CVE-2026-80587: mptcp: avoid combining some incoming suboptions

CVE-2026-80587: mptcp: avoid combining some incoming suboptions

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

mptcp: avoid combining some incoming suboptions

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 with a
small fix: an MP_FASTCLOSE can be used with a DSS when the sender picks
this option [1], which is not the case on Linux. Here are the rules:

  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   |  P   |  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 few points of attention:

 - In theory, an MP_CAPABLE 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.

 - Now, combining both an MP_CAPABLE and an MP_JOIN will no longer
   result to a reject of the two options, but only the second suboption
   is ignored. That seems OK to do that for this unexpected error. At
   least now all inconsistent combinations are handled the same way.
   This could change later in next. This also means the explicit checks
   for having both MPC + MPJ in subflow.c will now be unreachable.
   That's fine, they will be removed in a follow-up patch.

 - In case of conflicting combinations, the extra suboption(s) is/are
   ignored: having such combinations either means the remote peer is
   buggy, or is evil. The simplest action is then taken in this case:
   stop processing the current suboption.

 - 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.

 - An MP_CAPABLE ACK can carry a Data-Level Length, and an optional
   Checksum: they are the same as the ones found in a DSS, because a DSS
   cannot be used in parallel to an MP_CAPABLE. Similarly, even if there
   is room, a DSS cannot be used with an MP_JOIN.

The Linux kernel CVE team has assigned CVE-2026-80587 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.6 with commit eda7acddf8080bb2d022a8d4b8b2345eb80c63ec and fixed in 6.6.153 with commit dc1d8d3eb345c616fbe922a010fa391c72c54d52
	Issue introduced in 5.6 with commit eda7acddf8080bb2d022a8d4b8b2345eb80c63ec and fixed in 6.12.105 with commit 099bfcbd0c16ae9b50aba2a1bea033e63f895da7
	Issue introduced in 5.6 with commit eda7acddf8080bb2d022a8d4b8b2345eb80c63ec and fixed in 6.18.46 with commit a04dcc784959e4702048785d87e0d029bd2fbdcb
	Issue introduced in 5.6 with commit eda7acddf8080bb2d022a8d4b8b2345eb80c63ec and fixed in 7.1.10 with commit 6bab907292155513af397a12ccb488acbfc30d79
	Issue introduced in 5.6 with commit eda7acddf8080bb2d022a8d4b8b2345eb80c63ec and fixed in 7.2 with commit b6ee361524641f57b2e2363f7737f20e17f67827

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-80587
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/mptcp/options.c
	net/mptcp/protocol.h


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/dc1d8d3eb345c616fbe922a010fa391c72c54d52
	https://git.kernel.org/stable/c/099bfcbd0c16ae9b50aba2a1bea033e63f895da7
	https://git.kernel.org/stable/c/a04dcc784959e4702048785d87e0d029bd2fbdcb
	https://git.kernel.org/stable/c/6bab907292155513af397a12ccb488acbfc30d79
	https://git.kernel.org/stable/c/b6ee361524641f57b2e2363f7737f20e17f67827