[PATCH] media: cx23885: Fix use-after-free in cx23885_finidev due to race condition

In cx23885_v4l2_dev_notify_init, &dev->cx25840_work is bound with
cx23885_av_work_handler, and &dev->ir_rx_work and &dev->ir_tx_work
are bound with cx23885_ir_rx_work_handler and
cx23885_ir_tx_work_handler. cx23885_irq can schedule these works on
system_wq when an AV_CORE interrupt is received, and the IR subdevice
can also schedule the IR works from its interrupt service routine via
the v4l2_device notify callback.

If we remove the device, cx23885_finidev makes cleanup and the memory
allocated for dev is released by kfree(dev), while the works mentioned
above may still be pending or running. The sequence of operations that
may lead to a UAF bug is as follows:

CPU0                                      CPU1

                                          | cx23885_irq
                                          | schedule_work(&dev->cx25840_work)
cx23885_finidev                           |
cx23885_input_fini(dev)                   |
cx23885_ir_fini(dev)                      |
cx23885_shutdown(dev)                     |
free_irq(pci_dev->irq, dev)               |
pci_disable_device(pci_dev)               |
cx23885_dev_unregister(dev)               |
v4l2_device_unregister(v4l2_dev)          |
kfree(dev)                                |
// dev is freed                           |
                                          | cx23885_av_work_handler
                                          | // use dev (use-after-free)

Fix it by canceling the works after the IRQ handler that can schedule
them has been stopped, and before proceeding with the remaining
cleanup in cx23885_finidev.

Note that the flush_work() calls in cx23885_input_ir_stop() do not
close this race: they only wait for works that are already queued or
running at that moment, they do not prevent the IRQ handler, which is
still registered at that point, from scheduling the works again
afterwards, and they are skipped entirely when dev->sd_ir is NULL.
The cancel_work_sync() calls are therefore placed after free_irq(),
the only point at which no new work can be scheduled.

Fixes: e5514f104d87 ("V4L/DVB: cx23885: Move AV Core irq handling to a work handler")
Assisted-by: Codex:deepseek-v4-flash
Signed-off-by: Pei Xiao <[email protected]>
---
 drivers/media/pci/cx23885/cx23885-core.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/media/pci/cx23885/cx23885-core.c b/drivers/media/pci/cx23885/cx23885-core.c
index 5fb26285e4af..7498091176b7 100644
--- a/drivers/media/pci/cx23885/cx23885-core.c
+++ b/drivers/media/pci/cx23885/cx23885-core.c
@@ -2246,6 +2246,10 @@ static void cx23885_finidev(struct pci_dev *pci_dev)
 	/* unregister stuff */
 	free_irq(pci_dev->irq, dev);
 
+	cancel_work_sync(&dev->cx25840_work);
+	cancel_work_sync(&dev->ir_rx_work);
+	cancel_work_sync(&dev->ir_tx_work);
+
 	pci_disable_device(pci_dev);
 
 	cx23885_dev_unregister(dev);
-- 
2.25.1