Re: [RESEND PATCH v2] media: mtk-jpeg: Fix use after free bug due to uncanceled work

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

Hello Zheng,

On 7/7/23 12:24, Zheng Wang wrote:
> In mtk_jpeg_probe, &jpeg->job_timeout_work is bound with
> mtk_jpeg_job_timeout_work. Then mtk_jpeg_dec_device_run
> and mtk_jpeg_enc_device_run may be called to start the
> work.
> If we remove the module which will call mtk_jpeg_remove
> to make cleanup, there may be a unfinished work. The
> possible sequence is as follows, which will cause a
> typical UAF bug.
> 
> Fix it by canceling the work before cleanup in the mtk_jpeg_remove
> 
> CPU0                  CPU1
> 
>                     |mtk_jpeg_job_timeout_work
> mtk_jpeg_remove     |
>   v4l2_m2m_release  |
>     kfree(m2m_dev); |
>                     |
>                     | v4l2_m2m_get_curr_priv
>                     |   m2m_dev->curr_ctx //use
> Fixes: b2f0d2724ba4 ("[media] vcodec: mediatek: Add Mediatek JPEG Decoder Driver")
> Signed-off-by: Zheng Wang <zyytlz.wz@163.com>
> ---
> - v2: use cancel_delayed_work_sync instead of cancel_delayed_work suggested by Kyrie.
> ---
>  drivers/media/platform/mediatek/jpeg/mtk_jpeg_core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/media/platform/mediatek/jpeg/mtk_jpeg_core.c b/drivers/media/platform/mediatek/jpeg/mtk_jpeg_core.c
> index 0051f372a66c..6069ecf420b0 100644
> --- a/drivers/media/platform/mediatek/jpeg/mtk_jpeg_core.c
> +++ b/drivers/media/platform/mediatek/jpeg/mtk_jpeg_core.c
> @@ -1816,6 +1816,7 @@ static void mtk_jpeg_remove(struct platform_device *pdev)
>  {
>  	struct mtk_jpeg_dev *jpeg = platform_get_drvdata(pdev);
>  
> +	cancel_delayed_work_sync(&jpeg->job_timeout_work);
>  	pm_runtime_disable(&pdev->dev);
>  	video_unregister_device(jpeg->vdev);
>  	v4l2_m2m_release(jpeg->m2m_dev);

AFAICS, there is a fundamental problem here. The job_timeout_work uses
v4l2_m2m_get_curr_priv() and at the time when driver module is unloaded,
all the v4l contexts must be closed and released. Hence the
v4l2_m2m_get_curr_priv() shall return NULL and crash the kernel when
work is executed before cancel_delayed_work_sync().

At the time when mtk_jpeg_remove() is invoked, there shall be no
job_timeout_work running in background because all jobs should be
completed before context is released. If you'll look at
v4l2_m2m_cancel_job(), you can see that it waits for the task completion
before closing context.

You shouldn't be able to remove driver module while it has active/opened
v4l contexts. If you can do that, then this is yours bug that needs to
be fixed.

In addition to this all, the job_timeout_work is initialized only for
the single-core JPEG device. I'd expect this patch should crash
multi-core JPEG devices.

-- 
Best regards,
Dmitry