[PATCH 5.10 01/43] tcp: check space before adding MPTCP SYN options

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

5.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: MoYuanhao <moyuanhao3676@163.com>

commit 06d64ab46f19ac12f59a1d2aa8cd196b2e4edb5b 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 <moyuanhao3676@163.com>
Fixes: cec37a6e41aa ("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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv4/tcp_output.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -823,8 +823,10 @@ static unsigned int tcp_syn_options(stru
 		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;
+			}
 		}
 	}