linux-kernel - Re: [PATCH RFC v3] media: em28xx: Fix race condition between open and init function

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Date:   Thu, 15 Apr 2021 11:25:24 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Igor Matheus Andrade Torrente <igormtorrente@...il.com>,
        mchehab@...nel.org
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        kernel test robot <lkp@...el.com>,
        syzbot+b2391895514ed9ef4a8e@...kaller.appspotmail.com,
        Shuah Khan <skhan@...uxfoundation.org>,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH RFC v3] media: em28xx: Fix race condition between open and
 init function

On 4/15/21 8:07 AM, Igor Matheus Andrade Torrente wrote:
> Fixes a race condition - for lack of a more precise term - between
> em28xx_v4l2_open and em28xx_v4l2_init, by detaching the v4l2_dev,
> media_pad and vdev structs from the em28xx_v4l2, and managing the
> lifetime of those objects more dynamicaly.
> 
> The race happens when a thread[1] - containing the em28xx_v4l2_init()
> code - calls the v4l2_mc_create_media_graph(), and it return a error,
> if a thread[2] - running v4l2_open() - pass the verification point
> and reaches the em28xx_v4l2_open() before the thread[1] finishes
> the deregistration of v4l2 subsystem, the thread[1] will free all
> resources before the em28xx_v4l2_open() can process their things,
> because the em28xx_v4l2_init() has the dev->lock. And all this lead
> the thread[2] to cause a user-after-free.
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Reported-and-tested-by: syzbot+b2391895514ed9ef4a8e@...kaller.appspotmail.com
> Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@...il.com>
> ---
> 
> V2: Add v4l2_i2c_new_subdev null check
>      Deal with v4l2 subdevs dependencies
> 
> V3: Fix link error when compiled as a module
> 
> ---
>   drivers/media/usb/em28xx/em28xx-camera.c |   4 +-
>   drivers/media/usb/em28xx/em28xx-video.c  | 300 +++++++++++++++--------
>   drivers/media/usb/em28xx/em28xx.h        |   6 +-
>   3 files changed, 209 insertions(+), 101 deletions(-)
> 

The changes looks good to me. Have you tried building as a modules and
running modprobes and rmmods? You can do that without a device.

thanks,
-- Shuah