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

[Anatol Pomozov]

Hi

On Fri, Apr 12, 2013 at 4:47 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Apr 12, 2013 at 3:32 PM, Anatol Pomozov
> <anatol.pomozov@xxxxxxxxx> 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;

Does it make sense to move it to a separate function in kref.h?

/** Useful when kref_get is racing with kref_put and refcounter might be 0 */
int kref_get_not_zero(kref* ref) {
    return atomic_inc_not_zero(&kref->refcount);
}

or maybe instead change default behavior of kref_get() to
atomic_inc_not_zero and force callers check the return value from
kref_get()?

>
> 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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