tcp: check space before adding MPTCP SYN options

commit 06d64ab upstream.

Ensure there is enough space before adding MPTCP options in
tcp_syn_options().

Without this check, 'remaining' could underflow, and causes issues. If
there is not enough space, MPTCP should not be used.

Signed-off-by: MoYuanhao <[email protected]>
Fixes: cec37a6 ("mptcp: Handle MP_CAPABLE options for outgoing connections")
Cc: [email protected]
Acked-by: Matthieu Baerts (NGI0) <[email protected]>
[ Matt: Add Fixes, cc Stable, update Description ]
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: MoYuanhao

net/ipv4/tcp_output.c

@@ -827,8 +827,10 @@ static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
 		unsigned int size;
 
 		if (mptcp_syn_options(sk, skb, &size, &opts->mptcp)) {
-			opts->options |= OPTION_MPTCP;
-			remaining -= size;
+			if (remaining >= size) {
+				opts->options |= OPTION_MPTCP;
+				remaining -= size;
+			}
 		}
 	}