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Message-ID: <20250908101256.GA2015@horms.kernel.org>
Date: Mon, 8 Sep 2025 11:12:56 +0100
From: Simon Horman <horms@...nel.org>
To: Breno Leitao <leitao@...ian.org>
Cc: Andrew Lunn <andrew+netdev@...n.ch>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
Shuah Khan <shuah@...nel.org>, david decotigny <decot@...glers.com>,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
linux-kselftest@...r.kernel.org, asantostc@...il.com, efault@....de,
calvin@...nvd.org, kernel-team@...a.com, stable@...r.kernel.org,
jv@...sburgh.net
Subject: Re: [PATCH net v3 1/3] netpoll: fix incorrect refcount handling
causing incorrect cleanup
On Fri, Sep 05, 2025 at 10:25:07AM -0700, Breno Leitao wrote:
> 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>
Reviewed-by: Simon Horman <horms@...nel.org>
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