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

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Message-ID: <2a8efd6f-34d5-480f-8ca1-efef2959fffd@kernel.org>
Date: Thu, 13 Nov 2025 18:35:31 +0100
From: Matthieu Baerts <matttbe@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: 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+355158e7e301548a1424@...kaller.appspotmail.com,
 "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, MPTCP Linux <mptcp@...ts.linux.dev>
Subject: Re: [PATCH net] mptcp: fix race condition in mptcp_schedule_work()

Hi Eric,

(+cc MPTCP ML)

On 13/11/2025 11:39, Eric Dumazet wrote:
> syzbot reported use-after-free in mptcp_schedule_work() [1]
> 
> Issue here is that mptcp_schedule_work() schedules a work,
> then gets a refcount on sk->sk_refcnt if the work was scheduled.
> This refcount will be released by mptcp_worker().
> 
> [A] if (schedule_work(...)) {
> [B]     sock_hold(sk);
>         return true;
>     }
> 
> Problem is that mptcp_worker() can run immediately and complete before [B]
> 
> We need instead :
> 
>     sock_hold(sk);
>     if (schedule_work(...))
>         return true;
>     sock_put(sk);

Thank you for having released the syzbot issue with the fix! That's way
easier for us when the fix is provided with the bug report! :-D

The modifications look good to me:

Reviewed-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>

This patch can be applied to -net directly, ideally with:

Cc: stable@...r.kernel.org

(Just for me to be able to track issues with the backports. If it cannot
be added, I can also track it "manually" if preferred.)

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.