Re: [PATCH v2 net] mptcp: fix race in mptcp_pm_nl_flush_addrs_doit()

[Mat Martineau <martineau@kernel.org>]

On Fri, 23 Jan 2026, Eric Dumazet wrote:

> syzbot and Eulgyu Kim reported crashes in mptcp_pm_nl_get_local_id()
> and/or mptcp_pm_nl_is_backup()
>
> Root cause is list_splice_init() in mptcp_pm_nl_flush_addrs_doit()
> which is not RCU ready.
>
> list_splice_init_rcu() can not be called here while holding pernet->lock
> spinlock.
>
> Many thanks to Eulgyu Kim for providing a repro and testing our patches.
>
> Fixes: 141694df6573 ("mptcp: remove address when netlink flushes addrs")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reported-by: syzbot+5498a510ff9de39d37da@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6970a46d.a00a0220.3ad28e.5cf0.GAE@google.com/T/
> Reported-by: Eulgyu Kim <eulgyukim@snu.ac.kr>
> Cc: Geliang Tang <geliang@kernel.org>
> ---
> v2: Make sure the list was not empty, return early otherwise.

Thanks Eric, the v2 code changes LGTM. The netdev tooling wasn't able to 
apply the patch 
(https://patchwork.kernel.org/project/netdevbpf/patch/20260123030327.3041148-1-edumazet@google.com/), 
so Matthieu is planning to send a basically-identical v3 that 'git am' and 
the netdev CI will be happy with.

Reviewed-by: Mat Martineau <martineau@kernel.org>

> v1: https://lore.kernel.org/netdev/20260122131306.2119853-1-edumazet@google.com/
>
> net/mptcp/pm_kernel.c | 18 +++++++++++++++---
> 1 file changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/net/mptcp/pm_kernel.c b/net/mptcp/pm_kernel.c
> index 57570a44e4185370f531047fe97ce9f9fbd1480b..af23be6658ded4860133bb9495c7738014815d28 100644
> --- a/net/mptcp/pm_kernel.c
> +++ b/net/mptcp/pm_kernel.c
> @@ -1294,16 +1294,28 @@ static void __reset_counters(struct pm_nl_pernet *pernet)
> int mptcp_pm_nl_flush_addrs_doit(struct sk_buff *skb, struct genl_info *info)
> {
> 	struct pm_nl_pernet *pernet = genl_info_pm_nl(info);
> -	LIST_HEAD(free_list);
> +	struct list_head free_list;
>
> 	spin_lock_bh(&pernet->lock);
> -	list_splice_init(&pernet->endp_list, &free_list);
> +
> +	free_list = pernet->endp_list;
> +	INIT_LIST_HEAD_RCU(&pernet->endp_list);
> +
> 	__reset_counters(pernet);
> 	pernet->next_id = 1;
> 	bitmap_zero(pernet->id_bitmap, MPTCP_PM_MAX_ADDR_ID + 1);
> 	spin_unlock_bh(&pernet->lock);
> -	mptcp_nl_flush_addrs_list(sock_net(skb->sk), &free_list);
> +
> +	if (free_list.next == &pernet->endp_list)
> +		return 0;
> +
> 	synchronize_rcu();
> +
> +	/* Adjust the pointers to free_list instead of pernet->endp_list */
> +	free_list.prev->next = &free_list;
> +	free_list.next->prev = &free_list;
> +
> +	mptcp_nl_flush_addrs_list(sock_net(skb->sk), &free_list);
> 	__flush_addrs(&free_list);
> 	return 0;
> }
> --
> 2.52.0.457.g6b5491de43-goog
>
>