linux-kernel - Re: [RFC v1 3/3] cap11xx: fix potential user-after-free on module unload

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Date:   Tue, 5 Feb 2019 22:24:47 +0100
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Sven Van Asbroeck <thesven73@...il.com>
Cc:     Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, Sebastian Reichel <sre@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC v1 3/3] cap11xx: fix potential user-after-free on module
 unload

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?

> 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().

-- 
Best regards,
Jacek Anaszewski