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Subject: Re: [PATCH RFC v3] media: em28xx: Fix race condition between open and
 init function
To:     Shuah Khan <skhan@linuxfoundation.org>, mchehab@kernel.org
Cc:     linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        kernel test robot <lkp@intel.com>,
        syzbot+b2391895514ed9ef4a8e@syzkaller.appspotmail.com
References: <20210415140724.15398-1-igormtorrente@gmail.com>
 <d254bf85-5185-6b21-afc6-fb00a9278186@linuxfoundation.org>
From:   Igor Torrente <igormtorrente@gmail.com>
Message-ID: <c18180b5-52b9-e787-654c-6967ecb81dce@gmail.com>
Date:   Fri, 16 Apr 2021 16:33:59 -0300
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On 4/15/21 2:25 PM, Shuah Khan wrote:
> 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@intel.com>
>> Reported-and-tested-by: 
>> syzbot+b2391895514ed9ef4a8e@syzkaller.appspotmail.com
>> Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@gmail.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.
> 

I tried and everything worked fine.

> thanks,
> -- Shuah
> 

Thanks,
---
Igor Matheus Andrade Torrente