[PATCH 54/78] watchdog: Fix race condition in registration code

[Kamal Mostafa]

3.8.13.1 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Guenter Roeck <linux at roeck-us.net>

commit 60403f7a4d9368d187f79cba5e4672d01df37574 upstream.

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: Arkadiusz Miskiewicz <arekm at maven.pl>
Signed-off-by: Guenter Roeck <linux at roeck-us.net>
Tested-by: Arkadiusz Miskiewicz <arekm at maven.pl>
Signed-off-by: Wim Van Sebroeck <wim at iguana.be>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/watchdog/watchdog_dev.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index ef8edec..05a5310 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -524,6 +524,7 @@ 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) {
@@ -532,9 +533,9 @@ 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 */
-- 
1.8.1.2