ceph: fix NULL pointer dereference in ceph_mds_auth_match()

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

ceph: fix NULL pointer dereference in ceph_mds_auth_match()

The CephFS kernel client has regression starting from 6.18-rc1.

We have issue in ceph_mds_auth_match() if fs_name == NULL:

const char fs_name = mdsc->fsc->mount_options->mds_namespace;

...

if (auth->match.fs_name && strcmp(auth->match.fs_name, fs_name)) {

/ fsname mismatch, try next one */

return 0;

}

Patrick Donnelly suggested that: In summary, we should definitely start

decoding `fs_name` from the MDSMap and do strict authorizations checks

against it. Note that the `-o mds_namespace=foo` should only be used for

selecting the file system to mount and nothing else. It's possible

no mds_namespace is specified but the kernel will mount the only

file system that exists which may have name "foo".

This patch reworks ceph_mdsmap_decode() and namespace_equals() with

the goal of supporting the suggested concept. Now struct ceph_mdsmap

contains m_fs_name field that receives copy of extracted FS name

by ceph_extract_encoded_string(). For the case of "old" CephFS file

systems, it is used "cephfs" name.

[ idryomov: replace redundant %*pE with %s in ceph_mdsmap_decode(),

get rid of a series of strlen() calls in ceph_namespace_match(),

drop changes to namespace_equals() body to avoid treating empty

mds_namespace as equal, drop changes to ceph_mdsc_handle_fsmap()

as namespace_equals() isn't an equivalent substitution there ]