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 premat…

Overview

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.

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.