IB/uverbs: Fix race between ib_uverbs_open and remove_one

Fixes: 2a72f21 ("IB/uverbs: Remove dev_table")

Before this commit there was a device look-up table that was protected
by a spin_lock used by ib_uverbs_open and by ib_uverbs_remove_one. When
it was dropped and container_of was used instead, it enabled the race
with remove_one as dev might be freed just after:
dev = container_of(inode->i_cdev, struct ib_uverbs_device, cdev) but
before the kref_get.

In addition, this buggy patch added some dead code as
container_of(x,y,z) can never be NULL and so dev can never be NULL.
As a result the comment above ib_uverbs_open saying "the open method
will either immediately run -ENXIO" is wrong as it can never happen.

The solution follows Jason Gunthorpe suggestion from below URL:
https://www.mail-archive.com/linux-rdma@vger.kernel.org/msg25692.html

cdev will hold a kref on the parent (the containing structure,
ib_uverbs_device) and only when that kref is released it is
guaranteed that open will never be called again.

In addition, fixes the active count scheme to use an atomic
not a kref to prevent WARN_ON as pointed by above comment
from Jason.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Shachar Raindel <raindel@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/core/uverbs.h

@@ -85,7 +85,7 @@
  */
 
 struct ib_uverbs_device {
-	struct kref				ref;
+	atomic_t				refcount;
 	int					num_comp_vectors;
 	struct completion			comp;
 	struct device			       *dev;
@@ -94,6 +94,7 @@ struct ib_uverbs_device {
 	struct cdev			        cdev;
 	struct rb_root				xrcd_tree;
 	struct mutex				xrcd_tree_mutex;
+	struct kobject				kobj;
 };
 
 struct ib_uverbs_event_file {

drivers/infiniband/core/uverbs_main.c

@@ -130,14 +130,18 @@ static int (*uverbs_ex_cmd_table[])(struct ib_uverbs_file *file,
 static void ib_uverbs_add_one(struct ib_device *device);
 static void ib_uverbs_remove_one(struct ib_device *device, void *client_data);
 
-static void ib_uverbs_release_dev(struct kref *ref)
+static void ib_uverbs_release_dev(struct kobject *kobj)
 {
 	struct ib_uverbs_device *dev =
-		container_of(ref, struct ib_uverbs_device, ref);
+		container_of(kobj, struct ib_uverbs_device, kobj);
 
-	complete(&dev->comp);
+	kfree(dev);
 }
 
+static struct kobj_type ib_uverbs_dev_ktype = {
+	.release = ib_uverbs_release_dev,
+};
+
 static void ib_uverbs_release_event_file(struct kref *ref)
 {
 	struct ib_uverbs_event_file *file =
@@ -303,13 +307,19 @@ static int ib_uverbs_cleanup_ucontext(struct ib_uverbs_file *file,
 	return context->device->dealloc_ucontext(context);
 }
 
+static void ib_uverbs_comp_dev(struct ib_uverbs_device *dev)
+{
+	complete(&dev->comp);
+}
+
 static void ib_uverbs_release_file(struct kref *ref)
 {
 	struct ib_uverbs_file *file =
 		container_of(ref, struct ib_uverbs_file, ref);
 
 	module_put(file->device->ib_dev->owner);
-	kref_put(&file->device->ref, ib_uverbs_release_dev);
+	if (atomic_dec_and_test(&file->device->refcount))
+		ib_uverbs_comp_dev(file->device);
 
 	kfree(file);
 }
@@ -775,9 +785,7 @@ static int ib_uverbs_open(struct inode *inode, struct file *filp)
 	int ret;
 
 	dev = container_of(inode->i_cdev, struct ib_uverbs_device, cdev);
-	if (dev)
-		kref_get(&dev->ref);
-	else
+	if (!atomic_inc_not_zero(&dev->refcount))
 		return -ENXIO;
 
 	if (!try_module_get(dev->ib_dev->owner)) {
@@ -798,27 +806,32 @@ static int ib_uverbs_open(struct inode *inode, struct file *filp)
 	mutex_init(&file->mutex);
 
 	filp->private_data = file;
+	kobject_get(&dev->kobj);
 
 	return nonseekable_open(inode, filp);
 
 err_module:
 	module_put(dev->ib_dev->owner);
 
 err:
-	kref_put(&dev->ref, ib_uverbs_release_dev);
+	if (atomic_dec_and_test(&dev->refcount))
+		ib_uverbs_comp_dev(dev);
+
 	return ret;
 }
 
 static int ib_uverbs_close(struct inode *inode, struct file *filp)
 {
 	struct ib_uverbs_file *file = filp->private_data;
+	struct ib_uverbs_device *dev = file->device;
 
 	ib_uverbs_cleanup_ucontext(file, file->ucontext);
 
 	if (file->async_file)
 		kref_put(&file->async_file->ref, ib_uverbs_release_event_file);
 
 	kref_put(&file->ref, ib_uverbs_release_file);
+	kobject_put(&dev->kobj);
 
 	return 0;
 }
@@ -914,10 +927,11 @@ static void ib_uverbs_add_one(struct ib_device *device)
 	if (!uverbs_dev)
 		return;
 
-	kref_init(&uverbs_dev->ref);
+	atomic_set(&uverbs_dev->refcount, 1);
 	init_completion(&uverbs_dev->comp);
 	uverbs_dev->xrcd_tree = RB_ROOT;
 	mutex_init(&uverbs_dev->xrcd_tree_mutex);
+	kobject_init(&uverbs_dev->kobj, &ib_uverbs_dev_ktype);
 
 	spin_lock(&map_lock);
 	devnum = find_first_zero_bit(dev_map, IB_UVERBS_MAX_DEVICES);
@@ -944,6 +958,7 @@ static void ib_uverbs_add_one(struct ib_device *device)
 	cdev_init(&uverbs_dev->cdev, NULL);
 	uverbs_dev->cdev.owner = THIS_MODULE;
 	uverbs_dev->cdev.ops = device->mmap ? &uverbs_mmap_fops : &uverbs_fops;
+	uverbs_dev->cdev.kobj.parent = &uverbs_dev->kobj;
 	kobject_set_name(&uverbs_dev->cdev.kobj, "uverbs%d", uverbs_dev->devnum);
 	if (cdev_add(&uverbs_dev->cdev, base, 1))
 		goto err_cdev;
@@ -974,9 +989,10 @@ static void ib_uverbs_add_one(struct ib_device *device)
 		clear_bit(devnum, overflow_map);
 
 err:
-	kref_put(&uverbs_dev->ref, ib_uverbs_release_dev);
+	if (atomic_dec_and_test(&uverbs_dev->refcount))
+		ib_uverbs_comp_dev(uverbs_dev);
 	wait_for_completion(&uverbs_dev->comp);
-	kfree(uverbs_dev);
+	kobject_put(&uverbs_dev->kobj);
 	return;
 }
 
@@ -996,9 +1012,10 @@ static void ib_uverbs_remove_one(struct ib_device *device, void *client_data)
 	else
 		clear_bit(uverbs_dev->devnum - IB_UVERBS_MAX_DEVICES, overflow_map);
 
-	kref_put(&uverbs_dev->ref, ib_uverbs_release_dev);
+	if (atomic_dec_and_test(&uverbs_dev->refcount))
+		ib_uverbs_comp_dev(uverbs_dev);
 	wait_for_completion(&uverbs_dev->comp);
-	kfree(uverbs_dev);
+	kobject_put(&uverbs_dev->kobj);
 }
 
 static char *uverbs_devnode(struct device *dev, umode_t *mode)