CVE-2026-89653: ceph: reject export_targets ranks >= CEPH_MAX_MDS in mdsmap decode

CVE-2026-89653: ceph: reject export_targets ranks >= CEPH_MAX_MDS in mdsmap decode

[Greg Kroah-Hartman]

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

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

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

ceph: reject export_targets ranks >= CEPH_MAX_MDS in mdsmap decode

MDSMap export_targets entries are monitor controlled. check_new_map()
uses each entry as a bit number in a fixed stack bitmap, so a rank
outside the protocol namespace can make set_bit() write past the end of
the array.

Reject ranks outside CEPH_MAX_MDS while decoding the map. Do not
validate against possible_max_rank here because maps may legitimately
reference ranks beyond a temporarily reduced max_mds.

The Linux kernel CVE team has assigned CVE-2026-89653 to this issue.


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

	Issue introduced in 5.15 with commit d517b3983dd3106ca92d6c5d0d09415a4a09481c and fixed in 6.12.109 with commit 736adee11af36e407ed902264f8b2fb5cf94b62f
	Issue introduced in 5.15 with commit d517b3983dd3106ca92d6c5d0d09415a4a09481c and fixed in 6.18.50 with commit 4d298880f82c42383b36946bafde7ccf4d804c9b
	Issue introduced in 5.15 with commit d517b3983dd3106ca92d6c5d0d09415a4a09481c and fixed in 7.2.4 with commit 96c3f5fbb0d5386e7111426f047f98cec4586674
	Issue introduced in 5.15 with commit d517b3983dd3106ca92d6c5d0d09415a4a09481c and fixed in 7.3-rc1 with commit aedc9053d909508a5f56c3f49f885fc030df4730

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-2026-89653
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:
	fs/ceph/mdsmap.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/736adee11af36e407ed902264f8b2fb5cf94b62f
	https://git.kernel.org/stable/c/4d298880f82c42383b36946bafde7ccf4d804c9b
	https://git.kernel.org/stable/c/96c3f5fbb0d5386e7111426f047f98cec4586674
	https://git.kernel.org/stable/c/aedc9053d909508a5f56c3f49f885fc030df4730