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Subject: Re: [PATCH v3] media:exynos4-is: Fix a use after free in
 isp_video_release
To:     Lv Yunlong <lyl2019@mail.ustc.edu.cn>, s.nawrocki@samsung.com,
        mchehab@kernel.org, krzk@kernel.org
Cc:     linux-media@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-samsung-soc@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20210427132734.5212-1-lyl2019@mail.ustc.edu.cn>
From:   Hans Verkuil <hverkuil@xs4all.nl>
Message-ID: <44f264d9-e039-66b6-6e4b-1a5b3c386aa4@xs4all.nl>
Date:   Wed, 5 May 2021 11:31:04 +0200
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Hi Lv Yunlong,

On 27/04/2021 15:27, Lv Yunlong wrote:
> In isp_video_release, file->private_data is freed via
> _vb2_fop_release()->v4l2_fh_release(). But the freed
> file->private_data is still used in v4l2_fh_is_singular_file()
> ->v4l2_fh_is_singular(file->private_data), which is a use
> after free bug.
> 
> My patch sets file->private_data to NULL after _vb2_fop_release()
> to avoid the use after free, and uses a variable 'is_singular_file'
> to keep the original function unchanged.

Actually, it is the use of 'is_singular_file' that fixes the bug,
the 'file->private_data = NULL;' is unnecessary here.

That said, it would be a really good idea if in a separate patch you
make v4l2_fh_release() more robust by setting filp->private_data to
NULL after the kfree(fh).

Regards,

	Hans

> 
> Fixes: 34947b8aebe3f ("[media] exynos4-is: Add the FIMC-IS ISP capture DMA driver")
> Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
> ---
>  drivers/media/platform/exynos4-is/fimc-isp-video.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/platform/exynos4-is/fimc-isp-video.c b/drivers/media/platform/exynos4-is/fimc-isp-video.c
> index 612b9872afc8..c07dcb0bccc2 100644
> --- a/drivers/media/platform/exynos4-is/fimc-isp-video.c
> +++ b/drivers/media/platform/exynos4-is/fimc-isp-video.c
> @@ -306,17 +306,21 @@ static int isp_video_release(struct file *file)
>  	struct fimc_is_video *ivc = &isp->video_capture;
>  	struct media_entity *entity = &ivc->ve.vdev.entity;
>  	struct media_device *mdev = entity->graph_obj.mdev;
> +	bool is_singular_file;
>  
>  	mutex_lock(&isp->video_lock);
>  
> -	if (v4l2_fh_is_singular_file(file) && ivc->streaming) {
> +	is_singular_file = v4l2_fh_is_singular_file(file);
> +
> +	if (is_singular_file && ivc->streaming) {
>  		media_pipeline_stop(entity);
>  		ivc->streaming = 0;
>  	}
>  
>  	_vb2_fop_release(file, NULL);
> +	file->private_data = NULL;
>  
> -	if (v4l2_fh_is_singular_file(file)) {
> +	if (is_singular_file) {
>  		fimc_pipeline_call(&ivc->ve, close);
>  
>  		mutex_lock(&mdev->graph_mutex);
>