[PATCHv3 3/3] vfio/mdev: Synchronize device create/remove with parent removal

[Parav Pandit]

In following sequences, 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_add()
                                  parent_remove_sysfs_files()

/* BUG: device added by cpu-0
 * whose parent is getting removed
 * and it won't process this mdev.
 */

issue-2:
--------
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()
                                  parent device removed.
   [...]
   parents->ops->remove()
 /*
  * BUG: Accessing invalid parent.
  */

This is similar race like create() racing with mdev_unregister_device().

BUG: unable to handle kernel paging request at ffffffffc0585668
PGD e8f618067 P4D e8f618067 PUD e8f61a067 PMD 85adca067 PTE 0
Oops: 0000 [#1] SMP PTI
CPU: 41 PID: 37403 Comm: bash Kdump: loaded Not tainted 5.1.0-rc6-vdevbus+ #6
Hardware name: Supermicro SYS-6028U-TR4+/X10DRU-i+, BIOS 2.0b 08/09/2016
RIP: 0010:mdev_device_remove+0xfa/0x140 [mdev]
Call Trace:
 remove_store+0x71/0x90 [mdev]
 kernfs_fop_write+0x113/0x1a0
 vfs_write+0xad/0x1b0
 ksys_write+0x5a/0xe0
 do_syscall_64+0x5a/0x210
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Therefore, mdev core is improved as below to overcome above issues.

Wait for any ongoing mdev create() and remove() to finish before
unregistering parent device using refcount and completion.
This continues to allow multiple create and remove to progress in
parallel for different mdev devices as most common case.
At the same time guard parent removal while parent is being access by
create() and remove callbacks.

Code is simplified from kref to use refcount as unregister_device() has
to wait anyway for all create/remove to finish.

While removing mdev devices during parent unregistration, there isn't
need to acquire refcount of parent device, hence code is restructured
using mdev_device_remove_common() to avoid it.

Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
Signed-off-by: Parav Pandit <parav@xxxxxxxxxxxx>
---
 drivers/vfio/mdev/mdev_core.c    | 86 ++++++++++++++++++++------------
 drivers/vfio/mdev/mdev_private.h |  6 ++-
 2 files changed, 60 insertions(+), 32 deletions(-)

diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
index 0bef0cae1d4b..ca33246c1dc3 100644
--- a/drivers/vfio/mdev/mdev_core.c
+++ b/drivers/vfio/mdev/mdev_core.c
@@ -78,34 +78,41 @@ static struct mdev_parent *__find_parent_device(struct device *dev)
 	return NULL;
 }
 
-static void mdev_release_parent(struct kref *kref)
+static bool mdev_try_get_parent(struct mdev_parent *parent)
 {
-	struct mdev_parent *parent = container_of(kref, struct mdev_parent,
-						  ref);
-	struct device *dev = parent->dev;
-
-	kfree(parent);
-	put_device(dev);
+	if (parent)
+		return refcount_inc_not_zero(&parent->refcount);
+	return false;
 }
 
-static struct mdev_parent *mdev_get_parent(struct mdev_parent *parent)
+static void mdev_put_parent(struct mdev_parent *parent)
 {
-	if (parent)
-		kref_get(&parent->ref);
-
-	return parent;
+	if (parent && refcount_dec_and_test(&parent->refcount))
+		complete(&parent->unreg_completion);
 }
 
-static void mdev_put_parent(struct mdev_parent *parent)
+static void mdev_device_remove_common(struct mdev_device *mdev)
 {
-	if (parent)
-		kref_put(&parent->ref, mdev_release_parent);
+	struct mdev_parent *parent;
+	struct mdev_type *type;
+	int ret;
+
+	type = to_mdev_type(mdev->type_kobj);
+	mdev_remove_sysfs_files(&mdev->dev, type);
+	device_del(&mdev->dev);
+	parent = mdev->parent;
+	ret = parent->ops->remove(mdev);
+	if (ret)
+		dev_err(&mdev->dev, "Remove failed: err=%d\n", ret);
+
+	/* Balances with device_initialize() */
+	put_device(&mdev->dev);
 }
 
 static int mdev_device_remove_cb(struct device *dev, void *data)
 {
 	if (dev_is_mdev(dev))
-		mdev_device_remove(dev);
+		mdev_device_remove_common(to_mdev_device(dev));
 
 	return 0;
 }
@@ -147,7 +154,8 @@ int mdev_register_device(struct device *dev, const struct mdev_parent_ops *ops)
 		goto add_dev_err;
 	}
 
