linux-kernel - [PATCH V2] netrom: Prevent race conditions between multiple add route

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Message-ID: <20251020133456.3564833-1-lizhi.xu@windriver.com>
Date: Mon, 20 Oct 2025 21:34:56 +0800
From: Lizhi Xu <lizhi.xu@...driver.com>
To: <dan.carpenter@...aro.org>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <horms@...nel.org>,
        <kuba@...nel.org>, <linux-hams@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <lizhi.xu@...driver.com>,
        <netdev@...r.kernel.org>, <pabeni@...hat.com>,
        <syzbot+2860e75836a08b172755@...kaller.appspotmail.com>,
        <syzkaller-bugs@...glegroups.com>
Subject: [PATCH V2] netrom: Prevent race conditions between multiple add route


On Mon, 20 Oct 2025 15:25:40 +0300, Dan Carpenter wrote:
> Task0					Task1						Task2
> =====					=====						=====
> [97] nr_add_node()
> [113] nr_neigh_get_dev()		[97] nr_add_node()
> 					[214] nr_node_lock()
> 					[245] nr_node->routes[2].neighbour->count--
> 					[246] nr_neigh_put(nr_node->routes[2].neighbour);
> 					[248] nr_remove_neigh(nr_node->routes[2].neighbour)
> 					[283] nr_node_unlock()
> [214] nr_node_lock()
> [253] nr_node->routes[2].neighbour = nr_neigh
> [254] nr_neigh_hold(nr_neigh);							[97] nr_add_node()
> 											[XXX] nr_neigh_put()
>                                                                                         ^^^^^^^^^^^^^^^^^^^^
> 
> These charts are supposed to be chronological so [XXX] is wrong because the
> use after free happens on line [248].  Do we really need three threads to
> make this race work?
The UAF problem occurs in Task2. Task1 sets the refcount of nr_neigh to 1,
then Task0 adds it to routes[2]. Task2 releases routes[2].neighbour after
executing [XXX]nr_neigh_put().

BR,
Lizhi