CVE-2026-43455: mctp: route: hold key->lock in mctp_flow_prepare_output()

CVE-2026-43455: mctp: route: hold key->lock in mctp_flow_prepare_output()

[Greg Kroah-Hartman]

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

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

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

mctp: route: hold key->lock in mctp_flow_prepare_output()

mctp_flow_prepare_output() checks key->dev and may call
mctp_dev_set_key(), but it does not hold key->lock while doing so.

mctp_dev_set_key() and mctp_dev_release_key() are annotated with
__must_hold(&key->lock), so key->dev access is intended to be
serialized by key->lock. The mctp_sendmsg() transmit path reaches
mctp_flow_prepare_output() via mctp_local_output() -> mctp_dst_output()
without holding key->lock, so the check-and-set sequence is racy.

Example interleaving:

  CPU0                                  CPU1
  ----                                  ----
  mctp_flow_prepare_output(key, devA)
    if (!key->dev)  // sees NULL
                                        mctp_flow_prepare_output(
                                            key, devB)
                                          if (!key->dev)  // still NULL
                                          mctp_dev_set_key(devB, key)
                                            mctp_dev_hold(devB)
                                            key->dev = devB
    mctp_dev_set_key(devA, key)
      mctp_dev_hold(devA)
      key->dev = devA   // overwrites devB

Now both devA and devB references were acquired, but only the final
key->dev value is tracked for release. One reference can be lost,
causing a resource leak as mctp_dev_release_key() would only decrease
the reference on one dev.

Fix by taking key->lock around the key->dev check and
mctp_dev_set_key() call.

The Linux kernel CVE team has assigned CVE-2026-43455 to this issue.


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

	Issue introduced in 5.16 with commit 67737c457281dd199ceb9e31b6ba7efd3bfe566d and fixed in 6.1.167 with commit 47893166bc5611ee9a20de6b8d2933b2320fb772
	Issue introduced in 5.16 with commit 67737c457281dd199ceb9e31b6ba7efd3bfe566d and fixed in 6.6.130 with commit 86f5334fcb48a5b611c33364ab52ca684d0f6d91
	Issue introduced in 5.16 with commit 67737c457281dd199ceb9e31b6ba7efd3bfe566d and fixed in 6.12.78 with commit 0695712f3a6f1a48915f95767cfb42077683dcdc
	Issue introduced in 5.16 with commit 67737c457281dd199ceb9e31b6ba7efd3bfe566d and fixed in 6.18.19 with commit 925a5ffd99cddd7a7e41d5ad120c7a2c6d50260f
	Issue introduced in 5.16 with commit 67737c457281dd199ceb9e31b6ba7efd3bfe566d and fixed in 6.19.9 with commit 8d27d9b260dd19c1b519e1a13de6448f9984e30e
	Issue introduced in 5.16 with commit 67737c457281dd199ceb9e31b6ba7efd3bfe566d and fixed in 7.0 with commit 7d86aa41c073c4e7eb75fd2e674f1fd8f289728a

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-2026-43455
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/mctp/route.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/47893166bc5611ee9a20de6b8d2933b2320fb772
	https://git.kernel.org/stable/c/86f5334fcb48a5b611c33364ab52ca684d0f6d91
	https://git.kernel.org/stable/c/0695712f3a6f1a48915f95767cfb42077683dcdc
	https://git.kernel.org/stable/c/925a5ffd99cddd7a7e41d5ad120c7a2c6d50260f
	https://git.kernel.org/stable/c/8d27d9b260dd19c1b519e1a13de6448f9984e30e
	https://git.kernel.org/stable/c/7d86aa41c073c4e7eb75fd2e674f1fd8f289728a