AZL-93455

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) __netpollcleanup() is called 2) set dev->npinfo to NULL 3) callrcu() with rcucleanupnetpollinfo() 3.1) rcucleanupnetpollinfo() tries to cancel all workers with canceldelayedwork(), 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 canceldelayedwork_sync here, as we are in softirq", the TX worker can run after 4).

Tl;DR: queueprocess() is not an RCU reader, it reaches npinfo through the work item via containerof().

Use disabledelayedwork_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.