Re: [PATCH net v3 1/3] netpoll: fix incorrect refcount handling causing incorrect cleanup

[Simon Horman <horms@kernel.org>]

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: stable@vger.kernel.org
> Cc: jv@jvosburgh.net
> Fixes: efa95b01da18 ("netpoll: fix use after free")
> Signed-off-by: Breno Leitao <leitao@debian.org>

Reviewed-by: Simon Horman <horms@kernel.org>