Messages in this thread | | | Date | Fri, 12 Apr 2013 17:04:59 -0700 | Subject | Re: [PATCH] module: Fix race condition between load and unload module | From | Anatol Pomozov <> |
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Hi
On Fri, Apr 12, 2013 at 4:53 PM, Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote: > On Fri, Apr 12, 2013 at 04:47:50PM -0700, Linus Torvalds wrote: >> On Fri, Apr 12, 2013 at 3:32 PM, Anatol Pomozov >> <anatol.pomozov@gmail.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 ;) > > We "should" have some type of "higher-up" lock to prevent the > release/get races from happening, we have that in the driver core, and I > thought we had such a lock already in the module subsystem as well, > which will prevent any of this from being needed. > > Rusty, don't we have a lock for this somewhere? > > Linus, I think your patch will reduce the window the race could happen, > but it should still be there, although testing with it would be > interesting to see if the original problem can be triggered with it.
Linus patch should fix the module race condition. vfree(module) cannot be called while we keep kobj->kset->lock. vfree() is called in THREAD_B only after it acquires lock, removes kobj from list. So if kobj is found by THREAD_A in kset->list and we did not release lock then memory is not freed.
> > I'll look at it some more tomorrow, about to go to dinner now... > > thanks, > > greg k-h
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