[PATCH 1/2] libceph: reject an initial monmap with no monitors

[Yogesh Gaur <yogeshgaur.83@gmail.com>]

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.

Reported-by: syzbot+1dbed5969931c19d3eb5@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1dbed5969931c19d3eb5
Fixes: 7b19b4db5add ("ceph: new device mount syntax")
Assisted-by: LLM
Signed-off-by: Yogesh Gaur <yogeshgaur.83@gmail.com>
---
 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)
-- 
2.55.0.windows.5