linux-kernel - Re: [PATCH v2] Input: uinput: Avoid Object-Already-Free with a global lock

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Date:   Thu, 18 Apr 2019 01:43:21 +0000
From:   "dmitry.torokhov@...il.com" <dmitry.torokhov@...il.com>
To:     Mukesh Ojha <mojha@...eaurora.org>
Cc:     linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        Gaurav Kohli <gkohli@...eaurora.org>,
        Peter Hutterer <peter.hutterer@...-t.net>,
        Martin Kepplinger <martink@...teo.de>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [PATCH v2] Input: uinput: Avoid Object-Already-Free with a
 global lock

Hi Mukesh,

On Mon, Apr 15, 2019 at 03:35:51PM +0530, Mukesh Ojha wrote:
> 
> Hi Dmitry,
> 
> Can you please have a look at this patch ? as this seems to reproducing
> quite frequently
> 
> Thanks,
> Mukesh
> 
> On 4/10/2019 1:29 PM, Mukesh Ojha wrote:
> > uinput_destroy_device() gets called from two places. In one place,
> > uinput_ioctl_handler() where it is protected under a lock
> > udev->mutex but there is no protection on udev device from freeing
> > inside uinput_release().

uinput_release() should be called when last file handle to the uinput
instance is being dropped, so there should be no other users and thus we
can't be racing with anyone.

> > 
> > This can result in Object-Already-Free case where uinput parent
> > device already got freed while a child being inserted inside it.
> > That result in a double free case for parent while kernfs_put()
> > being done for child in a failure path of adding a node.

Can you please describe scenario in more detail? How do you free the
parent device while child input device is being registered?

Thanks.

- 
Dmitry