[PATCH net] mptcp: reject a DSS option that follows an incompatible suboption

All file:line references below are against v7.2-rc4 (ac5b0e5651b1). The
KMSAN trace was captured on 7.2.0-rc6-kmsan72rc6 (075b74841bd0), where the
same lines apply.

mptcp_parse_option() writes the DSS status flags before it validates the
option length. For MPTCPOPT_DSS it assigns dsn64, use_map, ack64 and
use_ack at net/mptcp/options.c:160-163, computes expected_opsize at
:165-180, and only then rejects a bad length with a plain break at
:190-192. That break leaves use_map set.

OPTION_MPTCP_DSS is a separate, sticky bit set at :194, and the mapping
fields data_seq, subflow_seq and data_len are written only inside the
use_map branch at :207-220, i.e. after the length check.

mptcp_get_options() clears only the four-byte status group
("*(u32 *)&mp_opt->status = 0", :370). data_seq, subflow_seq and data_len
are declared ahead of struct_group(status, ...) in struct
mptcp_options_received (net/mptcp/protocol.h:145-177), so they are left
uninitialised, and the caller declares "struct mptcp_options_received
mp_opt;" on the stack (:1137).

So a single segment carrying two DSS options -- first a well-formed
ACK32-only DSS, which sets OPTION_MPTCP_DSS but no mapping, then a
truncated DSS whose flags claim a mapping, which sets use_map and then
fails the length check -- ends parsing with OPTION_MPTCP_DSS set and
use_map == 1 while the mapping fields were never written.
mptcp_incoming_options() passes the OPTION_MPTCP_DSS test at :1210 and
copies them into the skb extension at :1253-1258; get_mapping_status()
then branches on the uninitialised data_len:

  BUG: KMSAN: uninit-value in mptcp_subflow_data_available+0x2428/0x4c70
   get_mapping_status (net/mptcp/subflow.c:1152)
   subflow_check_data_avail (net/mptcp/subflow.c:1369)
   mptcp_subflow_data_available (net/mptcp/subflow.c:1466)
   subflow_data_ready
   tcp_data_queue
   tcp_rcv_established
   tcp_v4_do_rcv

  Uninit was stored to memory at:
   mptcp_incoming_options (net/mptcp/options.c:1258)
   tcp_data_queue
   tcp_rcv_established
   tcp_v4_do_rcv

  Local variable mp_opt created at:
   mptcp_incoming_options (net/mptcp/options.c:1137)

  CPU: 0 UID: 1000 PID: 137 Comm: poc 7.2.0-rc6-kmsan72rc6 #1

Reject a DSS option when an incompatible suboption is already present. The
test runs before any DSS flag is read, so a malformed duplicate can no
longer mutate state retained from an earlier valid option. ADD_ADDR,
RM_ADDR, MP_PRIO and MP_FAIL stay permitted alongside DSS: once
mptcp_established_options_dss() has run, those are the only options
mptcp_established_options() can still add (:843-878).

Fixes: 648ef4b88673 ("mptcp: Implement MPTCP receive path")
Cc: stable@kernel.org
Reported-by: TencentOS Corvus AI <redacted>
Assisted-by: tencentos-corvus-ai:kimi-k3
Signed-off-by: Fourie Zhang <redacted>
---
A KMSAN reproducer for this issue is available if requested.

 net/mptcp/options.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index c664023d37ba..7ec18fa1bef6 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c

@@ -153,6 +153,13 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		break;
 
 	case MPTCPOPT_DSS:
+		/* Can be used with a restricted number of other options */
+		if ((mp_opt->suboptions & ~(OPTION_MPTCP_ADD_ADDR |
+					    OPTION_MPTCP_RM_ADDR |
+					    OPTION_MPTCP_PRIO |
+					    OPTION_MPTCP_FAIL)) != 0)
+			break;
+
 		pr_debug("DSS\n");
 		ptr++;
 

-- 
2.43.7