CVE-2025-68245

In the Linux kernel, the following vulnerability has been resolved:

net: netpoll: fix incorrect refcount handling causing incorrect cleanup

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.