From 94dbce2ae4fdb57bdb53ffa3456da777a5cfb981 Mon Sep 17 00:00:00 2001 From: Yogesh Gaur Date: Fri, 11 Sep 2026 19:01:09 +0530 Subject: [PATCH] libceph: reject an initial monmap with no monitors build_initial_monmap() takes the monitor count straight from ceph_options::num_mon and builds a monmap with that many entries without checking that there is at least one. ceph_parse_ips() writes *count only after it has parsed an address successfully, so a ceph_options that went through a failed parse still has num_mon == 0 and the resulting monmap has no monitors in it at all. The first thing the client does with that monmap is open a session, and pick_new_mon() asserts the invariant that it is non-empty: kernel BUG at net/ceph/mon_client.c:211! RIP: 0010:pick_new_mon net/ceph/mon_client.c:211 [inline] RIP: 0010:__open_session+0x60f/0x740 net/ceph/mon_client.c:245 Call Trace: ceph_monc_open_session+0x46/0x60 net/ceph/mon_client.c:534 __ceph_open_session+0x135/0x590 net/ceph/ceph_common.c:798 ceph_real_mount fs/ceph/super.c:1173 [inline] ceph_get_tree+0xd98/0x1fd0 fs/ceph/super.c:1362 vfs_get_tree+0x92/0x320 fs/super.c:1947 vfs_cmd_create+0xd7/0x2a0 fs/fsopen.c:231 __do_sys_fsconfig+0x55a/0xcb0 fs/fsopen.c:463 The reproducer gets there through the new device syntax: an fsconfig() mon_addr= that does not resolve ("libceph: resolve '127.0.0.' (ret=-3): failed") leaves fsopt->mon_addr set but num_mon at 0, so the "No monitor address" check in ceph_get_tree() passes and the mount proceeds with an empty monmap. The next patch stops that string from being kept, but the assertion should not be reachable from a mount option in the first place. Commit 40480eee361e ("libceph: Reject monmaps advertising zero monitors") made the same BUG_ON() unreachable from the wire by rejecting num_mon == 0 in ceph_monmap_decode(). Apply the same rule to the initial monmap, which is the only other way one gets built, and fail the mount with -EINVAL rather than assert afterwards. Cc: stable@vger.kernel.org Fixes: 7b19b4db5add ("ceph: new device mount syntax") Reported-by: syzbot+1dbed5969931c19d3eb5@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=1dbed5969931c19d3eb5 Assisted-by: Cursor Signed-off-by: Yogesh Gaur Reviewed-by: Alex Markuze Signed-off-by: Ilya Dryomov --- net/ceph/mon_client.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/net/ceph/mon_client.c b/net/ceph/mon_client.c index c56457378d00..bcbcf47506c9 100644 --- a/net/ceph/mon_client.c +++ b/net/ceph/mon_client.c @@ -1141,6 +1141,9 @@ static int build_initial_monmap(struct ceph_mon_client *monc) int num_mon = opt->num_mon; int i; + if (num_mon < 1 || num_mon > CEPH_MAX_MON) + return -EINVAL; + /* build initial monmap */ monc->monmap = kzalloc_flex(*monc->monmap, mon_inst, num_mon); if (!monc->monmap)