From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3E13D366541; Fri, 11 Sep 2026 09:33:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789119208; cv=none; b=nCxG/T+6gMuf4ta/SXCYZ4oWLsOSu6gtqGgyHUR10EjobSjQ2TyhshBK2q73qguSqhUCKa0SyKHPAlGedyYx+AauU5ky+4xywIOMszkGPAyhwfvvY3z6wN+vjqSZLcsqss9jty8zY/gT06DR/S2YOw5L6DGw1I7I3PP1S9GDj24= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789119208; c=relaxed/simple; bh=AlzyApuiY64di7nC0zO+/USsKeqgeY+2xmkOaZ77768=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Y+m/FYqVwzkM0gnrm5C1rwE+L5kvDImWldZr6k2ZOb+gp/7FB61uvDzcZFeydBP8ow8zQbVUxhM2PRderW7EO4VVxJ+yVpFeTSObWlyER/voVqNi0ZxYBilHumoIkFGCV59MNIPl+0RRXHrhYk6LbPJFCKJpS/wV3crBP+a2S3I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=XmQbddKo; 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="XmQbddKo" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E052E1F000FF; Fri, 11 Sep 2026 09:33:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789119206; bh=6nBHpxNfGFuIPudhHr2CHsfB5sBUG/Alec2QQ5asWYQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=XmQbddKouLkeWl5MKldThnW5TV06N5gICaSVGJILJqF7NEvLg+6CplQzvo4WWc8P7 LX65QaJ5j7TjES1G02RAhdMV08efLh3dkF8YiUa2S+xjIMa4e8/40khzzVNQyPbmn/ dXdDWAVuA1hQ2eVzWTrG4a0VqVfeHcyXnH8BTX5BCnN1Sa1SCnjhzqHDkN5n7t0aBZ waUeFHHbQQYeDPuOSppWzFbUiYtggAk66DOqwj+fzK/IjH98FHfWOfe9Cox8WPLSqE 8W5hc4URep6bIxzRDklQuEq9wM3WOmt1nHn1qfEE1qPxg/5hN4HLWp7c09K9eVeg8B EweS3AOuhMmpw== Message-ID: <88e537ec-7bf0-45e8-a275-48f80a8b862e@kernel.org> Date: Fri, 11 Sep 2026 11:33:22 +0200 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Beta Subject: Re: [PATCH v2] tcp: fastopen: check rsk_drop_req() in tcp_fastopen_create_child() 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> <2a0f06a0-f8d7-45d4-a495-a80631f90280@kernel.org> <845b1b97-706f-42f8-b08c-90e863edef0a@linux.dev> From: Matthieu Baerts Content-Language: fr Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit Hi Jiayuan, On 11/09/2026 04:25, Jiayuan Chen wrote: > > On 9/11/26 12:12 AM, Matthieu Baerts wrote: >> 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. > > > Should we accept this subflow ? > > It's just a SYN with MPJ + valid fastopen cookie, replying SYNACK > and fallback to 3-way handshake may be easier. Yes, better indeed. But then I guess we should not send a TFO cookie in the SYNACK for this case, right? Setting foc->len = -1 for MPJ in subflow_prep_synack()? > (I'm not sure whether RFC define it or not.) I don't think the MPTCP one mentions this case. Cheers, Matt -- Sponsored by the NGI0 Core fund.