Re: Possible race in dev_coredumpm()-del_timer() path

From: Mukesh Ojha
Date: Thu Apr 14 2022 - 07:21:14 EST


On Thu, Apr 14, 2022 at 12:38:13PM +0200, Thomas Gleixner wrote:
> On Wed, Apr 13 2022 at 12:58, Greg KH wrote:
> > On Wed, Apr 13, 2022 at 03:46:39PM +0530, Mukesh Ojha wrote:
> >> p1 p2(X)
> >>
> >> dev_coredump() uevent sent to userspace
> >> device_add() =========================> userspace process X reads the uevents
> >> writes to devcd fd which
> >> results into writes to
> >>
> >> devcd_data_write()
> >> mod_delayed_work()
> >> try_to_grab_pending()
> >> del_timer()
> >> debug_assert_init()
> >> INIT_DELAYED_WORK
> >> schedule_delayed_work
> >> debug_object_fixup()
> >
> > Why do you have object debugging enabled? That's going to take a LONG
> > time, and will find bugs in your code. Perhaps like this one?
>
> It's not finding bugs in his code. It finds bug in the upstream
> dev_coredump code.
>
> > And if you turn object debugging off, what happens?
>
> The debugobject splat goes away, but the problem persists.
>
> device_add() -> uevent
>
> Preemption or concurrency:
>
> devcd_data_write()
> mod_delayed_work(..., w, 0); <- Uninitialized.
>
> The dev_coredump code exposes the device before it is fully initialized
> and a write ending up in devcd_data_write() touches uninitialized work.
>
> It does not help to move the initialization before device_add() as that
> creates another problem:
>
> INIT_DELAYED_WORK(w)
> ...
> device_add() -> uevent
>
> Preemption or concurrency:
>
> devcd_data_write()
> mod_delayed_work(..., w, 0); <- Schedules work immediately
>
> work_queue_runs()
> devcd_del(w)
> device_del()
> put_device() <- Drops the last reference
>
> initialization continues...
>
> So, yes this needs serialization of some sort.

Hi Thomas,

Thanks for understanding the problem.
Can the patch mentioned at below link helps with the first problem ?


https://lore.kernel.org/lkml/57a04278-0a60-cc7d-7ce8-a75c2befd568@xxxxxxxxxxx/

>
> Same problem vs. disabled_store().

you mean, while userspace is reading the data and suddenly disable_store() done from
sysfs.

-Mukesh
>
> Thanks,
>
> tglx