[PATCH v2] ocfs2: Publish the filecheck owner before adding sysfs attributes

[Cen Zhang <zzzccc427@gmail.com>]

ocfs2_filecheck_create_sysfs() initializes the filecheck allocation
before calling kobject_init_and_add(), but assigns entry->fs_fcheck
only after that call returns. The default attribute group can already
be visible while registration is in progress. A concurrent read can
therefore enter ocfs2_filecheck_attr_show() with a NULL fs_fcheck and
try to lock its fc_lock.

Assign the initialized owner before adding the kobject, so every
activated attribute has valid state. Clear the pointer when addition
fails, preserving the existing teardown guard and error cleanup.

A controlled probe-instrumented test kernel with the original OCFS2
registration ordering produced this Oops during a concurrent sysfs read:

  KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
  RIP: 0010:kasan_byte_accessible+0x15/0x30
  Call Trace:
   <TASK>
   __kasan_check_byte+0x13/0x50
   lock_acquire+0x113/0x300
   _raw_spin_lock+0x31/0x40
   ocfs2_filecheck_attr_show+0x138/0x5f0
   ocfs2_filecheck_show+0x5d/0x90
   sysfs_kf_seq_show+0x1d3/0x360
   seq_read_iter+0x480/0x1110
   kernfs_fop_read_iter+0x419/0x5a0
   vfs_read+0x7d2/0xc20
   ksys_read+0x111/0x200
   do_syscall_64+0x114/0x620
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>
  Modules linked in:

Fixes: 5f483c4abb50 ("ocfs2: add kobject for online file check")
Assisted-by: LLM
Cc: <stable@vger.kernel.org>
Reviewed-by: Heming Zhao <heming.zhao@suse.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>

---
Changes in v2:
- Explain why fs_fcheck must be cleared on registration failure.
- Retain that cleanup guard and add an inline error-unwind comment.
- Rebase on 35eb6a2f; keep the original diagnostic unchanged.

Link to v1: https://lore.kernel.org/r/pm-ocfs2-objects-candidate-0086-v2-646731c4254a02823b4d@gmail.com

 fs/ocfs2/filecheck.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/ocfs2/filecheck.c b/fs/ocfs2/filecheck.c
index be713d7d24a76b9aa8fcded3d83b3d91d124eb6e..75334e4bd0823fd8154717290bfe57d2e5198176 100644
--- a/fs/ocfs2/filecheck.c
+++ b/fs/ocfs2/filecheck.c
@@ -179,17 +179,20 @@ int ocfs2_filecheck_create_sysfs(struct ocfs2_super *osb)
 	fcheck->fc_size = 0;
 	fcheck->fc_done = 0;
 
+	/* Publish the initialized owner before activating its sysfs callbacks. */
+	entry->fs_fcheck = fcheck;
 	entry->fs_kobj.kset = osb->osb_dev_kset;
 	init_completion(&entry->fs_kobj_unregister);
 	ret = kobject_init_and_add(&entry->fs_kobj, &ocfs2_ktype_filecheck,
 					NULL, "filecheck");
 	if (ret) {
+		/* Mount failure still calls ocfs2_filecheck_remove_sysfs(). */
+		entry->fs_fcheck = NULL;
 		kobject_put(&entry->fs_kobj);
 		kfree(fcheck);
 		return ret;
 	}
 
-	entry->fs_fcheck = fcheck;
 	return 0;
 }

Fixes: a860f6eb4c6a ("ocfs2: sysfile interfaces for online file check")