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mptcp: do not use Fast Open on MP_JOIN subflows

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tcp_fastopen_create_child() hands the SYN packet itself to
subflow_syn_recv_sock(). For an MP_JOIN request this takes the
fatal fallback: the cloned child is destroyed and handed back with
drop_req flagged, but tcp_fastopen_create_child() does not check
the flag and queues it, so accept() can expose the freed child.

MP_JOIN cannot use Fast Open: data on a subflow requires the
completed HMAC exchange (RFC 8684, sec. 3.2). Refuse it (only the
TFO path passes a SYN skb here) and let tcp_conn_request() fall
back to the regular MP_JOIN handshake; also strip the Fast Open
cookie from MP_JOIN SYN/ACKs.

Fixes: 90bf45134d ("mptcp: add new sock flag to deal with join subflows")
Cc: stable@vger.kernel.org
Reported-by: Kimi Security Team <bug-report@moonshot.ai>
Suggested-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Yilin Zhang <yilinzhang@moonshot.ai>
[ Moving code to avoid clearing TS and consolidating checks. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20261030-mptcp-tfo-mpj-v4-1-e6eff2027342@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Yilin Zhang 2026-10-30 23:03:51 +01:00 committed by Paolo Abeni
parent 8e069fd3fd
commit 6d2cf411ab
1 changed files with 11 additions and 0 deletions
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11
net/mptcp/subflow.c
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@ -379,6 +379,10 @@ static void subflow_prep_synack(const struct sock *sk, struct request_sock *req,
struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
struct inet_request_sock *ireq = inet_rsk(req);
/* MP_JOIN cannot use TFO, do not send a cookie in the SYN/ACK */
if (foc && mptcp_subflow_rsk(req)->mp_join)
foc->len = -1;
/* clear tstamp_ok, as needed depending on cookie */
if (foc && foc->len > -1)
ireq->tstamp_ok = 0;
@ -887,6 +891,13 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
fallback = true;
} else if (subflow_req->mp_join) {
/* a SYN skb here comes from TFO, which MP_JOIN cannot use: just
* fall back to the regular path.
*/
if ((TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_ACK)) ==
TCPHDR_SYN)
return NULL;
mptcp_get_options(skb, &mp_opt);
if (!(mp_opt.suboptions & OPTION_MPTCP_MPJ_ACK))
fallback = true;