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

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

On 8/24/23 11:20, Zheng Hacker wrote:
> Dmitry Osipenko <dmitry.osipenko@collabora.com> 于2023年8月23日周三 02:51写道：
> 
>>
>> 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().
>>
> 
> Hi Dmitry,
> 
> Thanks for your reply. I think you're right. As m2m_dev is freed in
> v4l2_m2m_release,
> the invoking in v4l2_m2m_get_curr_priv might cause either UAF or null
> pointer dereference
> bug. I am sure that context is closed when we invoke mtk_jpeg_remove.
> But I'm not sure if
> context is released when mtk_jpegdec_timeout_work running.
> 
>> 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.
> 
> Yes, so I think the better way is to put the cancel_delayed_work_sync
> invoking into
> v4l2_m2m_ctx_release function?

The v4l2_m2m_ctx_release() already should wait for the job_timeout_work
completion or for the interrupt fire. Apparently it doesn't work in
yours case. You'll need to debug why v4l job or job_timeout_work is
running after v4l2_m2m_ctx_release(), it shouldn't happen.

The interrupt handler cancels job_timeout_work, you shouldn't need to
flush the work.

Technically, interrupt handler may race with job_timeout_work, but the
timeout is set to 1 second and in practice should be difficult to
trigger the race. The interrupt handler needs to be threaded, it should
use cancel_delayed_work_sync() and check the return value of this function.

>>
>> 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.
>>
> 
> I think that's true. As I'm not familiar with the code here. Could you
> please give me some advice about the patch?

We'll need to understand why v4l2_m2m_ctx_release() doesn't work as
expected before thinking about the patch.

-- 
Best regards,
Dmitry