In the Linux kernel, the following vulnerability has been resolved: netpoll: fix a use-after-free on shutdown path There is a use-after-fr

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.

 1  BUG: KASAN: slab-use-after-free in _raw_spin_lock_irqsave+0x3b/0x80
 2  Read of size 1 at addr ... by task kworker/9:1
 3  Workqueue: events queue_process
 4  Call Trace:
 5   skb_dequeue+0x1e/0xb0
 6   queue_process+0x2c/0x600
 7   process_scheduled_works+0x4b6/0x850
 8   worker_thread+0x414/0x5a0
 9  Allocated by task 242:
10   __netpoll_setup+0x201/0x4a0
11   netpoll_setup+0x249/0x550
12   enabled_store+0x32f/0x380
13  Freed by task 0:
14   kfree+0x1b7/0x540
15   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.