On Fri, Sep 05, 2025 at 10:25:07AM -0700, Breno Leitao wrote:
> commit efa95b01da18 ("netpoll: fix use after free") incorrectly
> ignored the refcount and prematurely set dev->npinfo to NULL during
> netpoll cleanup, leading to improper behavior and memory leaks.
>
> Scenario causing lack of proper cleanup:
>
> 1) A netpoll is associated with a NIC (e.g., eth0) and netdev->npinfo is
> allocated, and refcnt = 1
> - Keep in mind that npinfo is shared among all netpoll instances. In
> this case, there is just one.
>
> 2) Another netpoll is also associated with the same NIC and
> npinfo->refcnt += 1.
> - Now dev->npinfo->refcnt = 2;
> - There is just one npinfo associated to the netdev.
>
> 3) When the first netpolls goes to clean up:
> - The first cleanup succeeds and clears np->dev->npinfo, ignoring
> refcnt.
> - It basically calls `RCU_INIT_POINTER(np->dev->npinfo, NULL);`
> - Set dev->npinfo = NULL, without proper cleanup
> - No ->ndo_netpoll_cleanup() is either called
>
> 4) Now the second target tries to clean up
> - The second cleanup fails because np->dev->npinfo is already NULL.
> * In this case, ops->ndo_netpoll_cleanup() was never called, and
> the skb pool is not cleaned as well (for the second netpoll
> instance)
> - This leaks npinfo and skbpool skbs, which is clearly reported by
> kmemleak.
>
> Revert commit efa95b01da18 ("netpoll: fix use after free") and adds
> clarifying comments emphasizing that npinfo cleanup should only happen
> once the refcount reaches zero, ensuring stable and correct netpoll
> behavior.
>
> Cc: sta...@vger.kernel.org
> Cc: j...@jvosburgh.net
> Fixes: efa95b01da18 ("netpoll: fix use after free")
> Signed-off-by: Breno Leitao <lei...@debian.org>
Reviewed-by: Simon Horman <ho...@kernel.org>
