media: rtl2832: fix use-after-free in rtl2832_remove()

cancel_delayed_work_sync() is called before i2c_mux_del_adapters()
in rtl2832_remove(). While the cancel waits for any running instance
of i2c_gate_work to finish, it does not prevent the timer from being
rescheduled by a concurrent thread.

During probe, the r820t_attach() call attempts I2C transfers through
the mux adapter. These transfers go through i2c_mux_master_xfer(),
which calls rtl2832_deselect() after the transfer completes,
rescheduling i2c_gate_work via schedule_delayed_work(). If this
transfer is still in flight when rtl2832_remove() runs,
rtl2832_deselect() can reschedule i2c_gate_work after it has been
cancelled, causing a use-after-free when kfree(dev) is called.

Fix this by calling i2c_mux_del_adapters() before
cancel_delayed_work_sync(). Once the mux adapter is unregistered, no
new I2C transfers can go through it, so rtl2832_deselect() can no
longer reschedule i2c_gate_work. The subsequent
cancel_delayed_work_sync() is then guaranteed to be final.

Fixes: cddcc40b1b ("[media] rtl2832: convert to use an explicit i2c mux core")
Cc: stable@vger.kernel.org
Reported-by: syzbot+019ced393ab913002b75@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=019ced393ab913002b75
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>

drivers/media/dvb-frontends/rtl2832.c

@@ -1115,10 +1115,10 @@ static void rtl2832_remove(struct i2c_client *client)
 
 	dev_dbg(&client->dev, "\n");
 
-	cancel_delayed_work_sync(&dev->i2c_gate_work);
-
 	i2c_mux_del_adapters(dev->muxc);
 
+	cancel_delayed_work_sync(&dev->i2c_gate_work);
+
 	regmap_exit(dev->regmap);
 
 	kfree(dev);