Re: [PATCH] kernel: audit_tree: Fix a sleep-in-atomic-context bug

On Wed 20-06-18 21:29:12, Matthew Wilcox wrote: > On Thu, Jun 21, 2018 at 11:32:45AM +0800, Jia-Ju Bai wrote: > > The kernel may sleep with holding a spinlock. > > The function call paths (from bottom to top) in Linux-4.16.7 are: > > > > [FUNC] kmem_cache_alloc(GFP_KERNEL) > > fs/notify/mark.c, 439: > > kmem_cache_alloc in fsnotify_attach_connector_to_object > > fs/notify/mark.c, 520: > > fsnotify_attach_connector_to_object in fsnotify_add_mark_list > > fs/notify/mark.c, 590: > > fsnotify_add_mark_list in fsnotify_add_mark_locked > > kernel/audit_tree.c, 437: > > fsnotify_add_mark_locked in tag_chunk > > kernel/audit_tree.c, 423: > > spin_lock in tag_chunk > > There are several locks here; your report would be improved by saying > which one is the problem. I'm assuming it's old_entry->lock. > > spin_lock(&old_entry->lock); > ... > if (fsnotify_add_inode_mark_locked(chunk_entry, > old_entry->connector->inode, 1)) { > ... > return fsnotify_add_mark_locked(mark, inode, NULL, allow_dups); > ... > ret = fsnotify_add_mark_list(mark, inode, mnt, allow_dups); > ... > if (inode) > connp = &inode->i_fsnotify_marks; > conn = fsnotify_grab_connector(connp); > if (!conn) { > err = fsnotify_attach_connector_to_object(connp, inode, mnt); > > It seems to me that this is safe because old_entry is looked up from > fsnotify_find_mark, and it can't be removed while its lock is held. > Therefore there's always a 'conn' returned from fsnotify_grab_connector(), > and so this path will never be taken. > > But this code path is confusing to me, and I could be wrong. Jan, please > confirm my analysis is correct? Yes, you are correct. The presence of another mark in the list (and the fact we pin it there using refcount & mark_mutex) guarantees we won't need to allocate the connector. I agree the audit code's use of fsnotify would deserve some cleanup.