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Tue, 02 Dec 2025 03:02:06 -0800 (PST) Received: from [192.168.88.32] ([212.105.155.136]) by smtp.gmail.com with ESMTPSA id 956f58d0204a3-6433bf5f366sm6125689d50.0.2025.12.02.03.02.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 Dec 2025 03:02:05 -0800 (PST) Message-ID: <29f61bac-ec6a-447d-a2f4-89328eaba688@redhat.com> Date: Tue, 2 Dec 2025 12:02:02 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] netrom: fix possible deadlock between nr_rt_ioctl() and nr_rt_device_down() To: Junjie Cao , davem@davemloft.net, edumazet@google.com, kuba@kernel.org, syzbot+14afda08dc3484d5db82@syzkaller.appspotmail.com Cc: horms@kernel.org, linux-hams@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, stable@vger.kernel.org References: <20251127084112.123837-1-junjie.cao@intel.com> Content-Language: en-US From: Paolo Abeni In-Reply-To: <20251127084112.123837-1-junjie.cao@intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/27/25 9:41 AM, Junjie Cao wrote: > syzbot reported a circular locking dependency involving > nr_neigh_list_lock, nr_node_list_lock and nr_node->node_lock in the > NET/ROM routing code [1]. > > One of the problematic scenarios looks like this: > > CPU0 CPU1 > ---- ---- > nr_rt_device_down() nr_rt_ioctl() > lock(nr_neigh_list_lock); nr_del_node() > ... lock(nr_node_list_lock); > lock(nr_node_list_lock); nr_remove_neigh(); > lock(nr_neigh_list_lock); > > This creates the following lock chain: > > nr_neigh_list_lock -> nr_node_list_lock -> &nr_node->node_lock > > while the ioctl path may acquire the locks in the opposite order via > nr_dec_obs()/nr_del_node(), which makes lockdep complain about a > possible deadlock. > > Refactor nr_rt_device_down() to avoid nested locking of > nr_neigh_list_lock and nr_node_list_lock. The function now performs > two separate passes: one that walks all nodes under nr_node_list_lock > and drops routes / reference counts, and a second one that removes > unused neighbours under nr_neigh_list_lock. > > This also fixes a reference count leak of nr_neigh in the node route > removal path. Please don't mix separate fixes; the latter need to go in a different patch to help reviewers. Also both of them need a suitable Fixes tag. Thanks, Paolo