mptcp: ensure snd_una is properly initialized on connect

jira LE-2015
cve CVE-2024-40931
Rebuild_History Non-Buildable kernel-5.14.0-427.42.1.el9_4
commit-author Paolo Abeni <pabeni@redhat.com>
commit 8031b58

This is strictly related to commit fb7a0d3 ("mptcp: ensure snd_nxt
is properly initialized on connect"). It turns out that syzkaller can
trigger the retransmit after fallback and before processing any other
incoming packet - so that snd_una is still left uninitialized.

Address the issue explicitly initializing snd_una together with snd_nxt
and write_seq.

	Suggested-by: Mat Martineau <martineau@kernel.org>
Fixes: 8fd7380 ("mptcp: fallback in case of simultaneous connect")
	Cc: stable@vger.kernel.org
	Reported-by: Christoph Paasch <cpaasch@apple.com>
Closes: multipath-tcp/mptcp_net-next#485
	Signed-off-by: Paolo Abeni <pabeni@redhat.com>
	Reviewed-by: Mat Martineau <martineau@kernel.org>
	Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240607-upstream-net-20240607-misc-fixes-v1-1-1ab9ddfa3d00@kernel.org
	Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit 8031b58)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

net/mptcp/protocol.c

@@ -3691,6 +3691,7 @@ static int mptcp_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 
 	WRITE_ONCE(msk->write_seq, subflow->idsn);
 	WRITE_ONCE(msk->snd_nxt, subflow->idsn);
+	WRITE_ONCE(msk->snd_una, subflow->idsn);
 	if (likely(!__mptcp_check_fallback(msk)))
 		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_MPCAPABLEACTIVE);