mptcp: safety check before fallback

JIRA: https://issues.redhat.com/browse/RHEL-115624
Upstream Status: net.git commit db75a16

commit db75a16
Author: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Date:   Mon Feb 24 19:11:52 2025 +0100

    mptcp: safety check before fallback

    Recently, some fallback have been initiated, while the connection was
    not supposed to fallback.

    Add a safety check with a warning to detect when an wrong attempt to
    fallback is being done. This should help detecting any future issues
    quicker.

    Acked-by: Paolo Abeni <pabeni@redhat.com>
    Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
    Link: https://patch.msgid.link/20250224-net-mptcp-misc-fixes-v1-3-f550f636b435@kernel.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Davide Caratti <dcaratti@redhat.com>

Author: dcaratti

net/mptcp/protocol.h

@@ -1175,6 +1175,8 @@ static inline void __mptcp_do_fallback(struct mptcp_sock *msk)
 		pr_debug("TCP fallback already done (msk=%p)\n", msk);
 		return;
 	}
+	if (WARN_ON_ONCE(!READ_ONCE(msk->allow_infinite_fallback)))
+		return;
 	set_bit(MPTCP_FALLBACK_DONE, &msk->flags);
 }