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mptcp: blackhole only if 1st SYN retrans w/o MPC is accepted

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The Fixes commit mentioned this:

> An MPTCP firewall blackhole can be detected if the following SYN
> retransmission after a fallback to "plain" TCP is accepted.

But in fact, this blackhole was detected if any following SYN
retransmissions after a fallback to TCP was accepted.

That's because 'mptcp_subflow_early_fallback()' will set 'request_mptcp'
to 0, and 'mpc_drop' will never be reset to 0 after.

This is an issue, because some not so unusual situations might cause the
kernel to detect a false-positive blackhole, e.g. a client trying to
connect to a server while the network is not ready yet, causing a few
SYN retransmissions, before reaching the end server.

Fixes: 27069e7cb3 ("mptcp: disable active MPTCP in case of blackhole")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Matthieu Baerts (NGI0) 2025-01-29 13:24:32 +01:00 • committed by Paolo Abeni
parent 3effcc04d8
commit e598d8981f
1 changed files with 2 additions and 2 deletions
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4
net/mptcp/ctrl.c
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@ -418,9 +418,9 @@ void mptcp_active_detect_blackhole(struct sock *ssk, bool expired)
MPTCP_INC_STATS(net, MPTCP_MIB_MPCAPABLEACTIVEDROP);
subflow->mpc_drop = 1;
mptcp_subflow_early_fallback(mptcp_sk(subflow->conn), subflow);
} else {
subflow->mpc_drop = 0;
}
} else if (ssk->sk_state == TCP_SYN_SENT) {
subflow->mpc_drop = 0;
}
}