Description

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.

References 8

• git.kernel.org https://git.kernel.org/stable/c/49c8d2c1f94cc2f4d1a108530d7ba52614b874c2
• git.kernel.org https://git.kernel.org/stable/c/4afd4ebbad52aa146838ec23082ba393e426a2bb
• git.kernel.org https://git.kernel.org/stable/c/890472d6fbf062e6de7fdd56642cb305ab79d669
• git.kernel.org https://git.kernel.org/stable/c/8e6a50edad11e3e1426e4c29e7aa6201f3468ac2
• git.kernel.org https://git.kernel.org/stable/c/9a51b5ccd1c79afec1c03a4e1e6688da52597556
• git.kernel.org https://git.kernel.org/stable/c/9b0bb18b4b9dc017c1825a2c5e763615e34a1593
• git.kernel.org https://git.kernel.org/stable/c/c645693180a98606c430825223d2029315d85e9d
• git.kernel.org https://git.kernel.org/stable/c/c79a6d9da29219616b118a3adce9a14cd30f9bd0

Remediation