Re: [PATCH net] netpoll: fix a use-after-free on shutdown path

On Mon, Jun 22, 2026 at 8:31 PM Breno Leitao [off-list ref] wrote:

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().

In reality, we can improve this cleanup path by a lot, but, given that
this is targeting net, just do the sane path:

1) set dev->npinfo to NULL
2) synchronize net / RCU
3) cancel_delayed_work_sync() any new worker (that potentially showed up
   after the grace period -- and should exit soon given they will see
   dev->npinfo = NULL)
4) then rcu_cleanup_netpoll_info() -> kfree() npinfo

In the future, we can do the cleanup inline here, and don't need
npinfo->rcu rcu_head, but that is net-next material.

Cc: stable@vger.kernel.org
Fixes: 38e6bc185d95 ("netpoll: make __netpoll_cleanup non-block")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 net/core/netpoll.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>