Commit 49c8d2c1 authored Nov 07, 2025 by Breno Leitao Committed by Jakub Kicinski Nov 10, 2025

net: netpoll: fix incorrect refcount handling causing incorrect cleanup



commit efa95b01 ("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 efa95b01 ("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> # 3.17.x
Cc: Jay Vosburgh <jv@jvosburgh.net>
Fixes: efa95b01 ("netpoll: fix use after free")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20251107-netconsole_torture-v10-1-749227b55f63@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>

parent e6ca8f53

net/core/netpoll.c

+5 −2

Original line number	Diff line number	Diff line
		@@ -811,6 +811,10 @@ static void __netpoll_cleanup(struct netpoll *np)
		if (!npinfo)
		return;

		/* At this point, there is a single npinfo instance per netdevice, and
		* its refcnt tracks how many netpoll structures are linked to it. We
		* only perform npinfo cleanup when the refcnt decrements to zero.
		*/
		if (refcount_dec_and_test(&npinfo->refcnt)) {
		const struct net_device_ops *ops;

		@@ -820,8 +824,7 @@ static void __netpoll_cleanup(struct netpoll *np)

		RCU_INIT_POINTER(np->dev->npinfo, NULL);
		call_rcu(&npinfo->rcu, rcu_cleanup_netpoll_info);
		} else
		RCU_INIT_POINTER(np->dev->npinfo, NULL);
		}

		skb_pool_flush(np);
		}