netdev - Re: [PATCH net] l2tp: avoid one data-race in l2tp_tunnel_del

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Message-ID: <aWjIm9SDLjHztAT-@debian>
Date: Thu, 15 Jan 2026 11:59:39 +0100
From: Guillaume Nault <gnault@...hat.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, netdev@...r.kernel.org,
	eric.dumazet@...il.com,
	syzbot+7312e82745f7fa2526db@...kaller.appspotmail.com,
	James Chapman <jchapman@...alix.com>
Subject: Re: [PATCH net] l2tp: avoid one data-race in l2tp_tunnel_del_work()

On Thu, Jan 15, 2026 at 09:21:39AM +0000, Eric Dumazet wrote:
> We should read sk->sk_socket only when dealing with kernel sockets.
> 
> syzbot reported the following data-race:
> 
> BUG: KCSAN: data-race in l2tp_tunnel_del_work / sk_common_release
> 
> write to 0xffff88811c182b20 of 8 bytes by task 5365 on cpu 0:
>   sk_set_socket include/net/sock.h:2092 [inline]
>   sock_orphan include/net/sock.h:2118 [inline]
>   sk_common_release+0xae/0x230 net/core/sock.c:4003
>   udp_lib_close+0x15/0x20 include/net/udp.h:325
>   inet_release+0xce/0xf0 net/ipv4/af_inet.c:437
>   __sock_release net/socket.c:662 [inline]
>   sock_close+0x6b/0x150 net/socket.c:1455
>   __fput+0x29b/0x650 fs/file_table.c:468
>   ____fput+0x1c/0x30 fs/file_table.c:496
>   task_work_run+0x131/0x1a0 kernel/task_work.c:233
>   resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
>   __exit_to_user_mode_loop kernel/entry/common.c:44 [inline]
>   exit_to_user_mode_loop+0x1fe/0x740 kernel/entry/common.c:75
>   __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
>   syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
>   syscall_exit_to_user_mode_work include/linux/entry-common.h:159 [inline]
>   syscall_exit_to_user_mode include/linux/entry-common.h:194 [inline]
>   do_syscall_64+0x1e1/0x2b0 arch/x86/entry/syscall_64.c:100
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> read to 0xffff88811c182b20 of 8 bytes by task 827 on cpu 1:
>   l2tp_tunnel_del_work+0x2f/0x1a0 net/l2tp/l2tp_core.c:1418
>   process_one_work kernel/workqueue.c:3257 [inline]
>   process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3340
>   worker_thread+0x582/0x770 kernel/workqueue.c:3421
>   kthread+0x489/0x510 kernel/kthread.c:463
>   ret_from_fork+0x149/0x290 arch/x86/kernel/process.c:158
>   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
> 
> value changed: 0xffff88811b818000 -> 0x0000000000000000
> 
> Fixes: d00fa9adc528 ("l2tp: fix races with tunnel socket close")

Reviewed-by: Guillaume Nault <gnault@...hat.com>