watchdog: Fix the race between the release of watchdog_core_data and cdev

The struct cdev is embedded in the struct watchdog_core_data. In the
current code, we manage the watchdog_core_data with a kref, but the
cdev is manged by a kobject. There is no any relationship between
this kref and kobject. So it is possible that the watchdog_core_data is
freed before the cdev is entirely released. We can easily get the
following call trace with CONFIG_DEBUG_KOBJECT_RELEASE and
CONFIG_DEBUG_OBJECTS_TIMERS enabled.
  ODEBUG: free active (active state 0) object type: timer_list hint: delayed_work_timer_fn+0x0/0x38
  WARNING: CPU: 23 PID: 1028 at lib/debugobjects.c:481 debug_print_object+0xb0/0xf0
  Modules linked in: softdog(-) deflate ctr twofish_generic twofish_common camellia_generic serpent_generic blowfish_generic blowfish_common cast5_generic cast_common cmac xcbc af_key sch_fq_codel openvswitch nsh nf_conncount nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4
  CPU: 23 PID: 1028 Comm: modprobe Not tainted 5.3.0-next-20190924-yoctodev-standard+ #180
  Hardware name: Marvell OcteonTX CN96XX board (DT)
  pstate: 00400009 (nzcv daif +PAN -UAO)
  pc : debug_print_object+0xb0/0xf0
  lr : debug_print_object+0xb0/0xf0
  sp : ffff80001cbcfc70
  x29: ffff80001cbcfc70 x28: ffff800010ea2128
  x27: ffff800010bad000 x26: 0000000000000000
  x25: ffff80001103c640 x24: ffff80001107b268
  x23: ffff800010bad9e8 x22: ffff800010ea2128
  x21: ffff000bc2c62af8 x20: ffff80001103c600
  x19: ffff800010e867d8 x18: 0000000000000060
  x17: 0000000000000000 x16: 0000000000000000
  x15: ffff000bd7240470 x14: 6e6968207473696c
  x13: 5f72656d6974203a x12: 6570797420746365
  x11: 6a626f2029302065 x10: 7461747320657669
  x9 : 7463612820657669 x8 : 3378302f3078302b
  x7 : 0000000000001d7a x6 : ffff800010fd5889
  x5 : 0000000000000000 x4 : 0000000000000000
  x3 : 0000000000000000 x2 : ffff000bff948548
  x1 : 276a1c9e1edc2300 x0 : 0000000000000000
  Call trace:
   debug_print_object+0xb0/0xf0
   debug_check_no_obj_freed+0x1e8/0x210
   kfree+0x1b8/0x368
   watchdog_cdev_unregister+0x88/0xc8
   watchdog_dev_unregister+0x38/0x48
   watchdog_unregister_device+0xa8/0x100
   softdog_exit+0x18/0xfec4 [softdog]
   __arm64_sys_delete_module+0x174/0x200
   el0_svc_handler+0xd0/0x1c8
   el0_svc+0x8/0xc

This is a common issue when using cdev embedded in a struct.
Fortunately, we already have a mechanism to solve this kind of issue.
Please see commit 233ed09d7f ("chardev: add helper function to
register char devs with a struct device") for more detail.

In this patch, we choose to embed the struct device into the
watchdog_core_data, and use the API provided by the commit 233ed09d7f
to make sure that the release of watchdog_core_data and cdev are
in sequence.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20191008112934.29669-1-haokexin@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

drivers/watchdog/watchdog_dev.c

@@ -34,7 +34,6 @@
 #include <linux/init.h>		/* For __init/__exit/... */
 #include <linux/hrtimer.h>	/* For hrtimers */
 #include <linux/kernel.h>	/* For printk/panic/... */
-#include <linux/kref.h>		/* For data references */
 #include <linux/kthread.h>	/* For kthread_work */
 #include <linux/miscdevice.h>	/* For handling misc devices */
 #include <linux/module.h>	/* For module stuff/... */
@@ -52,14 +51,14 @@
 
