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Commit 60403f7a authored by Guenter Roeck's avatar Guenter Roeck Committed by Wim Van Sebroeck
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watchdog: Fix race condition in registration code 



A race condition exists when registering the first watchdog device.
Sequence of events:

- watchdog_register_device calls watchdog_dev_register
- watchdog_dev_register creates the watchdog misc device by calling
  misc_register.
  At that time, the matching character device (/dev/watchdog0) does not yet
  exist, and old_wdd is not set either.
- Userspace gets an event and opens /dev/watchdog
- watchdog_open is called and sets wdd = old_wdd, which is still NULL,
  and tries to dereference it. This causes the kernel to panic.

Seen with systemd trying to open /dev/watchdog immediately after
it was created.

Reported-by: default avatarArkadiusz Miskiewicz <arekm@maven.pl>
Signed-off-by: default avatarGuenter Roeck <linux@roeck-us.net>
Tested-by: default avatarArkadiusz Miskiewicz <arekm@maven.pl>
Signed-off-by: default avatarWim Van Sebroeck <wim@iguana.be>
parent 6330c707

drivers/watchdog/watchdog_dev.c

+2 −1
Original line number Diff line number Diff line
@@ -523,6 +523,7 @@ int watchdog_dev_register(struct watchdog_device *watchdog) 
	int err, devno;



	if (watchdog->id == 0) {

		old_wdd = watchdog;

		watchdog_miscdev.parent = watchdog->parent;

		err = misc_register(&watchdog_miscdev);

		if (err != 0) {
@@ -531,9 +532,9 @@ int watchdog_dev_register(struct watchdog_device *watchdog) 
			if (err == -EBUSY)

				pr_err("%s: a legacy watchdog module is probably present.\n",

					watchdog->info->identity);

			old_wdd = NULL;

			return err;

		}

		old_wdd = watchdog;

	}



	/* Fill in the data structures */