mptcp: ensure snd_una is properly initialized on connect

commit 8031b58c3a9b1db3ef68b3bd749fbee2e1e1aaa3 upstream.

This is strictly related to commit fb7a0d334894 ("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 <[email protected]>
Fixes: 8fd7380 ("mptcp: fallback in case of simultaneous connect")
Cc: [email protected]
Reported-by: Christoph Paasch <[email protected]>
Closes: multipath-tcp/mptcp_net-next#485
Signed-off-by: Paolo Abeni <[email protected]>
Reviewed-by: Mat Martineau <[email protected]>
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
Link: https://lore.kernel.org/r/20240607-upstream-net-20240607-misc-fixes-v1-1-1ab9ddfa3d00@kernel.org
Signed-off-by: Jakub Kicinski <[email protected]>
[ Conflicts in protocol.c, similar to the ones from commit 99951b62bf20
  ("mptcp: ensure snd_nxt is properly initialized on connect"), with the
  same resolution. Note that in this version, 'snd_una' is an atomic64
  type, so use atomic64_set() instead, as it is done everywhere else. ]
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

net/mptcp/protocol.c

@@ -2649,6 +2649,7 @@ static int mptcp_stream_connect(struct socket *sock, struct sockaddr *uaddr,
 		mptcp_subflow_early_fallback(msk, subflow);
 
 	WRITE_ONCE(msk->write_seq, subflow->idsn);
+	atomic64_set(&msk->snd_una, msk->write_seq);
 
 do_connect:
 	err = ssock->ops->connect(ssock, uaddr, addr_len, flags);