linux-kernel - [PATCH net] netpoll: fix incorrect refcount handling causing incorrect cleanup

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Message-Id: <20250901-netpoll_memleak-v1-1-34a181977dfc@debian.org>
Date: Mon, 01 Sep 2025 07:29:13 -0700
From: Breno Leitao <leitao@...ian.org>
To: "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
 david decotigny <decot@...glers.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 kernel-team@...a.com, stable@...r.kernel.org, jv@...sburgh.net, 
 Breno Leitao <leitao@...ian.org>
Subject: [PATCH 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.

Cc: stable@...r.kernel.org
Cc: jv@...sburgh.net
Fixes: efa95b01da18 ("netpoll: fix use after free")
Signed-off-by: Breno Leitao <leitao@...ian.org>
---
I have a selftest that shows the memory leak when kmemleak is enabled
and I will be submitting to net-next.

Also, giving I am reverting commit efa95b01da18 ("netpoll: fix use
after free"), which was supposed to fix a problem on bonding, I am
copying Jay.
---
 net/core/netpoll.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 5f65b62346d4e..19676cd379640 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -815,6 +815,10 @@ 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;
 
@@ -824,8 +828,7 @@ 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);
 }

---
base-commit: 864ecc4a6dade82d3f70eab43dad0e277aa6fc78
change-id: 20250901-netpoll_memleak-90d0d4bc772c

Best regards,
--  
Breno Leitao <leitao@...ian.org>