 /*
  * struct watchdog_core_data - watchdog core internal data
- * @kref:	Reference count.
+ * @dev:	The watchdog's internal device
  * @cdev:	The watchdog's Character device.
  * @wdd:	Pointer to watchdog device.
  * @lock:	Lock for watchdog core.
  * @status:	Watchdog core internal status bits.
  */
 struct watchdog_core_data {
-	struct kref kref;
+	struct device dev;
 	struct cdev cdev;
 	struct watchdog_device *wdd;
 	struct mutex lock;
@@ -839,7 +838,7 @@ static int watchdog_open(struct inode *inode, struct file *file)
 	file->private_data = wd_data;
 
 	if (!hw_running)
-		kref_get(&wd_data->kref);
+		get_device(&wd_data->dev);
 
 	/*
 	 * open_timeout only applies for the first open from
@@ -860,11 +859,11 @@ static int watchdog_open(struct inode *inode, struct file *file)
 	return err;
 }
 
-static void watchdog_core_data_release(struct kref *kref)
+static void watchdog_core_data_release(struct device *dev)
 {
 	struct watchdog_core_data *wd_data;
 
-	wd_data = container_of(kref, struct watchdog_core_data, kref);
+	wd_data = container_of(dev, struct watchdog_core_data, dev);
 
 	kfree(wd_data);
 }
@@ -924,7 +923,7 @@ static int watchdog_release(struct inode *inode, struct file *file)
 	 */
 	if (!running) {
 		module_put(wd_data->cdev.owner);
-		kref_put(&wd_data->kref, watchdog_core_data_release);
+		put_device(&wd_data->dev);
 	}
 	return 0;
 }
@@ -943,17 +942,22 @@ static struct miscdevice watchdog_miscdev = {
 	.fops		= &watchdog_fops,
 };
 
+static struct class watchdog_class = {
+	.name =		"watchdog",
+	.owner =	THIS_MODULE,
+	.dev_groups =	wdt_groups,
+};
+
 /*
  *	watchdog_cdev_register: register watchdog character device
  *	@wdd: watchdog device
- *	@devno: character device number
  *
  *	Register a watchdog character device including handling the legacy
  *	/dev/watchdog node. /dev/watchdog is actually a miscdevice and
  *	thus we set it up like that.
  */
 
-static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
+static int watchdog_cdev_register(struct watchdog_device *wdd)
 {
 	struct watchdog_core_data *wd_data;
 	int err;
@@ -961,7 +965,6 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
 	wd_data = kzalloc(sizeof(struct watchdog_core_data), GFP_KERNEL);
 	if (!wd_data)
 		return -ENOMEM;
-	kref_init(&wd_data->kref);
 	mutex_init(&wd_data->lock);
 
 	wd_data->wdd = wdd;
@@ -990,23 +993,33 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
 		}
 	}
 
+	device_initialize(&wd_data->dev);
+	wd_data->dev.devt = MKDEV(MAJOR(watchdog_devt), wdd->id);
+	wd_data->dev.class = &watchdog_class;
+	wd_data->dev.parent = wdd->parent;
+	wd_data->dev.groups = wdd->groups;
+	wd_data->dev.release = watchdog_core_data_release;
+	dev_set_drvdata(&wd_data->dev, wdd);
+	dev_set_name(&wd_data->dev, "watchdog%d", wdd->id);
+
 	/* Fill in the data structures */
 	cdev_init(&wd_data->cdev, &watchdog_fops);
-	wd_data->cdev.owner = wdd->ops->owner;
 
 	/* Add the device */
-	err = cdev_add(&wd_data->cdev, devno, 1);
+	err = cdev_device_add(&wd_data->cdev, &wd_data->dev);
 	if (err) {
 		pr_err("watchdog%d unable to add device %d:%d\n",
 			wdd->id,  MAJOR(watchdog_devt), wdd->id);
 		if (wdd->id == 0) {
 			misc_deregister(&watchdog_miscdev);
 			old_wd_data = NULL;
-			kref_put(&wd_data->kref, watchdog_core_data_release);
+			put_device(&wd_data->dev);
 		}
 		return err;
 	}
 
