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

[Jan Kara]

On Fri 22-06-18 14:56:09, Paul Moore wrote:
> 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 ;)

Yeah, I was looking into it on Friday and today :). Currently I've got a
bit stuck because I think I've found some races in audit_tree code and I
haven't yet decided how to fix them. E.g. am I right the following can
happen?

CPU1                                    CPU2
tag_chunk(inode, tree1)                 tag_chunk(inode, tree2)
  old_entry = fsnotify_find_mark();       old_entry = fsnotify_find_mark();
  old = container_of(old_entry);          old = container_of(old_entry);
  chunk = alloc_chunk(old->count + 1);    chunk = alloc_chunk(old->count + 1);
  mutex_lock(&group->mark_mutex);
  adds new mark
  replaces chunk
  old->dead = 1;
  mutex_unlock(&group->mark_mutex);
                                          mutex_lock(&group->mark_mutex);
                                          if (!(old_entry->flags &
                                                FSNOTIFY_MARK_FLAG_ATTACHED)) {
                                            Check fails as old_entry is
                                            not yet destroyed
                                          adds new mark
                                          replaces old chunk again ->
                                            list corruption, lost refs, ...
                                          mutex_unlock(&group->mark_mutex);

Generally there's a bigger problem that audit_tree code can have multiple
marks attached to one inode but only one of them is the "valid" one (i.e.,
the one embedded in the latest chunk). This is only a temporary state until
fsnotify_destroy_mark() detaches the mark and then on last reference drop
we really remove the mark from inode's list but during that window it is
undefined which mark is returned from fsnotify_find_mark()...

So am I right the above can really happen or is there some higher level
synchronization I'm missing? If this can really happen, I think I'll need
to rework the code so that audit_tree has just one mark attached and
let it probably point to the current chunk.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR