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watchdog: Fix race condition in registration code

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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@maven.pl>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Arkadiusz Miskiewicz <arekm@maven.pl>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
This commit is contained in:
Guenter Roeck 2013-04-05 21:22:43 -07:00 committed by Wim Van Sebroeck
parent 6330c7070b
commit 60403f7a4d
1 changed files with 2 additions and 1 deletions
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3
drivers/watchdog/watchdog_dev.c
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@ -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 */