pstore: inode: Only d_invalidate() is needed

Unloading a modular pstore backend with records in pstorefs would
trigger the dput() double-drop warning:

  WARNING: CPU: 0 PID: 2569 at fs/dcache.c:762 dput.part.0+0x3f3/0x410

Using the combo of d_drop()/dput() (as mentioned in
Documentation/filesystems/vfs.rst) isn't the right approach here, and
leads to the reference counting problem seen above. Use d_invalidate()
and update the code to not bother checking for error codes that can
never happen.
Suggested-by: Alexander Viro <viro@zeniv.linux.org.uk>
Fixes: 609e28bb ("pstore: Remove filesystem records when backend is unregistered")
Signed-off-by: Kees Cook <keescook@chromium.org>
---
Cc: "Guilherme G. Piccoli" <gpiccoli@igalia.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-hardening@vger.kernel.org

fs/pstore/inode.c

@@ -307,7 +307,6 @@ int pstore_put_backend_records(struct pstore_info *psi)
 {
 	struct pstore_private *pos, *tmp;
 	struct dentry *root;
-	int rc = 0;
 
 	root = psinfo_lock_root();
 	if (!root)
@@ -317,11 +316,8 @@ int pstore_put_backend_records(struct pstore_info *psi)
 		list_for_each_entry_safe(pos, tmp, &records_list, list) {
 			if (pos->record->psi == psi) {
 				list_del_init(&pos->list);
-				rc = simple_unlink(d_inode(root), pos->dentry);
-				if (WARN_ON(rc))
-					break;
-				d_drop(pos->dentry);
-				dput(pos->dentry);
+				d_invalidate(pos->dentry);
+				simple_unlink(d_inode(root), pos->dentry);
 				pos->dentry = NULL;
 			}
 		}
@@ -329,7 +325,7 @@ int pstore_put_backend_records(struct pstore_info *psi)
 
 	inode_unlock(d_inode(root));
 
-	return rc;
+	return 0;
 }
 
 /*