media: mtk-jpeg: fix use-after-free in release path due to uncancelled work

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

Race condition:

    CPU 0 (release)                    CPU 1 (workqueue)
    ----------------                   ------------------
    close()
      mtk_jpeg_release()
                                       mtk_jpegenc_worker()
                                         ctx = work->data
                                         // accessing ctx

        kfree(ctx)  // freed!
                                         access ctx  // UAF!

The work is queued via queue_work() during JPEG encode/decode operations
(via mtk_jpeg_device_run). If the device is closed while work is pending
or running, the work handler will access freed memory.

Fix this by calling cancel_work_sync() BEFORE acquiring the mutex. This
ordering is critical: if cancel_work_sync() is called after mutex_lock(),
and the work handler also tries to acquire the same mutex, it would cause
a deadlock.

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 ioctl operations.

Fixes: 5fb1c2361e ("mtk-jpegenc: add jpeg encode worker interface")
Cc: stable@vger.kernel.org
Signed-off-by: Fan Wu <fanwu01@zju.edu.cn>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>

drivers/media/platform/mediatek/jpeg/mtk_jpeg_core.c

@@ -1202,6 +1202,7 @@ static int mtk_jpeg_release(struct file *file)
 	struct mtk_jpeg_dev *jpeg = video_drvdata(file);
 	struct mtk_jpeg_ctx *ctx = mtk_jpeg_file_to_ctx(file);
 
+	cancel_work_sync(&ctx->jpeg_work);
 	mutex_lock(&jpeg->lock);
 	v4l2_m2m_ctx_release(ctx->fh.m2m_ctx);
 	v4l2_ctrl_handler_free(&ctx->ctrl_hdl);