[PATCH net v2 1/8] mptcp: options: reset DSS fields in case of unexpected size

A remote peer could send a malformed DSS with a wrong size, followed by
another DSS or MPC + Data. In this case, the first suboption will be
ignored, but leaving some fields written, which could lead to
inconsistency or access uninitialized data.

Explicitly reset the fields that could have been modified in case of
unexpected size.

Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260728-net-mptcp-misc-fixes-7-2-rc6-v1-0-f7e2d229159d%40kernel.org?part=1
Fixes: 648ef4b88673 ("mptcp: Implement MPTCP receive path")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Cc: Davide Caratti <redacted>
Cc: Florian Westphal <fw@strlen.de>
Cc: Christoph Paasch <redacted>
Note: Peter Krystad's email address is no longer valid
v2: New. (Clashiko)
---
 net/mptcp/options.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index c664023d37ba..038eca33c6b1 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c

@@ -188,8 +188,14 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		 * RFC 8684 Section 3.3.0 checks later in subflow_data_ready
 		 */
 		if (opsize != expected_opsize &&
-		    opsize != expected_opsize + TCPOLEN_MPTCP_DSS_CHECKSUM)
+		    opsize != expected_opsize + TCPOLEN_MPTCP_DSS_CHECKSUM) {
+			mp_opt->dsn64 = 0;
+			mp_opt->use_map = 0;
+			mp_opt->ack64 = 0;
+			mp_opt->use_ack = 0;
+			mp_opt->data_fin = 0;
 			break;
+		}
 
 		mp_opt->suboptions |= OPTION_MPTCP_DSS;
 		if (mp_opt->use_ack) {

-- 
2.53.0