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 Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-4-8 Signed-off-by: Michael Bommarito --- 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/ Link to v2: https://lore.kernel.org/all/20260707171730.2679013-1-michael.bommarito@gmail.com/ 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