netdev - Re: [PATCH net] mptcp: fix race in mptcp_pm_nl_flush_addrs

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Message-ID: <CANn89i+wyDiyq13nn4_NkckA0A9z7ybeKRXaC4A-VS0yFc9GQw@mail.gmail.com>
Date: Thu, 22 Jan 2026 14:54:46 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Matthieu Baerts <matttbe@...nel.org>, Mat Martineau <martineau@...nel.org>, 
	Geliang Tang <geliang.tang@...ux.dev>, Florian Westphal <fw@...len.de>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, syzbot+5498a510ff9de39d37da@...kaller.appspotmail.com, 
	Eulgyu Kim <eulgyukim@....ac.kr>, Geliang Tang <geliang@...nel.org>
Subject: Re: [PATCH net] mptcp: fix race in mptcp_pm_nl_flush_addrs_doit()

On Thu, Jan 22, 2026 at 2:13 PM Eric Dumazet <edumazet@...gle.com> 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@...gle.com>
> Reported-by: syzbot+5498a510ff9de39d37da@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6970a46d.a00a0220.3ad28e.5cf0.GAE@google.com/T/
> Reported-by: Eulgyu Kim <eulgyukim@....ac.kr>
> Cc: Geliang Tang <geliang@...nel.org>
> ---
>  net/mptcp/pm_kernel.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/net/mptcp/pm_kernel.c b/net/mptcp/pm_kernel.c
> index 57570a44e4185370f531047fe97ce9f9fbd1480b..1a97d0eafa2b0c9e4275b90d4a576f837dc286a9 100644
> --- a/net/mptcp/pm_kernel.c
> +++ b/net/mptcp/pm_kernel.c
> @@ -1294,16 +1294,24 @@ 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);
>         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;


We have to test if the list was empty, and avoid the synchronize_rcu
in this case.

I will squash in V2, unless someone complains.

diff --git a/net/mptcp/pm_kernel.c b/net/mptcp/pm_kernel.c
index 1a97d0eafa2b0c9e4275b90d4a576f837dc286a9..af23be6658ded4860133bb9495c7738014815d28
100644
--- a/net/mptcp/pm_kernel.c
+++ b/net/mptcp/pm_kernel.c
@@ -1305,6 +1305,10 @@ int mptcp_pm_nl_flush_addrs_doit(struct sk_buff
*skb, struct genl_info *info)
        pernet->next_id = 1;
        bitmap_zero(pernet->id_bitmap, MPTCP_PM_MAX_ADDR_ID + 1);
        spin_unlock_bh(&pernet->lock);
+
+       if (free_list.next == &pernet->endp_list)
+               return 0;
+
        synchronize_rcu();

        /* Adjust the pointers to free_list instead of pernet->endp_list */