[PATCH 6.1 247/609] mptcp: only set DATA_FIN when a mapping is present

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

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

From: Michael Bommarito <[email protected]>

commit b2ff91b752b0d85e8815e7f44fd85205c4268094 upstream.

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 <[email protected]>
Cc: [email protected]
Signed-off-by: Michael Bommarito <[email protected]>
Reviewed-by: Matthieu Baerts (NGI0) <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 net/mptcp/options.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -152,17 +152,11 @@ static void mptcp_parse_option(const str
 		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) {
@@ -173,12 +167,18 @@ static void mptcp_parse_option(const str
 		}
 
 		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
 		 */