CVE-2026-68155: libceph: Reject monmaps advertising zero monitors

CVE-2026-68155: libceph: Reject monmaps advertising zero monitors

[Greg Kroah-Hartman]

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

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

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

libceph: Reject monmaps advertising zero monitors

A message of type CEPH_MSG_MON_MAP contains a monmap that is sent from a
monitor to the client. This monmap contains information about the
existing monitors in the cluster. Currently, a monmap indicating that
there are zero monitors in the cluster is treated as valid. However, it
is impossible to have zero monitors in the cluster and still receive a
valid monmap from a monitor. Therefore, such a monmap must be corrupted
and should be treated as invalid. Furthermore, a monmap with a monitor
count of zero can subsequently crash the client when attempting to open
a session with a monitor in __open_session(). This happens because the
"BUG_ON(monc->monmap->num_mon < 1)" assertion in pick_new_mon() is
triggered.

This patch extends a check in ceph_monmap_decode() to also reject
arriving mon_maps with num_mon == 0 rather than only with
num_mon > CEPH_MAX_MON.

[ idryomov: drop "log output for unusual values of num_mon" part ]

The Linux kernel CVE team has assigned CVE-2026-68155 to this issue.


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

	Fixed in 6.6.148 with commit 0591a15815b498be628a937146e44487d599ba33
	Fixed in 6.12.101 with commit cd0d41bc569632eaaeccde9d2a6bc919ec00c407
	Fixed in 6.18.42 with commit e67e8b694872c9bc66996040f9de9242f6236ed9
	Fixed in 7.1.6 with commit 3b249546f59c3d6d3592c10657f82bc3f1faa07c
	Fixed in 7.2-rc5 with commit 40480eee361ed9676b3f844d532ac28b47251634

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-2026-68155
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/ceph/mon_client.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/0591a15815b498be628a937146e44487d599ba33
	https://git.kernel.org/stable/c/cd0d41bc569632eaaeccde9d2a6bc919ec00c407
	https://git.kernel.org/stable/c/e67e8b694872c9bc66996040f9de9242f6236ed9
	https://git.kernel.org/stable/c/3b249546f59c3d6d3592c10657f82bc3f1faa07c
	https://git.kernel.org/stable/c/40480eee361ed9676b3f844d532ac28b47251634