize() */
> +	put_device(&mdev->dev);
>  	return ret;
>  }
>  
> -/*
> - * mdev_device_remove_ops gets called from sysfs's 'remove' and when parent
> - * device is being unregistered from mdev device framework.
> - * - 'force_remove' is set to 'false' when called from sysfs's 'remove' which
> - *   indicates that if the mdev device is active, used by VMM or userspace
> - *   application, vendor driver could return error then don't remove the device.
> - * - 'force_remove' is set to 'true' when called from mdev_unregister_device()
> - *   which indicate that parent device is being removed from mdev device
> - *   framework so remove mdev device forcefully.
> - */
> -static int mdev_device_remove_ops(struct mdev_device *mdev, bool force_remove)
> -{
> -	struct mdev_parent *parent = mdev->parent;
> -	int ret;
> -
> -	/*
> -	 * Vendor driver can return error if VMM or userspace application is
> -	 * using this mdev device.
> -	 */
> -	ret = parent->ops->remove(mdev);
> -	if (ret && !force_remove)
> -		return ret;
> -
> -	sysfs_remove_groups(&mdev->dev.kobj, parent->ops->mdev_attr_groups);
> -	return 0;
> -}
> -
>  static int mdev_device_remove_cb(struct device *dev, void *data)
>  {
>  	if (dev_is_mdev(dev))
> -		mdev_device_remove(dev, true);
> -
> +		mdev_device_must_remove(to_mdev_device(dev));
>  	return 0;
>  }
>  
> @@ -194,6 +169,7 @@ int mdev_register_device(struct device *dev, const struct mdev_parent_ops *ops)
>  	}
>  
>  	kref_init(&parent->ref);
> +	init_srcu_struct(&parent->unreg_srcu);
>  
>  	parent->dev = dev;
>  	parent->ops = ops;
> @@ -214,6 +190,7 @@ int mdev_register_device(struct device *dev, const struct mdev_parent_ops *ops)
>  	if (ret)
>  		dev_warn(dev, "Failed to create compatibility class link\n");
>  
> +	rcu_assign_pointer(parent->self, parent);
>  	list_add(&parent->next, &parent_list);
>  	mutex_unlock(&parent_list_lock);
>  
> @@ -244,21 +221,36 @@ void mdev_unregister_device(struct device *dev)
>  
>  	mutex_lock(&parent_list_lock);
>  	parent = __find_parent_device(dev);
> -
>  	if (!parent) {
>  		mutex_unlock(&parent_list_lock);
>  		return;
>  	}
> +	list_del(&parent->next);
> +	mutex_unlock(&parent_list_lock);
> +
>  	dev_info(dev, "MDEV: Unregistering\n");
>  
> -	list_del(&parent->next);
> +	/* Publish that this mdev parent is unregistering. So any new
> +	 * create/remove cannot start on this parent anymore by user.
> +	 */
> +	rcu_assign_pointer(parent->self, NULL);
> +
> +	/*
> +	 * Wait for any active create() or remove() mdev ops on the parent
> +	 * to complete.
> +	 */
> +	synchronize_srcu(&parent->unreg_srcu);
> +
> +	/* At this point it is confirmed that any pending user initiated
> +	 * create or remove callbacks accessing the parent are completed.
> +	 * It is safe to remove the parent now.
> +	 */
>  	class_compat_remove_link(mdev_bus_compat_class, dev, NULL);
>  
>  	device_for_each_child(dev, NULL, mdev_device_remove_cb);
>  
>  	parent_remove_sysfs_files(parent);
>  
> -	mutex_unlock(&parent_list_lock);
>  	mdev_put_parent(parent);
>  }
>  EXPORT_SYMBOL(mdev_unregister_device);
> @@ -278,14 +270,24 @@ static void mdev_device_release(struct device *dev)
>  int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
>  {
>  	int ret;
> +	struct mdev_parent *valid_parent;
>  	struct mdev_device *mdev, *tmp;
>  	struct mdev_parent *parent;
>  	struct mdev_type *type = to_mdev_type(kobj);
> +	int srcu_idx;
>  
>  	parent = mdev_get_parent(type->parent);
>  	if (!parent)
>  		return -EINVAL;
>  
> +	srcu_idx = srcu_read_lock(&parent->unreg_srcu);
> +	valid_parent = srcu_dereference(parent->self, &parent->unreg_srcu);
> +	if (!valid_parent) {
> +		/* parent is undergoing unregistration */
> +		ret = -ENODEV;
> +		goto mdev_fail;
> +	}
> +
>  	mutex_lock(&mdev_list_lock);
>  
>  	/* Check for duplicate */
> @@ -310,68 +312,76 @@ int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
>  
>  	mdev->parent = parent;
>  
> +	device_initialize(&mdev->dev);
