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

[Zheng Wang]

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,
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@xxxxxxx>
---
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