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

commit 45f1458a85 upstream.

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.

Cc: stable@vger.kernel.org
Fixes: 38e6bc185d ("netpoll: make __netpoll_cleanup non-block")
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260625-netpoll_rcu_fix-v2-1-0748ffac1e98@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/core/netpoll.c

@@ -812,14 +812,6 @@ static void rcu_cleanup_netpoll_info(struct rcu_head *rcu_head)
 			container_of(rcu_head, struct netpoll_info, rcu);
 
 	skb_queue_purge(&npinfo->txq);
-
-	/* we can't call cancel_delayed_work_sync here, as we are in softirq */
-	cancel_delayed_work(&npinfo->tx_work);
-
-	/* clean after last, unfinished work */
-	__skb_queue_purge(&npinfo->txq);
-	/* now cancel it again */
-	cancel_delayed_work(&npinfo->tx_work);
 	kfree(npinfo);
 }
 
@@ -843,6 +835,7 @@ static void __netpoll_cleanup(struct netpoll *np)
 			ops->ndo_netpoll_cleanup(np->dev);
 
 		RCU_INIT_POINTER(np->dev->npinfo, NULL);
+		disable_delayed_work_sync(&npinfo->tx_work);
 		call_rcu(&npinfo->rcu, rcu_cleanup_netpoll_info);
 	}