>  	mdev->dev.parent  = dev;
>  	mdev->dev.bus     = &mdev_bus_type;
>  	mdev->dev.release = mdev_device_release;
> +	mdev->dev.groups = type->parent->ops->mdev_attr_groups;
>  	dev_set_name(&mdev->dev, "%pUl", uuid.b);
>  
> -	ret = device_register(&mdev->dev);
> +	ret = type->parent->ops->create(kobj, mdev);
>  	if (ret)
> -		goto mdev_fail;
> +		goto create_fail;
>  
> -	ret = mdev_device_create_ops(kobj, mdev);
> +	ret = device_add(&mdev->dev);
>  	if (ret)
> -		goto create_fail;
> +		goto dev_fail;
>  
>  	ret = mdev_create_sysfs_files(&mdev->dev, type);
> -	if (ret) {
> -		mdev_device_remove_ops(mdev, true);
> -		goto create_fail;
> -	}
> +	if (ret)
> +		goto sysfs_fail;
>  
>  	mdev->type_kobj = kobj;
>  	mdev->active = true;
>  	dev_dbg(&mdev->dev, "MDEV: created\n");
> +	srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
>  
>  	return 0;
>  
> +sysfs_fail:
> +	device_del(&mdev->dev);
> +dev_fail:
> +	type->parent->ops->remove(mdev);
>  create_fail:
> -	device_unregister(&mdev->dev);
> +	put_device(&mdev->dev);
>  mdev_fail:
> +	srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
>  	mdev_put_parent(parent);
>  	return ret;
>  }
>  
> -int mdev_device_remove(struct device *dev, bool force_remove)
> +int mdev_device_remove(struct device *dev)
>  {
> +	struct mdev_parent *valid_parent;
>  	struct mdev_device *mdev;
>  	struct mdev_parent *parent;
> -	struct mdev_type *type;
> +	int srcu_idx;
>  	int ret;
>  
>  	mdev = to_mdev_device(dev);
> +	parent = mdev->parent;
> +	srcu_idx = srcu_read_lock(&parent->unreg_srcu);
> +	valid_parent = srcu_dereference(parent->self, &parent->unreg_srcu);
> +	if (!valid_parent) {
> +		srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
> +		/* parent is undergoing unregistration */
> +		return -ENODEV;
> +	}
> +
> +	mutex_lock(&mdev_list_lock);
>  	if (!mdev->active) {
>  		mutex_unlock(&mdev_list_lock);
> -		return -EAGAIN;
> +		srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
> +		return -ENODEV;
>  	}
> -
>  	mdev->active = false;
>  	mutex_unlock(&mdev_list_lock);
>  
> -	type = to_mdev_type(mdev->type_kobj);
> -	parent = mdev->parent;
> -
> -	ret = mdev_device_remove_ops(mdev, force_remove);
> -	if (ret) {
> -		mdev->active = true;
> -		return ret;
> -	}
> +	ret = mdev_device_must_remove(mdev);
> +	srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
>  
> -	mdev_remove_sysfs_files(dev, type);
> -	device_unregister(dev);
>  	mdev_put_parent(parent);
> -
> -	return 0;
> +	return ret;
>  }
>  
>  static int __init mdev_init(void)
> diff --git a/drivers/vfio/mdev/mdev_private.h b/drivers/vfio/mdev/mdev_private.h
> index 84b2b6c..3d17db9 100644
> --- a/drivers/vfio/mdev/mdev_private.h
> +++ b/drivers/vfio/mdev/mdev_private.h
> @@ -23,6 +23,11 @@ struct mdev_parent {
>  	struct list_head next;
>  	struct kset *mdev_types_kset;
>  	struct list_head type_list;
> +	/* Protects unregistration to wait until create/remove
> +	 * are completed.
> +	 */
> +	struct srcu_struct unreg_srcu;
> +	struct mdev_parent __rcu *self;
>  };
>  
>  struct mdev_device {
> @@ -58,6 +63,6 @@ struct mdev_type {
>  void mdev_remove_sysfs_files(struct device *dev, struct mdev_type *type);
>  
>  int  mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid);
> -int  mdev_device_remove(struct device *dev, bool force_remove);
> +int  mdev_device_remove(struct device *dev);
>  
>  #endif /* MDEV_PRIVATE_H */
> diff --git a/drivers/vfio/mdev/mdev_sysfs.c b/drivers/vfio/mdev/mdev_sysfs.c
> index c782fa9..68a8191 100644
> --- a/drivers/vfio/mdev/mdev_sysfs.c
> +++ b/drivers/vfio/mdev/mdev_sysfs.c
> @@ -236,11 +236,9 @@ static ssize_t remove_store(struct device *dev, struct device_attribute *attr,
>  	if (val && device_remove_file_self(dev, attr)) {
>  		int ret;
>  
> -		ret = mdev_device_remove(dev, false);
> -		if (ret) {
> -			device_create_file(dev, attr);
> +		ret = mdev_device_remove(dev);
> +		if (ret)
>  			return ret;
> -		}
>  	}
>  
>  	return count;
>