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:
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.
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.
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
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