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

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Message-ID: <CA+55aFzxxGBU3AOeNg3UEicHrx3nzWRyo5DOd=hh6SHJfYvB3g@mail.gmail.com>
Date:	Fri, 12 Apr 2013 16:47:50 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Anatol Pomozov <anatol.pomozov@...il.com>
Cc:	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>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] module: Fix race condition between load and unload module

On Fri, Apr 12, 2013 at 3:32 PM, Anatol Pomozov
<anatol.pomozov@...il.com> wrote:
>
> Here is timeline for the crash in case if kset_find_obj() searches for
> an object tht nobody holds and other thread is doing kobject_put()
> on the same kobject:
>
> THREAD A (calls kset_find_obj())     THREAD B (calls kobject_put())
> splin_lock()
>                                      atomic_dec_return(kobj->kref), counter gets zero here
>                                      ... starts kobject cleanup ....
>                                      spin_lock() // WAIT thread A in kobj_kset_leave()
> iterate over kset->list
> atomic_inc(kobj->kref) (counter becomes 1)
> spin_unlock()
>                                      spin_lock() // taken
>                                      // it does not know that thread A increased counter so it
>                                      remove obj from list
>                                      spin_unlock()
>                                      vfree(module) // frees module object with containing kobj
>
> // kobj points to freed memory area!!
> koubject_put(kobj) // OOPS!!!!

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 ;)

                  Linus

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