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

[Paul Moore]

On Fri, Jun 22, 2018 at 5:23 AM Jan Kara <jack@xxxxxxx> wrote:
> 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.

I'm always open to suggestions and patches (hint, hint) from the
fsnotify experts ;)

-- 
paul moore
www.paul-moore.com