Commit 60403f7a authored by Guenter Roeck's avatar Guenter Roeck Committed by Wim Van Sebroeck

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
...@@ -523,6 +523,7 @@ int watchdog_dev_register(struct watchdog_device *watchdog) ...@@ -523,6 +523,7 @@ int watchdog_dev_register(struct watchdog_device *watchdog)
int err, devno; int err, devno;
if (watchdog->id == 0) { if (watchdog->id == 0) {
old_wdd = watchdog;
watchdog_miscdev.parent = watchdog->parent; watchdog_miscdev.parent = watchdog->parent;
err = misc_register(&watchdog_miscdev); err = misc_register(&watchdog_miscdev);
if (err != 0) { if (err != 0) {
...@@ -531,9 +532,9 @@ int watchdog_dev_register(struct watchdog_device *watchdog) ...@@ -531,9 +532,9 @@ int watchdog_dev_register(struct watchdog_device *watchdog)
if (err == -EBUSY) if (err == -EBUSY)
pr_err("%s: a legacy watchdog module is probably present.\n", pr_err("%s: a legacy watchdog module is probably present.\n",
watchdog->info->identity); watchdog->info->identity);
old_wdd = NULL;
return err; return err;
} }
old_wdd = watchdog;
} }
/* Fill in the data structures */ /* Fill in the data structures */
......
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