audit: fix refcounting in audit-tree

Refcounting of fsnotify_mark in audit tree is broken.  E.g:

                              refcount
create_chunk
  alloc_chunk                 1
  fsnotify_add_mark           2

untag_chunk
  fsnotify_get_mark           3
  fsnotify_destroy_mark
    audit_tree_freeing_mark   2
  fsnotify_put_mark           1
  fsnotify_put_mark           0
  via destroy_list
    fsnotify_mark_destroy    -1

This was reported by various people as triggering Oops when stopping auditd.

We could just remove the put_mark from audit_tree_freeing_mark() but that would
break freeing via inode destruction.  So this patch simply omits a put_mark
after calling destroy_mark or adds a get_mark before.

The additional get_mark is necessary where there's no other put_mark after
fsnotify_destroy_mark() since it assumes that the caller is holding a reference
(or the inode is keeping the mark pinned, not the case here AFAICS).

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Reported-by: Valentin Avram <aval13@gmail.com>
Reported-by: Peter Moody <pmoody@google.com>
Acked-by: Eric Paris <eparis@redhat.com>
CC: stable@vger.kernel.org

kernel/audit_tree.c

@@ -250,7 +250,6 @@ static void untag_chunk(struct node *p)
 		spin_unlock(&hash_lock);
 		spin_unlock(&entry->lock);
 		fsnotify_destroy_mark(entry);
-		fsnotify_put_mark(entry);
 		goto out;
 	}
 
@@ -293,7 +292,6 @@ static void untag_chunk(struct node *p)
 	spin_unlock(&hash_lock);
 	spin_unlock(&entry->lock);
 	fsnotify_destroy_mark(entry);
-	fsnotify_put_mark(entry);
 	goto out;
 
 Fallback:
@@ -332,6 +330,7 @@ static int create_chunk(struct inode *inode, struct audit_tree *tree)
 		spin_unlock(&hash_lock);
 		chunk->dead = 1;
 		spin_unlock(&entry->lock);
+		fsnotify_get_mark(entry);
 		fsnotify_destroy_mark(entry);
 		fsnotify_put_mark(entry);
 		return 0;
@@ -412,6 +411,7 @@ static int tag_chunk(struct inode *inode, struct audit_tree *tree)
 		spin_unlock(&chunk_entry->lock);
 		spin_unlock(&old_entry->lock);
 
+		fsnotify_get_mark(chunk_entry);
 		fsnotify_destroy_mark(chunk_entry);
 
 		fsnotify_put_mark(chunk_entry);
@@ -445,7 +445,6 @@ static int tag_chunk(struct inode *inode, struct audit_tree *tree)
 	spin_unlock(&old_entry->lock);
 	fsnotify_destroy_mark(old_entry);
 	fsnotify_put_mark(old_entry); /* pair to fsnotify_find mark_entry */
-	fsnotify_put_mark(old_entry); /* and kill it */
 	return 0;
 }