linux-kernel - Re: [PATCH] batman-adv: Fix orig node refcnt leak when creating neigh node

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Date:   Fri, 18 Sep 2020 08:22:43 +0200
From:   Sven Eckelmann <sven@...fation.org>
To:     Marek Lindner <mareklindner@...mailbox.ch>,
        Simon Wunderlich <sw@...onwunderlich.de>,
        Antonio Quartulli <a@...table.cc>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        b.a.t.m.a.n@...ts.open-mesh.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xiyu Yang <xiyuyang19@...an.edu.cn>
Cc:     yuanxzhang@...an.edu.cn, kjlu@....edu,
        Xiyu Yang <xiyuyang19@...an.edu.cn>,
        Xin Tan <tanxin.ctf@...il.com>
Subject: Re: [PATCH] batman-adv: Fix orig node refcnt leak when creating neigh node

On Friday, 18 September 2020 05:03:19 CEST Xiyu Yang wrote:
> batadv_neigh_node_create() is used to create a neigh node object, whose
> fields will be initialized with the specific object. When a new
> reference of the specific object is created during the initialization,
> its refcount should be increased.
> 
> However, when "neigh_node" object initializes its orig_node field with
> the "orig_node" object, the function forgets to hold the refcount of the
> "orig_node", causing a potential refcount leak and use-after-free issue
> for the reason that the object can be freed in other places.
> 
> Fix this issue by increasing the refcount of orig_node object during the
> initialization and adding corresponding batadv_orig_node_put() in
> batadv_neigh_node_release().


I will most likely not add this patch because I have concerns that this would 
need an active garbage collector to fix the reference counter loop.

Please check batadv_neigh_node::orig_node (whose reference counter you've just 
incremented) and batadv_orig_node::neigh_list (with batadv_neigh_node). And at 
the same time the batadv_neigh_node_release and batadv_orig_node_release. So 
the originator will only free the reference (and thus potentially call 
batadv_neigh_node_release) when its own reference counter is zero. But it 
cannot become zero because the neigh_node is holding a reference to this 
originator.

Kind regards,
	Sven

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