Subject: Re: [PATCH net 2/2] net/sched: flower: fix error handler on replace

From: Simon Horman <[email-masked]>
Date: Fri, 28 Apr 2023 09:11:56 +0200

On Wed, Apr 26, 2023 at 05:39:09PM +0200, Ivan Vecera wrote:
> 
> 
> On 26. 04. 23 16:46, Vlad Buslov wrote:
> > On Wed 26 Apr 2023 at 11:22, Pedro Tammela <[email-masked]> wrote:
> > > On 26/04/2023 09:14, Vlad Buslov wrote:
> > > > When replacing a filter (i.e. 'fold' pointer is not NULL) the insertion of
> > > > new filter to idr is postponed until later in code since handle is already
> > > > provided by the user. However, the error handling code in fl_change()
> > > > always assumes that the new filter had been inserted into idr. If error
> > > > handler is reached when replacing existing filter it may remove it from idr
> > > > therefore making it unreachable for delete or dump afterwards. Fix the
> > > > issue by verifying that 'fold' argument wasn't provided by caller before
> > > > calling idr_remove().
> > > > Fixes: 08a0063df3ae ("net/sched: flower: Move filter handle initialization
> > > > earlier")
> > > > Signed-off-by: Vlad Buslov <[email-masked]>
> > > > ---
> > > >    net/sched/cls_flower.c | 3 ++-
> > > >    1 file changed, 2 insertions(+), 1 deletion(-)
> > > > diff --git a/net/sched/cls_flower.c b/net/sched/cls_flower.c
> > > > index 1844545bef37..a1c4ee2e0be2 100644
> > > > --- a/net/sched/cls_flower.c
> > > > +++ b/net/sched/cls_flower.c
> > > > @@ -2339,7 +2339,8 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
> > > >    errout_mask:
> > > >    	fl_mask_put(head, fnew->mask);
> > > >    errout_idr:
> > > > -	idr_remove(&head->handle_idr, fnew->handle);
> > > > +	if (!fold)
> > > > +		idr_remove(&head->handle_idr, fnew->handle);
> > > >    	__fl_put(fnew);
> > > >    errout_tb:
> > > >    	kfree(tb);
> > > 
> > > Actually this seems to be fixing the same issue:
> > > https://lore.kernel.org/all/[email-masked]/
> > 
> > Indeed it does, I've missed that patch. However, it seems there
> > is an issue with Ivan's approach. Consider what would happen when
> > fold!=NULL && in_ht==false and rhashtable_insert_fast() fails here:
> > 
> > 
> >          if (fold) {
> >                  /* Fold filter was deleted concurrently. Retry lookup. */
> >                  if (fold->deleted) {
> >                          err = -EAGAIN;
> >                          goto errout_hw;
> >                  }
> > 
> >                  fnew->handle = handle; // <-- fnew->handle is assigned
> > 
> >                  if (!in_ht) {
> >                          struct rhashtable_params params =
> >                                  fnew->mask->filter_ht_params;
> > 
> >                          err = rhashtable_insert_fast(&fnew->mask->ht,
> >                                                       &fnew->ht_node,
> >                                                       params);
> >                          if (err)
> >                                  goto errout_hw; /* <-- err is set, go to
> >                                                       error handler here */
> >                          in_ht = true;
> >                  }
> > 
> >                  refcount_inc(&fnew->refcnt);
> >                  rhashtable_remove_fast(&fold->mask->ht,
> >                                         &fold->ht_node,
> >                                         fold->mask->filter_ht_params);
> >                  /* !!! we never get to insert fnew into idr here, if ht insertion fails */
> >                  idr_replace(&head->handle_idr, fnew, fnew->handle);
> >                  list_replace_rcu(&fold->list, &fnew->list);
> >                  fold->deleted = true;
> > 
> >                  spin_unlock(&tp->lock);
> > 
> >                  fl_mask_put(head, fold->mask);
> >                  if (!tc_skip_hw(fold->flags))
> >                          fl_hw_destroy_filter(tp, fold, rtnl_held, NULL);
> >                  tcf_unbind_filter(tp, &fold->res);
> >                  /* Caller holds reference to fold, so refcnt is always > 0
> >                   * after this.
> >                   */
> >                  refcount_dec(&fold->refcnt);
> >                  __fl_put(fold);
> >          }
> > 
> > ...
> > 
> >   errout_ht:
> >           spin_lock(&tp->lock);
> >   errout_hw:
> >           fnew->deleted = true;
> >           spin_unlock(&tp->lock);
> >           if (!tc_skip_hw(fnew->flags))
> >                   fl_hw_destroy_filter(tp, fnew, rtnl_held, NULL);
> >           if (in_ht)
> >                   rhashtable_remove_fast(&fnew->mask->ht, &fnew->ht_node,
> >                                          fnew->mask->filter_ht_params);
> >   errout_mask:
> >           fl_mask_put(head, fnew->mask);
> >   errout_idr:
> >           /* !!! On next line we remove handle that we don't actually own */
> >           idr_remove(&head->handle_idr, fnew->handle);
> >           __fl_put(fnew);
> >   errout_tb:
> >           kfree(tb);
> >   errout_mask_alloc:
> >           tcf_queue_work(&mask->rwork, fl_uninit_mask_free_work);
> >   errout_fold:
> >           if (fold)
> >                   __fl_put(fold);
> >           return err;
> > 
> > 
> > Also, if I understood the idea behind Ivan's fix correctly, it relies on
> > the fact that calling idr_remove() with handle==0 is a noop. I prefer my
> > approach slightly better as it is more explicit IMO.
> > 
> > Thoughts?
> 
> Yes, your approach is better...
> 
> Acked-by: Ivan Vecera <[email-masked]>

In the meantime it seems that Ivan's patch has been accepted into net.

- [net] net/sched: flower: Fix wrong handle assignment during filter change
  https://git.kernel.org/netdev/net/c/32eff6bacec2

Is some adjustment to this patch required to take that into account?