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

[Jan Kara <jack@suse.cz>]

On Mon 25-06-18 11:22:57, Jan Kara wrote:
> On Fri 22-06-18 14:56:09, Paul Moore wrote:
> > On Fri, Jun 22, 2018 at 5:23 AM Jan Kara <jack@suse.cz> 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.

Also am I right to assume that if two tag_chunk() calls race, both try to
add new fsnotify mark in create_chunk() and one of them fails, then the
resulting ENOSPC error from create_chunk() is actually a bug? Because from
looking at the code it seems that the desired functionality is that
tag_chunk() should add 'tree' to the chunk, expanding chunk as needed.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR