Some MPTCP suboptions are mutually exclusive according to the RFC8684,
but also because in different places, the code doesn't expect some
combinations to be present. That's specially true for suboptions that
would be present twice, but with different attributes.

The new restrictions are the same as the ones applied on the output
side, with mptcp_write_options. The same rules can be reused:

  Which options can be used together?

  X: mutually exclusive
  O: often used together
  C: can be used together in some cases
  P: could be used together but we prefer not to (optimisations)

  | Opt: | MPC  | MPJ  | DSS  | ADD  |  RM  | PRIO | FAIL |  FC  |
  |------|------|------|------|------|------|------|------|------|
  | MPC  |------|------|------|------|------|------|------|------|
  | MPJ  |  X   |------|------|------|------|------|------|------|
  | DSS  |  X   |  X   |------|------|------|------|------|------|
  | ADD  |  X   |  X   |  P   |------|------|------|------|------|
  | RM   |  C   |  C   |  C   |  P   |------|------|------|------|
  | PRIO |  X   |  C   |  C   |  C   |  C   |------|------|------|
  | FAIL |  X   |  X   |  C   |  X   |  X   |  X   |------|------|
  | FC   |  X   |  X   |  X   |  X   |  X   |  X   |  X   |------|
  | RST  |  X   |  X   |  X   |  X   |  X   |  X   |  O   |  O   |
  |------|------|------|------|------|------|------|------|------|

The only difference is with the 'P': another stack could send and
ADD_ADDR with other suboptions (DSS, RM_ADDR), and this should be
allowed.

A point of attention is with the MP_CAPABLE: it could be used with a
RM_ADDR, but there is no reason to add it with a SYN. Note that even
with a 4th ACK, it doesn't seem to be useful, except when IDs are known
in advance via another channel. Better not to break that.

Also, in mp_opt->suboptions, there is also a bit reserved to the
checksum, which can be used in an MP_CAPABLE and a DSS. Each time a DSS
option can be used in parallel with another option, the checksum can be
set, so the verification is combined into a new OPTIONS_MPTCP_DSS macro.

Fixes: eda7acddf808 ("mptcp: Handle MPTCP TCP options")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Changes in v2:
- Take into account OPTION_MPTCP_CSUMREQD. (Sashiko)
- Link to v1: https://patch.msgid.link/20260709-mptcp-harden-combine-opt-v1-1-378b0a47c1b5@kernel.org
---
 net/mptcp/options.c  | 55 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 net/mptcp/protocol.h |  1 +
 2 files changed, 56 insertions(+)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index d78f55df538a..d6b009319839 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -50,6 +50,14 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 			}
 		}
 
+		/* Only the MPC + ACK can be used with a RM_ADDR */
+		if (subopt == OPTION_MPTCP_MPC_ACK) {
+			if ((mp_opt->suboptions & ~OPTION_MPTCP_RM_ADDR) != 0)
+				break;
+		} else if (mp_opt->suboptions != 0) {
+			break;
+		}
+
 		/* Cfr RFC 8684 Section 3.3.0:
 		 * If a checksum is present but its use had
 		 * not been negotiated in the MP_CAPABLE handshake, the receiver MUST
@@ -122,6 +130,11 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		break;
 
 	case MPTCPOPT_MP_JOIN:
+		/* Can be used with a restricted number of other options */
+		if ((mp_opt->suboptions & ~(OPTION_MPTCP_RM_ADDR |
+					    OPTION_MPTCP_PRIO)) != 0)
+			break;
+
 		if (opsize == TCPOLEN_MPTCP_MPJ_SYN) {
 			mp_opt->suboptions |= OPTION_MPTCP_MPJ_SYN;
 			mp_opt->backup = *ptr++ & MPTCPOPT_BACKUP;
@@ -153,6 +166,13 @@ 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++;
 
@@ -234,6 +254,12 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		break;
 
 	case MPTCPOPT_ADD_ADDR:
+		/* Can be used with a restricted number of other options */
+		if ((mp_opt->suboptions & ~(OPTIONS_MPTCP_DSS |
+					    OPTION_MPTCP_RM_ADDR |
+					    OPTION_MPTCP_PRIO)) != 0)
+			break;
+
 		mp_opt->echo = (*ptr++) & MPTCP_ADDR_ECHO;
 		if (!mp_opt->echo) {
 			if (opsize == TCPOLEN_MPTCP_ADD_ADDR ||
@@ -293,6 +319,14 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		break;
 
 	case MPTCPOPT_RM_ADDR:
+		/* Can be used with a restricted number of other options */
+		if ((mp_opt->suboptions & ~(OPTION_MPTCP_MPC_ACK |
+					    OPTIONS_MPTCP_MPJ |
+					    OPTIONS_MPTCP_DSS |
+					    OPTION_MPTCP_ADD_ADDR |
+					    OPTION_MPTCP_PRIO)) != 0)
+			break;
+
 		if (opsize < TCPOLEN_MPTCP_RM_ADDR_BASE + 1 ||
 		    opsize > TCPOLEN_MPTCP_RM_ADDR_BASE + MPTCP_RM_IDS_MAX)
 			break;
@@ -307,6 +341,13 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		break;
 
 	case MPTCPOPT_MP_PRIO:
+		/* Can be used with a restricted number of other options */
+		if ((mp_opt->suboptions & ~(OPTIONS_MPTCP_MPJ |
+					    OPTIONS_MPTCP_DSS |
+					    OPTION_MPTCP_ADD_ADDR |
+					    OPTION_MPTCP_RM_ADDR)) != 0)
+			break;
+
 		if (opsize != TCPOLEN_MPTCP_PRIO)
 			break;
 
@@ -316,6 +357,10 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		break;
 
 	case MPTCPOPT_MP_FASTCLOSE:
+		/* Can be used only with RST */
+		if ((mp_opt->suboptions & ~OPTION_MPTCP_RST) != 0)
+			break;
+
 		if (opsize != TCPOLEN_MPTCP_FASTCLOSE)
 			break;
 
@@ -327,6 +372,11 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		break;
 
 	case MPTCPOPT_RST:
+		/* Can be used with a restricted number of other options */
+		if ((mp_opt->suboptions & ~(OPTION_MPTCP_FAIL |
+					    OPTION_MPTCP_FASTCLOSE)) != 0)
+			break;
+
 		if (opsize != TCPOLEN_MPTCP_RST)
 			break;
 
@@ -342,6 +392,11 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		break;
 
 	case MPTCPOPT_MP_FAIL:
+		/* Can be used with a restricted number of other options */
+		if ((mp_opt->suboptions & ~(OPTIONS_MPTCP_DSS |
+					    OPTION_MPTCP_RST)) != 0)
+			break;
+
 		if (opsize != TCPOLEN_MPTCP_FAIL)
 			break;
 
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index da40c6f3705f..c7ead31bade5 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -37,6 +37,7 @@
 				 OPTION_MPTCP_MPC_ACK)
 #define OPTIONS_MPTCP_MPJ	(OPTION_MPTCP_MPJ_SYN | OPTION_MPTCP_MPJ_SYNACK | \
 				 OPTION_MPTCP_MPJ_ACK)
+#define OPTIONS_MPTCP_DSS	(OPTION_MPTCP_DSS | OPTION_MPTCP_CSUMREQD)
 
 /* MPTCP option subtypes */
 #define MPTCPOPT_MP_CAPABLE	0

---
base-commit: f984839d1257b5a7d840038d8a92ef7b08301779
change-id: 20260709-mptcp-harden-combine-opt-da101e4c5247

Best regards,
--  
Matthieu Baerts (NGI0) <matttbe@kernel.org>

Hi Matthieu,

Thank you for your modifications, that's great!

Our CI did some validations and here is its report:

- KVM Validation: normal (except selftest_mptcp_join): Success! ✅
- KVM Validation: normal (only selftest_mptcp_join): Success! ✅
- KVM Validation: debug (except selftest_mptcp_join): Success! ✅
- KVM Validation: debug (only selftest_mptcp_join): Success! ✅
- KVM Validation: btf-normal (only bpftest_all): Success! ✅
- KVM Validation: btf-debug (only bpftest_all): Success! ✅
- Task: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/29876515722

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/b327335614d4
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1131982


If there are some issues, you can reproduce them using the same environment as
the one used by the CI thanks to a docker image, e.g.:

    $ cd [kernel source code]
    $ docker run -v "${PWD}:${PWD}:rw" -w "${PWD}" --privileged --rm -it \
        --pull always mptcp/mptcp-upstream-virtme-docker:latest \
        auto-normal

For more details:

    https://github.com/multipath-tcp/mptcp-upstream-virtme-docker


Please note that despite all the efforts that have been already done to have a
stable tests suite when executed on a public CI like here, it is possible some
reported issues are not due to your modifications. Still, do not hesitate to
help us improve that ;-)

Cheers,
MPTCP GH Action bot
Bot operated by Matthieu Baerts (NGI0 Core)