[PATCH] mptcp: only honor zero-length DATA_FIN when a mapping is present

[Michael Bommarito <michael.bommarito@gmail.com>]

mptcp_get_options() initializes only the status group of struct
mptcp_options_received; data_seq, subflow_seq and data_len are set by
mptcp_parse_option() only inside the DSS mapping block, which runs when
the DSS M (mapping present) bit is set.

A peer can send a DSS option with DATA_FIN set but the mapping bit clear.
The parser then sets mp_opt.data_fin while leaving data_len and data_seq
uninitialized, and for a zero-length segment mptcp_incoming_options()
reads them; KMSAN reports an uninit-value in mptcp_incoming_options().

Impact: a remote peer that has completed the MPTCP handshake makes
mptcp_incoming_options() read uninitialized data_len and data_seq (KMSAN
uninit-value) by sending a DSS option with DATA_FIN set and the mapping
bit clear.

A DATA_FIN is always sent with a mapping (mptcp_write_data_fin()), so
gating this path on the mapping bit drops only the malformed no-map case
and leaves valid DATA_FIN handling unchanged.

Fixes: 43b54c6ee382 ("mptcp: Use full MPTCP-level disconnect state machine")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
The stale data_seq then reaches mptcp_update_rcv_data_fin(); no further
consequence is demonstrated, so this is a robustness fix. The read site
for a zero-length segment is:

	if (mp_opt.data_fin && mp_opt.data_len == 1 &&
	    mptcp_update_rcv_data_fin(msk, mp_opt.data_seq, mp_opt.dsn64))

Reproduced under KMSAN: a no-map DSS DATA_FIN crafted on the wire and
injected over a TUN device into a real MPTCP listener produces twelve
"uninit-value in mptcp_incoming_options" reports on stock and none on the patched build (the unrelated mm-side
KMSAN boot noise present on both trees is not affected); a well-formed mapped DATA_FIN
(control) drives the same branch with no report. Harness on request.

 net/mptcp/options.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index dff3fd5d3b559..e40efa26a6694 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -1227,7 +1227,7 @@ bool mptcp_incoming_options(struct sock *sk, struct sk_buff *skb)
 	 * present, needs to be updated here before the skb is freed.
 	 */
 	if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
-		if (mp_opt.data_fin && mp_opt.data_len == 1 &&
+		if (mp_opt.use_map && mp_opt.data_fin && mp_opt.data_len == 1 &&
 		    mptcp_update_rcv_data_fin(msk, mp_opt.data_seq, mp_opt.dsn64))
 			mptcp_schedule_work((struct sock *)msk);
 
-- 
2.53.0