-	kref_init(&parent->ref);
+	refcount_set(&parent->refcount, 1);
+	init_completion(&parent->unreg_completion);
 
 	parent->dev = dev;
 	parent->ops = ops;
@@ -206,14 +214,27 @@ void mdev_unregister_device(struct device *dev)
 	dev_info(dev, "MDEV: Unregistering\n");
 
 	list_del(&parent->next);
+	mutex_unlock(&parent_list_lock);
+
+	/* Release the initial reference so that new create cannot start */
+	mdev_put_parent(parent);
+
+	/*
+	 * Wait for all the create and remove references to drop.
+	 */
+	wait_for_completion(&parent->unreg_completion);
+
+	/*
+	 * New references cannot be taken and all users are done
+	 * using the parent. So it is safe to unregister parent.
+	 */
 	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);
+	kfree(parent);
+	put_device(dev);
 }
 EXPORT_SYMBOL(mdev_unregister_device);
 
@@ -237,10 +258,11 @@ int mdev_device_create(struct kobject *kobj,
 	struct mdev_parent *parent;
 	struct mdev_type *type = to_mdev_type(kobj);
 
-	parent = mdev_get_parent(type->parent);
-	if (!parent)
+	if (!mdev_try_get_parent(type->parent))
 		return -EINVAL;
 
+	parent = type->parent;
+
 	mutex_lock(&mdev_list_lock);
 
 	/* Check for duplicate */
@@ -287,6 +309,7 @@ int mdev_device_create(struct kobject *kobj,
 
 	mdev->active = true;
 	dev_dbg(&mdev->dev, "MDEV: created\n");
+	mdev_put_parent(parent);
 
 	return 0;
 
@@ -306,7 +329,6 @@ int mdev_device_remove(struct device *dev)
 	struct mdev_device *mdev, *tmp;
 	struct mdev_parent *parent;
 	struct mdev_type *type;
-	int ret;
 
 	mdev = to_mdev_device(dev);
 
@@ -330,15 +352,17 @@ int mdev_device_remove(struct device *dev)
 	mutex_unlock(&mdev_list_lock);
 
 	type = to_mdev_type(mdev->type_kobj);
-	mdev_remove_sysfs_files(dev, type);
-	device_del(&mdev->dev);
-	parent = mdev->parent;
-	ret = parent->ops->remove(mdev);
-	if (ret)
-		dev_err(&mdev->dev, "Remove failed: err=%d\n", ret);
+	if (!mdev_try_get_parent(type->parent)) {
+		/*
+		 * Parent unregistration have started.
+		 * No need to remove here.
+		 */
+		mutex_unlock(&mdev_list_lock);
+		return -ENODEV;
+	}
 
-	/* Balances with device_initialize() */
-	put_device(&mdev->dev);
+	parent = mdev->parent;
+	mdev_device_remove_common(mdev);
 	mdev_put_parent(parent);
 
 	return 0;
diff --git a/drivers/vfio/mdev/mdev_private.h b/drivers/vfio/mdev/mdev_private.h
index 924ed2274941..55ebab0af7b0 100644
--- a/drivers/vfio/mdev/mdev_private.h
+++ b/drivers/vfio/mdev/mdev_private.h
@@ -19,7 +19,11 @@ void mdev_bus_unregister(void);
 struct mdev_parent {
 	struct device *dev;
 	const struct mdev_parent_ops *ops;
-	struct kref ref;
+	/* Protects unregistration to wait until create/remove
+	 * are completed.
+	 */
+	refcount_t refcount;
+	struct completion unreg_completion;
 	struct list_head next;
 	struct kset *mdev_types_kset;
 	struct list_head type_list;
-- 
2.19.2