+	wd_data->cdev.owner = wdd->ops->owner;
+
 	/* Record time of most recent heartbeat as 'just before now'. */
 	wd_data->last_hw_keepalive = ktime_sub(ktime_get(), 1);
 	watchdog_set_open_deadline(wd_data);
@@ -1017,7 +1030,7 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
 	 */
 	if (watchdog_hw_running(wdd)) {
 		__module_get(wdd->ops->owner);
-		kref_get(&wd_data->kref);
+		get_device(&wd_data->dev);
 		if (handle_boot_enabled)
 			hrtimer_start(&wd_data->timer, 0, HRTIMER_MODE_REL);
 		else
@@ -1040,7 +1053,7 @@ static void watchdog_cdev_unregister(struct watchdog_device *wdd)
 {
 	struct watchdog_core_data *wd_data = wdd->wd_data;
 
-	cdev_del(&wd_data->cdev);
+	cdev_device_del(&wd_data->cdev, &wd_data->dev);
 	if (wdd->id == 0) {
 		misc_deregister(&watchdog_miscdev);
 		old_wd_data = NULL;
@@ -1059,15 +1072,9 @@ static void watchdog_cdev_unregister(struct watchdog_device *wdd)
 	hrtimer_cancel(&wd_data->timer);
 	kthread_cancel_work_sync(&wd_data->work);
 
-	kref_put(&wd_data->kref, watchdog_core_data_release);
+	put_device(&wd_data->dev);
 }
 
-static struct class watchdog_class = {
-	.name =		"watchdog",
-	.owner =	THIS_MODULE,
-	.dev_groups =	wdt_groups,
-};
-
 static int watchdog_reboot_notifier(struct notifier_block *nb,
 				    unsigned long code, void *data)
 {
@@ -1098,27 +1105,14 @@ static int watchdog_reboot_notifier(struct notifier_block *nb,
 
 int watchdog_dev_register(struct watchdog_device *wdd)
 {
-	struct device *dev;
-	dev_t devno;
 	int ret;
 
-	devno = MKDEV(MAJOR(watchdog_devt), wdd->id);
+	ret = watchdog_cdev_register(wdd);
-
-	ret = watchdog_cdev_register(wdd, devno);
 	if (ret)
 		return ret;
 
-	dev = device_create_with_groups(&watchdog_class, wdd->parent,
-					devno, wdd, wdd->groups,
-					"watchdog%d", wdd->id);
-	if (IS_ERR(dev)) {
-		watchdog_cdev_unregister(wdd);
-		return PTR_ERR(dev);
-	}
-
 	ret = watchdog_register_pretimeout(wdd);
 	if (ret) {
-		device_destroy(&watchdog_class, devno);
 		watchdog_cdev_unregister(wdd);
 		return ret;
 	}
@@ -1126,7 +1120,8 @@ int watchdog_dev_register(struct watchdog_device *wdd)
 	if (test_bit(WDOG_STOP_ON_REBOOT, &wdd->status)) {
 		wdd->reboot_nb.notifier_call = watchdog_reboot_notifier;
 
-		ret = devm_register_reboot_notifier(dev, &wdd->reboot_nb);
+		ret = devm_register_reboot_notifier(&wdd->wd_data->dev,
+						    &wdd->reboot_nb);
 		if (ret) {
 			pr_err("watchdog%d: Cannot register reboot notifier (%d)\n",
 			       wdd->id, ret);
@@ -1148,7 +1143,6 @@ int watchdog_dev_register(struct watchdog_device *wdd)
 void watchdog_dev_unregister(struct watchdog_device *wdd)
 {
 	watchdog_unregister_pretimeout(wdd);
-	device_destroy(&watchdog_class, wdd->wd_data->cdev.dev);
 	watchdog_cdev_unregister(wdd);
 }