tcp: check space before adding MPTCP SYN options

JIRA: https://issues.redhat.com/browse/RHEL-115628

Upstream commit:
commit 06d64ab
Author: MoYuanhao <moyuanhao3676@163.com>
Date:   Mon Dec 9 13:28:14 2024 +0100

    tcp: check space before adding MPTCP SYN options

    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 <moyuanhao3676@163.com>
    Fixes: cec37a6 ("mptcp: Handle MP_CAPABLE options for outgoing connections")
    Cc: stable@vger.kernel.org
    Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
    [ Matt: Add Fixes, cc Stable, update Description ]
    Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
    Reviewed-by: Eric Dumazet <edumazet@google.com>
    Link: https://patch.msgid.link/20241209-net-mptcp-check-space-syn-v1-1-2da992bb6f74@kernel.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Paolo Abeni

net/ipv4/tcp_output.c

@@ -828,8 +828,10 @@ 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;
+			}
 		}
 	}