netpoll: fix a use-after-free on shutdown path

Summary

In the Linux kernel, the following vulnerability has been resolved: netpoll: fix a use-after-free on shutdown path There is a use-after-free error on netpoll, which is clearly detected by KASAN. BUG: KASAN: slab-use-after-free in _raw_spin_lock_irqsave+0x3b/0x80 Read of size 1 at addr ... by task kworker/9:1 Workqueue: events queue_process Call Trace: skb_dequeue+0x1e/0xb0 queue_process+0x2c/0x600 process_scheduled_works+0x4b6/0x850 worker_thread+0x414/0x5a0 Allocated by task 242: __netpoll_setup+0x201/0x4a0 netpoll_setup+0x249/0x550 enabled_store+0x32f/0x380 Freed by task 0: kfree+0x1b7/0x540 rcu_core+0x3f8/0x7a0 The problem happens when there is a pending TX worker running in parallel with the cleanup path. This is what happens on netpoll shutdown path: 1) __netpoll_cleanup() is called 2) set dev->npinfo to NULL 3) call_rcu() with rcu_cleanup_netpoll_info() 3.1) rcu_cleanup_netpoll_info() tries to cancel all workers with cancel_delayed_work(), but doesn't wait for the worker to finish 4) and kfree(npinfo); Because 3.1) doesn't really cancel the work, as the comment says "we can't call cancel_delayed_work_sync here, as we are in softirq", the TX worker can run after 4). Tl;DR: queue_process() is not an RCU reader, it reaches npinfo through the work item via container_of(). Use disable_delayed_work_sync() to ensure the worker is completely stopped and prevent any future re-arming attempts. Once npinfo is set to NULL, senders will bail out and not queue new work. The disable flag ensures any in-flight re-arming attempts also fail silently. In the future, we can do the cleanup inline here without needing the npinfo->rcu rcu_head, but that is net-next material.