Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 5 Feb 2019 13:43:51 -0800
From:   Dmitry Torokhov <dmitry.torokhov@gmail.com>
To:     Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc:     Sven Van Asbroeck <thesven73@gmail.com>, Tejun Heo <tj@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-kernel@vger.kernel.org, Sebastian Reichel <sre@kernel.org>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [RFC v1 3/3] cap11xx: fix potential user-after-free on module
 unload
Message-ID: <20190205214351.GB19151@dtor-ws>
References: <20190204220952.30761-1-TheSven73@googlemail.com>
 <20190204220952.30761-4-TheSven73@googlemail.com>
 <20190205081846.GA118684@dtor-ws>
 <6565ec19-b629-c289-2fed-a5f404763b74@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <6565ec19-b629-c289-2fed-a5f404763b74@gmail.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Feb 05, 2019 at 10:24:47PM +0100, Jacek Anaszewski wrote:
> Hi Dmitry,
> 
> On 2/5/19 9:18 AM, Dmitry Torokhov wrote:
> > Hi Sven,
> > 
> > On Mon, Feb 04, 2019 at 05:09:52PM -0500, Sven Van Asbroeck wrote:
> > > The work which is scheduled by led_classdev->brightness_set() is
> > > potentially left pending or running until after the driver module
> > > is unloaded.
> > > 
> > > Fix by using resource-controlled version of INIT_WORK().
> > 
> > I believe this is wrong way of fixing this. The LED classdev objects are
> > refcounted, and may live beyond the point where we unwibd devm stack,
> > so we are still left with the same use-after-free that we currently
> > have.
> 
> Could you please share what LED classdev objects refcounting
> do you have on mind?

My mind was in a state of confusion when I wrote the above ;)

> 
> > This is a general issue with LED subsystem as it provides no callback
> > for properly tearing down device structures, but I think in this
> > particular case we can simply switch from set_brightness() to
> > set_brightness_blocking() which will use the work item internal to the
> > LED classdev and that one is being shut down properly.
> > 
> > Jacek, does the above sound right?
> 
> Yes, since the introduction of brightness_set_blocking op there is no
> need for out-of-led-core workqueues for deferring brightness setting.
> And we do flush_work() in led_classdev_unregister().

OK, great, I'll write up a patch for cap11xx and others if I find them.

Thanks.

-- 
Dmitry