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

A peer can send a DSS option with the DATA_FIN flag set but the mapping
bit clear. The parser then records mp_opt->data_fin while leaving
data_len and data_seq uninitialized. For a zero-length segment
mptcp_incoming_options() evaluates

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

which reads the uninitialized data_len and data_seq; KMSAN reports an
uninit-value in mptcp_incoming_options(). The stale data_seq can also be
fed into the receive-side DATA_FIN sequence tracking.

Record the DATA_FIN flag only when the DSS option carries a mapping, so
data_fin is never set without data_seq and data_len also being present.
data_fin is part of the status group that mptcp_get_options() clears up
front, so on the no-map path it stays zero and the zero-length DATA_FIN
branch is simply skipped. A DATA_FIN is always transmitted together with
a mapping (mptcp_write_data_fin() sets use_map along with data_seq and
data_len), so legitimate DATA_FIN handling is unaffected.

Move the pr_debug() that logs the parsed DSS flags below the mapping
block, so it reports the final data_fin value instead of the stale one
it would otherwise print before the assignment.

Fixes: 43b54c6ee382 ("mptcp: Use full MPTCP-level disconnect state machine")
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <redacted>
---
v3:
 - Move the pr_debug() DSS flag log below the mapping block so it prints
   the final data_fin value (Gang Yan).  data_fin is now assigned only
   inside the use_map block, so logging it earlier would report a stale
   value.  No functional change to the fix.
v2: adopt Paolo Abeni's suggested approach - do not set mp_opt->data_fin
    at all unless a mapping is present, rather than gating the consumer in
    mptcp_incoming_options() (v1).  data_fin then defaults to the value
    mptcp_get_options() already clears it to (0) on the no-map path, so
    the uninitialized data_len/data_seq are never read.

Link to v1: https://lore.kernel.org/all/20260617215725.1116295-1-michael.bommarito@gmail.com/ (local)
Link to v2: https://lore.kernel.org/all/20260707171730.2679013-1-michael.bommarito@gmail.com/ (local)

 net/mptcp/options.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index dff3fd5d3b559..1b74ca5b6a595 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c

@@ -157,17 +157,11 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		ptr++;
 
 		flags = (*ptr++) & MPTCP_DSS_FLAG_MASK;
-		mp_opt->data_fin = (flags & MPTCP_DSS_DATA_FIN) != 0;
 		mp_opt->dsn64 = (flags & MPTCP_DSS_DSN64) != 0;
 		mp_opt->use_map = (flags & MPTCP_DSS_HAS_MAP) != 0;
 		mp_opt->ack64 = (flags & MPTCP_DSS_ACK64) != 0;
 		mp_opt->use_ack = (flags & MPTCP_DSS_HAS_ACK);
 
-		pr_debug("data_fin=%d dsn64=%d use_map=%d ack64=%d use_ack=%d\n",
-			 mp_opt->data_fin, mp_opt->dsn64,
-			 mp_opt->use_map, mp_opt->ack64,
-			 mp_opt->use_ack);
-
 		expected_opsize = TCPOLEN_MPTCP_DSS_BASE;
 
 		if (mp_opt->use_ack) {

@@ -178,12 +172,18 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		}
 
 		if (mp_opt->use_map) {
+			mp_opt->data_fin = (flags & MPTCP_DSS_DATA_FIN) != 0;
 			if (mp_opt->dsn64)
 				expected_opsize += TCPOLEN_MPTCP_DSS_MAP64;
 			else
 				expected_opsize += TCPOLEN_MPTCP_DSS_MAP32;
 		}
 
+		pr_debug("data_fin=%d dsn64=%d use_map=%d ack64=%d use_ack=%d\n",
+			 mp_opt->data_fin, mp_opt->dsn64,
+			 mp_opt->use_map, mp_opt->ack64,
+			 mp_opt->use_ack);
+
 		/* Always parse any csum presence combination, we will enforce
 		 * RFC 8684 Section 3.3.0 checks later in subflow_data_ready
 		 */

-- 
2.53.0