[PATCH v2] ocfs2: drain fs_state readers before journal teardown

[Cen Zhang <zzzccc427@gmail.com>]

The per-mount fs_state open callback formats journal fields through
osb->journal. Its debugfs active-user guard protects the open callback,
but does not keep the journal allocated after journal shutdown.

Both unmount and the mount-error path in ocfs2_mount_volume() can free
and clear the journal before the debugfs subtree is removed:

  fs_state reader                       Mount cleanup
  ---------------                       -------------
  enter ocfs2_osb_debug_open()
    ocfs2_osb_dump()
      load osb->journal
                                        ocfs2_journal_shutdown()
                                          free journal; clear pointer
      read journal fields
  leave the open callback
                                        remove the debugfs subtree

Remove fs_state before cleanup releases its backing state in both
callers of ocfs2_journal_shutdown(). Debugfs removal prevents new opens
and waits for admitted open callbacks to finish. Later reads and release
use only the per-open snapshot buffer, so they do not need the journal.

Keep removal in the caller paths while the per-mount debugfs parent is
still present. The remaining debug files retain their teardown order.

KASAN report as below:

    ==================================================================
    BUG: KASAN: slab-use-after-free in ocfs2_osb_debug_open+0x967/0x15e0
    Read of size 4 at addr ffff888116a56028 by task cat/502

    Call Trace:
     <TASK>
     dump_stack_lvl+0x93/0xd0
     print_report+0xce/0x630
     kasan_report+0xe0/0x110
     kasan_check_range+0x105/0x1b0
     ocfs2_osb_debug_open+0x967/0x15e0
     full_proxy_open_regular+0x193/0x310
     do_dentry_open+0x595/0x12d0
     vfs_open_consume+0xd1/0x400
     path_openat+0x18d0/0x2020
     do_file_open+0x21e/0x470
     do_sys_openat2+0xf4/0x1b0
     __x64_sys_openat+0x136/0x1e0
     do_syscall_64+0x114/0x620
     entry_SYSCALL_64_after_hwframe+0x77/0x7f
     </TASK>

    Allocated by task 495:
     kasan_save_stack+0x33/0x60
     kasan_save_track+0x14/0x30
     __kasan_kmalloc+0xaa/0xb0
     __kmalloc_cache_noprof+0x28d/0x610
     ocfs2_journal_alloc+0x91/0x450
     ocfs2_initialize_super.isra.0+0x11b8/0x3070
     ocfs2_fill_super+0x801/0x38f0
     get_tree_bdev_flags+0x35d/0x5f0
     vfs_get_tree+0x93/0x320
     fc_mount+0x1d/0x200
     path_mount+0x592/0x1e00
     __x64_sys_mount+0x24b/0x2d0
     do_syscall_64+0x114/0x620
     entry_SYSCALL_64_after_hwframe+0x77/0x7f

    Freed by task 504:
     kasan_save_stack+0x33/0x60
     kasan_save_track+0x14/0x30
     kasan_save_free_info+0x3b/0x60
     __kasan_slab_free+0x5f/0x80
     kfree+0x308/0x580
     ocfs2_journal_shutdown+0x10a/0xc20
     ocfs2_dismount_volume+0x315/0x940
     generic_shutdown_super+0x15f/0x490
     kill_block_super+0x40/0xa0
     deactivate_locked_super+0xae/0x120
     deactivate_super+0xa2/0xc0
     cleanup_mnt+0x217/0x450
     task_work_run+0x144/0x230
     exit_to_user_mode_loop+0x17d/0x570
     do_syscall_64+0x4d1/0x620
     entry_SYSCALL_64_after_hwframe+0x77/0x7f

    The buggy address belongs to the object at ffff888116a56000
     which belongs to the cache kmalloc-512 of size 512
    The buggy address is located 40 bytes inside of
     freed 512-byte region [ffff888116a56000, ffff888116a56200)
    ==================================================================

Fixes: da5e7c87827e ("ocfs2: cleanup journal init and shutdown")
Cc: stable@vger.kernel.org
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---
Changes in v2:
- Address the osb dereference before BUG_ON(!osb) raised by Sashiko:
  https://sashiko.dev/#/patchset/98583
- Remove fs_state after BUG_ON(!osb) in ocfs2_dismount_volume().
- Trim the KASAN report.

Assisted-by: LLM

Link to v1: https://lore.kernel.org/r/pm-ocfs2-objects-candidate-0003-v6-2f69dbb7417fcd537be6@gmail.com

 fs/ocfs2/super.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/ocfs2/super.c b/fs/ocfs2/super.c
index 0af2245ae303..c02896e127e5 100644
--- a/fs/ocfs2/super.c
+++ b/fs/ocfs2/super.c
@@ -1783,6 +1783,9 @@ static int ocfs2_mount_volume(struct super_block *sb)
 	return 0;
 
 out_system_inodes:
+	/* Drain fs_state readers before releasing their backing state. */
+	debugfs_lookup_and_remove("fs_state", osb->osb_debug_root);
+
 	if (osb->local_alloc_state == OCFS2_LA_ENABLED)
 		ocfs2_shutdown_local_alloc(osb);
 	/* Drain pending suballoc reclaim work before the journal goes away */
@@ -1812,6 +1815,9 @@ static void ocfs2_dismount_volume(struct super_block *sb, int mnt_err)
 	osb = OCFS2_SB(sb);
 	BUG_ON(!osb);
 
+	/* Drain fs_state readers before releasing their backing state. */
+	debugfs_lookup_and_remove("fs_state", osb->osb_debug_root);
+
 	/* Remove file check sysfs related directories/files,
 	 * and wait for the pending file check operations */
 	ocfs2_filecheck_remove_sysfs(osb);