ocfs2/dlm: require a ref for locking_state debugfs open

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

ocfs2/dlm: require a ref for locking_state debugfs open

debug_lockres_open() copies inode->i_private into struct debug_lockres and
debug_lockres_release() later drops that pointer with dlm_put(). That
only works if open successfully pins the struct dlm_ctxt.

Today open calls dlm_grab(dlm) but ignores its return value. Once the
last domain unregister has removed the context from dlm_domains,
dlm_grab() returns NULL, yet open still stores the raw pointer and returns
success. The later release path is outside the debugfs removal barrier,
so it can call dlm_put() after dlm_free_ctxt_mem() has freed the context.
KASAN reports this as a slab-use-after-free in dlm_put() called from
debug_lockres_release().

Fail the open when dlm_grab() cannot acquire the reference and unwind the
seq_file private state before returning. That keeps locking_state from
handing out a file descriptor whose release path does not own the
dlm_ctxt.

The buggy scenario involves two paths, with each column showing the order
within that path:

locking_state debugfs open: last domain unregister:
1. debug_lockres_open() reads 1. dlm_unregister_domain() calls
inode->i_private. dlm_complete_dlm_shutdown().
2. debug_lockres_open() calls 2. shutdown removes the dlm_ctxt from
dlm_grab(dlm) and gets NULL. dlm_domains.
3. open still stores the raw dlm 3. final teardown reaches
pointer in dl->dl_ctxt and dlm_free_ctxt_mem() and frees it.
returns success.
4. debug_lockres_release() later
calls dlm_put(dl->dl_ctxt).

Validation reproduced this kernel report:
KASAN slab-use-after-free in dlm_put+0x82/0x200
RIP: 0033:0x7f4d349bc9e0
The buggy address belongs to the object at ffff888103a3c000 which belongs
to the cache kmalloc-2k of size 2048
The buggy address is located 816 bytes inside of freed 2048-byte region
[ffff888103a3c000, ffff888103a3c800)
Write of size 4
Call trace:
dump_stack_lvl+0x66/0xa0 (?:?)
print_report+0xd0/0x630 (?:?)
dlm_put+0x82/0x200 (?:?)
srso_alias_return_thunk+0x5/0xfbef5 (?:?)
__virt_addr_valid+0x188/0x2f0 (?:?)
kasan_report+0xe4/0x120 (?:?)
kasan_check_range+0x105/0x1b0 (?:?)
debug_lockres_release+0x53/0x80 (fs/ocfs2/dlm/dlmdebug.c:587)
dlm_put+0x9/0x200 (?:?)
debug_lockres_release+0x5c/0x80 (fs/ocfs2/dlm/dlmdebug.c:587)
full_proxy_release+0x67/0x90 (?:?)
__fput+0x1df/0x4b0 (?:?)
do_raw_spin_lock+0x10f/0x1b0 (?:?)
fput_close_sync+0xd2/0x170 (?:?)
__x64_sys_close+0x55/0x90 (?:?)
do_syscall_64+0x10c/0x640 (arch/x86/entry/syscall_64.c:87)
irqentry_exit+0xac/0x6e0 (?:?)
entry_SYSCALL_64_after_hwframe+0x77/0x7f (?:?)
Freed by task stack:
kasan_save_stack+0x33/0x60 (?:?)
kasan_save_track+0x14/0x30 (?:?)
kasan_save_free_info+0x3b/0x60 (?:?)
__kasan_slab_free+0x5f/0x80 (?:?)
kfree+0x30f/0x580 (?:?)
dlm_put+0x1ce/0x200 (?:?)
dlm_unregister_domain+0xf6/0xb30 (?:?)
o2cb_cluster_disconnect+0x6b/0x90 (?:?)
ocfs2_cluster_disconnect+0x41/0x70 (?:?)
ocfs2_dlm_shutdown+0x1c4/0x220 (?:?)
ocfs2_dismount_volume+0x38a/0x550 (?:?)
generic_shutdown_super+0xc3/0x220 (?:?)
kill_block_super+0x29/0x60 (?:?)
deactivate_locked_super+0x66/0xe0 (?:?)
cleanup_mnt+0x13d/0x210 (?:?)
task_work_run+0xfa/0x170 (?:?)
exit_to_user_mode_loop+0xd6/0x430 (?:?)
do_syscall_64+0x3cb/0x640 (arch/x86/entry/syscall_64.c:87)
entry_SYSCALL_64_after_hwframe+0x77/0x7f (?:?)