linux-kernel - Re: [PATCH] module: Fix race condition between load and unload module

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Date:	Sat, 13 Apr 2013 21:42:06 -0700
From:	Anatol Pomozov <anatol.pomozov@...il.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Salman Qazi <sqazi@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] module: Fix race condition between load and unload module

Hi

On Sat, Apr 13, 2013 at 8:35 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Fri, Apr 12, 2013 at 04:47:50PM -0700, Linus Torvalds wrote:
>> This is a much more generic bug in kobjects, and I would hate to add
>> some random workaround for just one case of this bug like you do. The
>> more fundamental bug needs to be fixed too.
>>
>> I think the more fundamental bugfix is to just fix kobject_get() to
>> return NULL if the refcount was zero, because in that case the kobject
>> no longer really exists.
>>
>> So instead of having
>>
>>     kref_get(&kobj->kref);
>>
>> it should do
>>
>>     if (!atomic_inc_not_zero(&kobj->kref.refcount))
>>         kobj = NULL;
>>
>> and I think that should fix your race automatically, no? Proper patch
>> attached (but TOTALLY UNTESTED - it seems to compile, though).
>>
>> The problem is that we lose the warning for when the refcount is zero
>> and somebody does a kobject_get(), but that is ok *assuming* that
>> people actually check the return value of kobject_get() rather than
>> just "know" that if they passed in a non-NULL kobj, they'll get it
>> right back.
>>
>> Greg - please take a look... I'm adding Al to the discussion too,
>> because Al just *loooves* these kinds of races ;)
>
> Unless I'm misreading what's going on, we have the following to thank for that:
>         /* remove from sysfs if the caller did not do it */
>         if (kobj->state_in_sysfs) {
>                 pr_debug("kobject: '%s' (%p): auto cleanup kobject_del\n",
>                          kobject_name(kobj), kobj);
>                 kobject_del(kobj);
>         }
> in kobject_cleanup().  Why don't we require kobject_del() before the final
> kobject_put(), if the sucker had been added?  FWIW, I thought it *was*
> required all along...

But kobject_release/kobject_cleanup function is called as a result of
atomic decrement_compare. Until we perform the atomic operation we
don't know whether it is final kobject_put() or not.

kobject_put() {
    if (atomic_sub_and_test(kobj->kref->refcount)) {
        // refcounter is decremented to 0 so cleanup sysfs
        kobject_release(kobj)
    }
}
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