[PATCH 5.10.y] mptcp: only set DATA_FIN when a mapping is present

From: Michael Bommarito <[email protected]>

[ Upstream commit b2ff91b752b0d85e8815e7f44fd85205c4268094 ]

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: Sasha Levin <[email protected]>
---
 net/mptcp/options.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 8bc8812f75266..a63288ec5af6a 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -142,17 +142,11 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		 */
 		mp_opt->mpc_map = 0;
 		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) {
@@ -163,12 +157,18 @@ 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);
+
 		/* RFC 6824, Section 3.3:
 		 * If a checksum is present, but its use had
 		 * not been negotiated in the MP_CAPABLE handshake,
@@ -300,6 +300,7 @@ void mptcp_get_options(const struct sk_buff *skb,
 	mp_opt->port = 0;
 	mp_opt->rm_addr = 0;
 	mp_opt->dss = 0;
+	mp_opt->data_fin = 0;
 
 	length = (th->doff * 4) - sizeof(struct tcphdr);
 	ptr = (const unsigned char *)(th + 1);
-- 
2.53.0