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smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=DliSWd+I; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="DliSWd+I" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2C18C1F000FF; Thu, 10 Sep 2026 16:12:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789056741; bh=oi3W/VPNME6kUeLxOeFaZGn5HifZxroNnX6q4H5FTvE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=DliSWd+IS2Qmm0LdVZGL+LgJLMir6UO3MqAvAB/376Tm46Sjy+Xuvm7bEF+wZmYyk Nc6qTL9SMrXo7Bk5fSNnjfbFOO+n4tUuZgALylSfI+4tJVpzgA3E1wWTUaH9PR82Ll iwsB8ssFE3onMNjAQuUNrr6sOTvFSwgB5RYp0N4L2nCzsIhmuQ8HyXK0SNkhX3rit2 gdfDjkP8+qKErBKtWD/A8JsGZqOP5hxjcObQsgfnmRb4orNVY6Rxzyo1IgFFn2H7me G641OGL9C/t3rLN7atlk2ML9nRx26YB3myLEtg4L0nHpkzL24n2SekYQcB95CvyIAA ed7PedeS9ySXg== Message-ID: <2a0f06a0-f8d7-45d4-a495-a80631f90280@kernel.org> Date: Thu, 10 Sep 2026 18:12:18 +0200 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Thunderbird Daily Subject: Re: [PATCH v2] tcp: fastopen: check rsk_drop_req() in tcp_fastopen_create_child() Content-Language: fr To: Jiayuan Chen , Paolo Abeni Cc: netdev@vger.kernel.org, mptcp@lists.linux.dev, Kimi Security Team , Yilin Zhang , Mat Martineau References: <20260903094010.4066892-1-yilinzhang@moonshot.ai> From: Matthieu Baerts Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit Hi Jiayuan, On 10/09/2026 14:15, Jiayuan Chen wrote: > > On 9/10/26 6:13 PM, Matthieu Baerts wrote: >> Hi Paolo, >> >> Thank you for having checked! >> >> On 10/09/2026 10:12, Paolo Abeni wrote: >>> On 9/3/26 11:40 AM, Yilin Zhang wrote: >>>> subflow_syn_recv_sock() sets drop_req when an MP_JOIN SYN takes the >>>> fatal >>>> fallback and destroys the cloned child. tcp_fastopen_create_child() >>>> ignored the flag and could queue the destroyed child. >>>> >>>> With an MPTCP listener using server-side Fast Open, a valid-cookie >>>> MP_JOIN SYN could then expose the freed child through accept(). >>>> >>>> Release the locked child and drop the request before tcp_conn_request() >>>> sends a SYN-ACK. Initialize drop_req when allocating the request so the >>>> check cannot observe stale state after request-socket reuse. >>>> >>>> Changes in v2: >>>> - unlock the child before dropping its reference >>>> - drop the request instead of sending a SYN-ACK after the MPTCP reset, >>>>    as suggested by Jiayuan Chen >>>> >>>> Fixes: 90bf45134d55 ("mptcp: add new sock flag to deal with join >>>> subflows") >>>> Reported-by: Kimi Security Team >>>> Suggested-by: Jiayuan Chen >>>> Signed-off-by: Yilin Zhang >>> This looks like the wrong fix. IIRC fastopen is not compatible with MPJ >>> - as the latter must accept data only after the 4way handshake >>> completion. >>> >>> @Mat(s): could you please double check the above ^^^ statement??? >>> >>> If so mptcp should reject entirely MPJ + fastopen and no addtional code >>> required on the TCP side. >> I didn't check in details, but when I try with this packetdrill repro... >> >>   https://lore.kernel.org/20260903070649.3965366-1-yilinzhang@moonshot.ai >> >> ... it looks like the kernel replies with an MP_RST, but also a SYN+ACK, >> and a WARN: >> >>    refcount_t: underflow; use-after-free. >>    (...) >>    inet_csk_listen_stop (net/ipv4/inet_connection_sock.c:1533) >>    ? mptcp_subflow_queue_clean (net/mptcp/subflow.c:1925) >>    mptcp_check_listen_stop.part.0 (include/net/sock.h:1486 >>                                    net/mptcp/protocol.c:3482) >>    __mptcp_close (net/mptcp/protocol.c:3534) >>    mptcp_close (net/mptcp/protocol.c:3572) >>    inet_release (net/ipv4/af_inet.c:425) >> >> With this v2, I don't see the SYN+ACK, only the RST. >> >> >> If I'm not mistaken, with TFO, syn_recv_sock will be called first >> (tcp_conn_request -> tcp_fastopen_create_child -> subflow_syn_recv_sock) >> then MPTCP will only check TFO when the SYN+ACK is being sent >> (tcp_conn_request -> subflow_v(46)_send_synack). >> >> In subflow_syn_recv_sock(), I don't think we have a strong indicator > > > You inspired me. Maybe we can use such code: > >     if (subflow_req->mp_join && >         (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) >             return NULL; Good idea! Indeed, having a SYN here only happens with TFO. It would be good to add a comment there then. > We now skip RST and SYN+ACK also can be sent by tcp_conn_request(). Yes, I guess returning NULL is not enough, a reset should probably be sent as well (prohibit), a MIB counter incremented (MPJ rejected?), and the request dropped. Just to avoid a deadlock: @Yilin: will you send a v3 with this suggestion? Cheers, Matt -- Sponsored by the NGI0 Core fund.