[PATCH 5.10] libceph: Reject monmaps advertising zero monitors

[Roman Demidov <roman.demidov.nn@gmail.com>]

From: Raphael Zimmer <raphael.zimmer@tu-ilmenau.de>

commit 40480eee361ed9676b3f844d532ac28b47251634 upstream.

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 ]

Signed-off-by: Raphael Zimmer <raphael.zimmer@tu-ilmenau.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Roman Demidov <roman.demidov.nn@gmail.com>
---
Backport fix for CVE-2026-68155

 net/ceph/mon_client.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ceph/mon_client.c b/net/ceph/mon_client.c
index e1f01aef263e..fac52fe7a587 100644
--- a/net/ceph/mon_client.c
+++ b/net/ceph/mon_client.c
@@ -59,7 +59,7 @@ static struct ceph_monmap *ceph_monmap_decode(void *p, void *end)
 
 	num_mon = ceph_decode_32(&p);
 
-	if (num_mon > CEPH_MAX_MON)
+	if (num_mon == 0 || num_mon > CEPH_MAX_MON)
 		goto bad;
 	m = kmalloc(struct_size(m, mon_inst, num_mon), GFP_NOFS);
 	if (m == NULL)
-- 
2.53.0