[PATCH] media: mediatek: vcodec: fix use-after-free in encoder release path

[Fan Wu]

The fops_vcodec_release() function frees the context structure (ctx)
without first cancelling any pending or running work in ctx->encode_work.
This creates a race window where the workqueue handler (mtk_venc_worker)
may still be accessing the context memory after it has been freed.

Race condition:

    CPU 0 (release path)               CPU 1 (workqueue)
    ---------------------               ------------------
    fops_vcodec_release()
      v4l2_m2m_ctx_release()
        v4l2_m2m_cancel_job()
        // waits for m2m job "done"
                                        mtk_venc_worker()
                                          v4l2_m2m_job_finish()
                                          // m2m job "done"
                                          // BUT worker still running!
                                          // post-job_finish access:
                                        other ctx dereferences
                                          // UAF if ctx already freed
        // returns (job "done")
      kfree(ctx)  // ctx freed

Root cause: The v4l2_m2m_ctx_release() only waits for the m2m job
lifecycle (via TRANS_RUNNING flag), not the workqueue lifecycle.
After v4l2_m2m_job_finish() is called, the m2m framework considers
the job complete and v4l2_m2m_ctx_release() returns, but the worker
function continues executing and may still access ctx.

The work is queued during encode operations via:
  queue_work(ctx->dev->encode_workqueue, &ctx->encode_work)
The worker function accesses ctx->m2m_ctx, ctx->dev, and other ctx
fields even after calling v4l2_m2m_job_finish().

This vulnerability was confirmed with KASAN by running an instrumented
test module that widens the post-job_finish race window. KASAN detected:

  BUG: KASAN: slab-use-after-free in mtk_venc_worker+0x159/0x180
  Read of size 4 at addr ffff88800326e000 by task kworker/u8:0/12

  Workqueue: mtk_vcodec_enc_wq mtk_venc_worker

  Allocated by task 47:
    __kasan_kmalloc+0x7f/0x90
    fops_vcodec_open+0x85/0x1a0

  Freed by task 47:
    __kasan_slab_free+0x43/0x70
    kfree+0xee/0x3a0
    fops_vcodec_release+0xb7/0x190

Fix this by calling cancel_work_sync(&ctx->encode_work) before kfree(ctx).
This ensures the workqueue handler is both cancelled (if pending) and
synchronized (waits for any running handler to complete) before the
context is freed.

Placement rationale: The fix is placed after v4l2_ctrl_handler_free()
and before list_del_init(&ctx->list). At this point, all m2m operations
are done (v4l2_m2m_ctx_release() has returned), and we need to ensure
the workqueue is synchronized before removing ctx from the list and
freeing it.

Note: The open error path does NOT need cancel_work_sync() because
INIT_WORK() only initializes the work structure - it does not schedule
it. Work is only scheduled later during device_run() operations.

Fixes: 0934d3759615 ("media: mediatek: vcodec: separate decoder and encoder")
Signed-off-by: Fan Wu <fanwu01 at zju.edu.cn>
---
 .../mediatek/vcodec/encoder/mtk_vcodec_enc_drv.c         | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/media/platform/mediatek/vcodec/encoder/mtk_vcodec_enc_drv.c b/drivers/media/platform/mediatek/vcodec/encoder/mtk_vcodec_enc_drv.c
index c977ed0c09b6..4e4541b2fc8e 100644
--- a/drivers/media/platform/mediatek/vcodec/encoder/mtk_vcodec_enc_drv.c
+++ b/drivers/media/platform/mediatek/vcodec/encoder/mtk_vcodec_enc_drv.c
@@ -215,6 +215,15 @@ static int fops_vcodec_release(struct file *file)
 	v4l2_fh_exit(&ctx->fh);
 	v4l2_ctrl_handler_free(&ctx->ctrl_hdl);
 
+	/*
+	 * Cancel any pending encode work before freeing the context.
+	 * Although v4l2_m2m_ctx_release() waits for m2m job completion,
+	 * the workqueue handler (mtk_venc_worker) may still be accessing
+	 * the context after v4l2_m2m_job_finish() returns. Without this,
+	 * a use-after-free occurs when the worker accesses ctx after kfree.
+	 */
+	cancel_work_sync(&ctx->encode_work);
+
 	spin_lock_irqsave(&dev->dev_ctx_lock, flags);
 	list_del_init(&ctx->list);
 	spin_unlock_irqrestore(&dev->dev_ctx_lock, flags);
-- 
2.34.1