CVE-2026-68155

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

libceph: Reject monmaps advertising zero monitors

A message of type CEPHMSGMON_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 _opensession(). This happens because the "BUGON(monc->monmap->nummon < 1)" assertion in picknewmon() is triggered.

This patch extends a check in cephmonmapdecode() to also reject arriving monmaps with nummon == 0 rather than only with nummon > CEPHMAX_MON.

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