linux-kernel - Re: [PATCH V4] netrom: Preventing the use of abnormal neighbor

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Message-ID: <785c8add-ab09-47b2-94bf-a4bfe8c13388@redhat.com>
Date: Tue, 28 Oct 2025 15:13:37 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Lizhi Xu <lizhi.xu@...driver.com>
Cc: dan.carpenter@...aro.org, davem@...emloft.net, edumazet@...gle.com,
 horms@...nel.org, kuba@...nel.org, linux-hams@...r.kernel.org,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 syzbot+2860e75836a08b172755@...kaller.appspotmail.com,
 syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH V4] netrom: Preventing the use of abnormal neighbor

On 10/23/25 3:50 PM, Lizhi Xu wrote:
> The root cause of the problem is that multiple different tasks initiate
> SIOCADDRT & NETROM_NODE commands to add new routes, there is no lock
> between them to protect the same nr_neigh.
> 
> Task0 can add the nr_neigh.refcount value of 1 on Task1 to routes[2].
> When Task2 executes nr_neigh_put(nr_node->routes[2].neighbour), it will
> release the neighbour because its refcount value is 1.
> 
> In this case, the following situation causes a UAF on Task2:
> 
> Task0					Task1						Task2
> =====					=====						=====
> nr_add_node()
> nr_neigh_get_dev()			nr_add_node()
> 					nr_node_lock()
> 					nr_node->routes[2].neighbour->count--
> 					nr_neigh_put(nr_node->routes[2].neighbour);
> 					nr_remove_neigh(nr_node->routes[2].neighbour)
> 					nr_node_unlock()
> nr_node_lock()
> nr_node->routes[2].neighbour = nr_neigh
> nr_neigh_hold(nr_neigh);								nr_add_node()
> 											nr_neigh_put()
> 											if (nr_node->routes[2].neighbour->count
> Description of the UAF triggering process:
> First, Task 0 executes nr_neigh_get_dev() to set neighbor refcount to 3.
> Then, Task 1 puts the same neighbor from its routes[2] and executes
> nr_remove_neigh() because the count is 0. After these two operations,
> the neighbor's refcount becomes 1. Then, Task 0 acquires the nr node
> lock and writes it to its routes[2].neighbour.
> Finally, Task 2 executes nr_neigh_put(nr_node->routes[2].neighbour) to
> release the neighbor. The subsequent execution of the neighbor->count
> check triggers a UAF.

I looked at the code quite a bit and I think this could possibly avoid
the above mentioned race, but this whole area looks quite confusing to me.

I think it would be helpful if you could better describe the relevant
scenario starting from the initial setup (no nodes, no neighs).

Thanks,

Paolo