linux-kernel - Re: rtc: fix chardev initialization races

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Date:   Tue, 22 May 2018 14:09:36 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Uwe Kleine-König <uwe@...ine-koenig.org>
cc:     Ales Novak <alnovak@...e.cz>, a.zummo@...ertech.it,
        akpm@...ux-foundation.org, rtc-linux@...glegroups.com,
        linux-kernel@...r.kernel.org
Subject: Re: rtc: fix chardev initialization races

On Mon, 21 May 2018, Uwe Kleine-König wrote:

> > The race looks like that (thanks Jiri):
> > 
> > CPU0:                                CPU1:
> > sys_load_module()
> >  do_init_module()
> >   do_one_initcall()
> >    cmos_do_probe()
> >     rtc_device_register()
> >      __register_chrdev()
> >      cdev->owner = struct module*
> >                                      open("/dev/rtc0")
> >     rtc_device_unregister()
> >   module_put()
> >   free_module()
> >    module_free(mod->module_core)
> >    /* struct module *module is now
> >       freed */
> >                                       chrdev_open()
> >                                        spin_lock(cdev_lock)
> >                                        cdev_get()
> >                                         try_module_get()
> >                                          module_is_live()
> >                                          /* dereferences already
> >                                             freed struct module* */
> 
> [Context: For a patch to rtc-pcf2127.c Alexandre Belloni asked not to
> fail after rtc_device_register successfully finished and pointed to this
> reasoning as explaination.]
> 
> If there is really such a race then (I hope) there is
> something in the cdev code that needs fixing. According to my
> understanding, when rtc_device_unregister returned, the cdev is gone and
> so chrdev_open is supposed to fail.

Oh wow, hello back to 4 years ago!

Looking at the current code, I don't think there is no such race any more, 
as the last thing cmos_do_probe() -> __rtc_register_device() does that can 
potentially fail is the chardev creation itself.

IOW if it exists, it's guaranteed to not go away on a probe error handling 
path (and the rest is protected via ops_lock mutex).

-- 
Jiri Kosina
SUSE Labs