Re: [PATCH] watchdog: Fix race condition in registration code

[Arkadiusz Miskiewicz <a.miskiewicz@gmail.com>]

On Saturday 06 of April 2013, Guenter Roeck wrote:
> 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 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 <a.miskiewicz@gmail.com>

Please use
Reported-by: Arkadiusz Miśkiewicz <arekm@maven.pl>

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> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> ---
> Arkadiusz,
> 
> would be great if you can test this in your system.

Did few reboots without oops but this test isn't reliable. Previously I wasn't 
able to reproduce this on demand. It just happens sometime. If any problem 
popup I'll let you know.

So for now
Tested-by: Arkadiusz Miśkiewicz <arekm@maven.pl>

> 
>  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 08b48bb..faf4e18 100644
> --- a/drivers/watchdog/watchdog_dev.c
> +++ b/drivers/watchdog/watchdog_dev.c
> @@ -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 */


-- 
Arkadiusz Miśkiewicz, arekm / maven.pl