From: Mukesh Ojha <quic_mojha@quicinc.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: <gregkh@linuxfoundation.org>, <linux-kernel@vger.kernel.org>,
<sboyd@kernel.org>, <johannes@sipsolutions.net>,
<rafael@kernel.org>
Subject: Re: Possible race in dev_coredumpm()-del_timer() path
Date: Thu, 14 Apr 2022 16:50:55 +0530 [thread overview]
Message-ID: <20220414112055.GA14124@hu-mojha-hyd.qualcomm.com> (raw)
In-Reply-To: <87pmlkdk6i.ffs@tglx>
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@quicinc.com/
>
> Same problem vs. disabled_store().
you mean, while userspace is reading the data and suddenly disable_store() done from
sysfs.
-Mukesh
>
> Thanks,
>
> tglx
next prev parent reply other threads:[~2022-04-14 11:21 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-13 5:29 Possible race in dev_coredumpm()-del_timer() path Mukesh Ojha
2022-04-13 5:34 ` Greg KH
2022-04-13 6:41 ` Johannes Berg
2022-04-13 10:16 ` Mukesh Ojha
2022-04-13 10:58 ` Greg KH
2022-04-13 11:21 ` Mukesh Ojha
2022-04-13 13:01 ` Greg KH
2022-04-13 14:17 ` Mukesh Ojha
2022-04-13 14:18 ` Mukesh Ojha
2022-04-14 10:38 ` Thomas Gleixner
2022-04-14 11:20 ` Mukesh Ojha [this message]
2022-04-14 11:31 ` Thomas Gleixner
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