[PATCHv1 7/7] vfio/mdev: Fix race conditions with mdev device life cycle APIs

From: Parav Pandit <parav@mellanox.com>
To: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	kwankhede@nvidia.com, alex.williamson@redhat.com
Cc: cjia@nvidia.com, parav@mellanox.com
Subject: [PATCHv1 7/7] vfio/mdev: Fix race conditions with mdev device life cycle APIs
Date: Tue, 26 Mar 2019 22:45:45 -0500	[thread overview]
Message-ID: <1553658345-43995-8-git-send-email-parav@mellanox.com> (raw)
In-Reply-To: <1553658345-43995-1-git-send-email-parav@mellanox.com>

Below race condition and call trace exist with current device life cycle
sequence.

1. 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()

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()
                                    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
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 to overcome above issues.

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.

mdev_device_remove() is refactored to not block on srcu when device is
removed as part of parent removal.

Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
Signed-off-by: Parav Pandit <parav@mellanox.com>
---
 drivers/vfio/mdev/mdev_core.c    | 83 ++++++++++++++++++++++++++++++++++------
 drivers/vfio/mdev/mdev_private.h |  6 +++
 2 files changed, 77 insertions(+), 12 deletions(-)

diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
index aefcf34..fa233c8 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);
 }
@@ -147,10 +148,30 @@ static int mdev_device_remove_ops(struct mdev_device *mdev, bool force_remove)
 	return 0;
 }
 
+static int mdev_device_remove_common(struct mdev_device *mdev,
+				     bool force_remove)
+{
+	struct mdev_type *type;
+	int ret;
+
+	type = to_mdev_type(mdev->type_kobj);
+
+	ret = mdev_device_remove_ops(mdev, force_remove);
+	if (ret && !force_remove) {
+		mutex_lock(&mdev_list_lock);
+		mdev->active = true;
+		mutex_unlock(&mdev_list_lock);
+		return ret;
+	}
+	mdev_remove_sysfs_files(&mdev->dev, type);
+	device_unregister(&mdev->dev);
+	return ret;
+}
+
 static int mdev_device_remove_cb(struct device *dev, void *data)
 {
 	if (dev_is_mdev(dev))
-		mdev_device_remove(dev, true);
+		mdev_device_remove_common(to_mdev_device(dev), true);
 
 	return 0;
 }
@@ -193,6 +214,7 @@ 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;
@@ -213,6 +235,7 @@ 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);
 
@@ -251,13 +274,31 @@ void mdev_unregister_device(struct device *dev)
 	dev_info(dev, "MDEV: Unregistering\n");
 
 	list_del(&parent->next);
+	mutex_unlock(&parent_list_lock);
+
+	/*
+	 * 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 +319,24 @@ int mdev_device_create(struct kobject *kobj,
 		       struct device *dev, const guid_t *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 */
@@ -334,44 +385,52 @@ int mdev_device_create(struct kobject *kobj,
 	mdev->type_kobj = kobj;
 	mdev->active = true;
 	dev_dbg(&mdev->dev, "MDEV: created\n");
+	srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
 
 	return 0;
 
 create_fail:
 	device_unregister(&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)
 {
+	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);
+		srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
 		return -EAGAIN;
 	}
 
 	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;
+	ret = mdev_device_remove_common(mdev, force_remove);
+	srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
+	if (ret)
 		return ret;
-	}
 
-	mdev_remove_sysfs_files(dev, type);
-	device_unregister(dev);
 	mdev_put_parent(parent);
 
 	return 0;
diff --git a/drivers/vfio/mdev/mdev_private.h b/drivers/vfio/mdev/mdev_private.h
index ddcf9c7..b799978 100644
--- a/drivers/vfio/mdev/mdev_private.h
+++ b/drivers/vfio/mdev/mdev_private.h
@@ -23,6 +23,12 @@ 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 {
-- 
1.8.3.1

