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SubjectRe: rtc: fix chardev initialization races
On Tue, 22 May 2018, Uwe Kleine-König wrote:
> [adding linux-rtc ML and Alexandre to Cc:]
>
> Hello,
>
> On Tue, May 22, 2018 at 02:09:36PM +0200, Jiri Kosina wrote:
> > 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.
>
> OK, so you agree that it's also save to do something in a driver's probe
> after rtc_device_register() and call rtc_device_unregister() in the error
> path, right?

Hmm, not really; that's what the code apparently did 4 years ago (judging
from the scenario in the old mail, I of course forgot all the details),
but doesn't do it any more.

Looking at the current code, if you call rtc_device_unregister() in the
probe path, where is the guarantee that cdev_get() will not derefernce
already freed struct module*?

Thanks,

--
Jiri Kosina
SUSE Labs

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