[PATCH 6.4 259/292] mptcp: do not rely on implicit state check in mptcp_listen()

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From: Paolo Abeni <pabeni@xxxxxxxxxx>

commit 0226436acf2495cde4b93e7400e5a87305c26054 upstream.

Since the blamed commit, closing the first subflow resets the first
subflow socket state to SS_UNCONNECTED.

The current mptcp listen implementation relies only on such
state to prevent touching not-fully-disconnected sockets.

Incoming mptcp fastclose (or paired endpoint removal) unconditionally
closes the first subflow.

All the above allows an incoming fastclose followed by a listen() call
to successfully race with a blocking recvmsg(), potentially causing the
latter to hit a divide by zero bug in cleanup_rbuf/__tcp_select_window().

Address the issue explicitly checking the msk socket state in
mptcp_listen(). An alternative solution would be moving the first
subflow socket state update into mptcp_disconnect(), but in the long
term the first subflow socket should be removed: better avoid relaying
on it for internal consistency check.

Fixes: b29fcfb54cd7 ("mptcp: full disconnect implementation")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Christoph Paasch <cpaasch@xxxxxxxxx>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/414
Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
Reviewed-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
Signed-off-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/mptcp/protocol.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3697,6 +3697,11 @@ static int mptcp_listen(struct socket *s
 	pr_debug("msk=%p", msk);
 
 	lock_sock(sk);
+
+	err = -EINVAL;
+	if (sock->state != SS_UNCONNECTED || sock->type != SOCK_STREAM)
+		goto unlock;
+
 	ssock = __mptcp_nmpc_socket(msk);
 	if (IS_ERR(ssock)) {
 		err = PTR_ERR(ssock);





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