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 5028F396B9D; Thu, 10 Sep 2026 10:13:38 +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=1789035219; cv=none; b=aFk3bPR2T9Pa0j9xtC35h6F7bk9apNYY+mY8TtX6JHd5EESLIvYXAL68TiiIM7banQ/3sW5sYuzY9uq7LNPeeHssLgcHn0/TCkalmRo8hencEFIbsB6HFJWTnfIg25FLePaFdKc423BV2ofYo4vzLEDYhsoaVSNim5dUczkFDto= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789035219; c=relaxed/simple; bh=t7XBJTdeA5LYyiKBQx1hBo7BxbXRQJjB0d9EpiSTDYE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=uwpLJZbLpISacoLcPQqdyOLJzCqI4JpVHSReBwYznvi2RSKwaTvxe9hV6yMaD7GC/BKo0Ily6IhM3XLHk+M8836fHU1ksg5/uKL35svAb27OjAqqRkpMDR+6hX4psxkYuUqjhSMrsxIGRrNjqSKtch7UaqFaPFdte1Ayhs1hk8c= ARC-Authentication-Results:i=1; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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 that the caller is TFO. It looks like we would also need to change the TCP stack to pass this info, no? Cheers, Matt -- Sponsored by the NGI0 Core fund.