[PATCH v3] media: hantro: fix use after free bug in hantro_remove due to race condition

[Ezequiel Garcia]

Hi Zheng,

On Mon, Mar 13, 2023 at 12:42 PM Zheng Wang <zyytlz.wz at 163.com> wrote:
>
> In hantro_probe, vpu->watchdog_work is bound with
> hantro_watchdog. Then hantro_end_prepare_run may
> be called to start the work.
>
> If we close the file or remove the module which will
> call hantro_release and hantro_remove to make cleanup,

It's not possible to close the file or remove the module while a watchdog is
scheduled.

That's because the watchdog is active only during a mem2mem job,
and the file won't be closed until the job is done.

v4l2_m2m_ctx_release calls v4l2_m2m_cancel_jobw
which waits until the job is done.

If you can confirm it's possible to remove or close the file
while a job is running, that would be a driver bug.

Thanks for the patch, but it's not needed.

Regards,
Ezequiel

> there may be an unfinished work. The possible sequence
> is as follows, which will cause a typical UAF bug.
>
> The same thing will happen in hantro_release, and use
> ctx after freeing it.
>
> Fix it by canceling the work before cleanup in hantro_release.
>
> CPU0                  CPU1
>
>                     |hantro_watchdog
> hantro_remove     |
>   v4l2_m2m_release  |
>     kfree(m2m_dev); |
>                     |
>                     | v4l2_m2m_get_curr_priv
>                     |   m2m_dev->curr_ctx //use
>
> Signed-off-by: Zheng Wang <zyytlz.wz at 163.com>
> ---
> v3:
> - use cancel_delayed_work_sync instead of cancel_delayed_work and add it to
> hantro_release suggested by Hans Verkuil
>
> v2:
> - move the cancel-work-related code to hantro_remove suggested by Hans Verkuil
> ---
>  drivers/media/platform/verisilicon/hantro_drv.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/media/platform/verisilicon/hantro_drv.c b/drivers/media/platform/verisilicon/hantro_drv.c
> index b0aeedae7b65..86a4c0fa8c7d 100644
> --- a/drivers/media/platform/verisilicon/hantro_drv.c
> +++ b/drivers/media/platform/verisilicon/hantro_drv.c
> @@ -597,6 +597,7 @@ static int hantro_release(struct file *filp)
>         struct hantro_ctx *ctx =
>                 container_of(filp->private_data, struct hantro_ctx, fh);
>
> +       cancel_delayed_work_sync(&ctx->dev->watchdog_work);
>         /*
>          * No need for extra locking because this was the last reference
>          * to this file.
> @@ -1099,6 +1100,7 @@ static int hantro_remove(struct platform_device *pdev)
>
>         v4l2_info(&vpu->v4l2_dev, "Removing %s\n", pdev->name);
>
> +       cancel_delayed_work_sync(&vpu->watchdog_work);
>         media_device_unregister(&vpu->mdev);
>         hantro_remove_dec_func(vpu);
>         hantro_remove_enc_func(vpu);
> --
> 2.25.1
>