CVE-2026-89554: mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction

CVE-2026-89554: mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction

[Greg Kroah-Hartman]

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

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

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

mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction

mptcp_token_join_cookie_init_state() restores remote_nonce, local_nonce,
backup, join_id, token and msk from the saved cookie entry when rebuilding
the request socket for a MP_JOIN 4th-ACK handled under SYN cookies, but it
does not restore local_id, even though the SYN path saved it.
subflow_ulp_clone() then reads that uninitialized field and stores it as
the joined subflow's address-ID. Because the request-sock slab is
SLAB_TYPESAFE_BY_RCU and not zeroed on allocation, the value is the stale
byte of a previously freed request socket, which an off-path peer can
influence by sending concurrent MP_JOIN SYNs. This corrupts the path
manager's id-based subflow bookkeeping for the connection.

Restore subflow_req->local_id from the cookie entry, as done for the other
fields.

The Linux kernel CVE team has assigned CVE-2026-89554 to this issue.


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

	Issue introduced in 5.9 with commit 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 and fixed in 6.12.109 with commit 64f2c5dd49b956a542c8c02b8dad5d262a462bba
	Issue introduced in 5.9 with commit 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 and fixed in 6.18.50 with commit 51887ccd88791ddaa8755a7c614fda031ecf7982
	Issue introduced in 5.9 with commit 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 and fixed in 7.2.4 with commit bf19d166337c6488b39cb03eeeffb30a941d6326
	Issue introduced in 5.9 with commit 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 and fixed in 7.3-rc1 with commit b878dfdd12d7a5b8722a78d35e313506140ca3d9

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-2026-89554
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/syncookies.c


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/64f2c5dd49b956a542c8c02b8dad5d262a462bba
	https://git.kernel.org/stable/c/51887ccd88791ddaa8755a7c614fda031ecf7982
	https://git.kernel.org/stable/c/bf19d166337c6488b39cb03eeeffb30a941d6326
	https://git.kernel.org/stable/c/b878dfdd12d7a5b8722a78d35e313506140ca3d9