Re: [PATCH net v2] cls_u32: fix use after free in u32_destroy_key()

[Paolo Abeni <pabeni@redhat.com>]

Hi,
On Fri, 2018-02-02 at 13:52 -0800, Cong Wang wrote:
> On Fri, Feb 2, 2018 at 6:30 AM, Paolo Abeni <pabeni@redhat.com> wrote:
> > The problem is that the htnode is freed before the linked knodes and the
> > latter will try to access the first at u32_destroy_key() time.
> > This change addresses the issue using the htnode refcnt to guarantee
> > the correct free order. While at it also add a RCU annotation,
> > to keep sparse happy.
> > 
> > v1 -> v2: use rtnl_derefence() instead of RCU read locks
> > 
> > Reported-by: Li Shuang <shuali@redhat.com>
> > Fixes: c0d378ef1266 ("net_sched: use tcf_queue_work() in u32 filter")
> > Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> > ---
> >  net/sched/cls_u32.c | 19 +++++++++++++++----
> >  1 file changed, 15 insertions(+), 4 deletions(-)
> > 
> > diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
> > index 60c892c36a60..10440fbf3c68 100644
> > --- a/net/sched/cls_u32.c
> > +++ b/net/sched/cls_u32.c
> > @@ -398,10 +398,12 @@ static int u32_init(struct tcf_proto *tp)
> >  static int u32_destroy_key(struct tcf_proto *tp, struct tc_u_knode *n,
> >                            bool free_pf)
> >  {
> > +       struct tc_u_hnode *ht = rtnl_dereference(n->ht_down);
> > +
> >         tcf_exts_destroy(&n->exts);
> >         tcf_exts_put_net(&n->exts);
> > -       if (n->ht_down)
> > -               n->ht_down->refcnt--;
> > +       if (ht && ht->refcnt-- == 0)
> > +               kfree(ht);
> >  #ifdef CONFIG_CLS_U32_PERF
> >         if (free_pf)
> >                 free_percpu(n->pf);
> > @@ -624,7 +626,12 @@ static int u32_destroy_hnode(struct tcf_proto *tp, struct tc_u_hnode *ht,
> >                         idr_destroy(&ht->handle_idr);
> >                         idr_remove_ext(&tp_c->handle_idr, ht->handle);
> >                         RCU_INIT_POINTER(*hn, ht->next);
> > -                       kfree_rcu(ht, rcu);
> > +
> > +                       /* u32_destroy_key() will will later free ht for us, if
> > +                        * it's still referenced by some knode
> > +                        */
> > +                       if (ht->refcnt == 0)
> > +                               kfree_rcu(ht, rcu);
> 
> 
> Isn't u32_destroy_hnode() always called with ht->refcnt==0 ?
> So no need this check at all?
> 
> 
> >                         return 0;
> >                 }
> >         }
> > @@ -667,7 +674,11 @@ static void u32_destroy(struct tcf_proto *tp, struct netlink_ext_ack *extack)
> > 
> >                 while ((ht = rtnl_dereference(tp_c->hlist)) != NULL) {
> >                         RCU_INIT_POINTER(tp_c->hlist, ht->next);
> > -                       kfree_rcu(ht, rcu);
> > +                       /* u32_destroy_key() will will later free ht for us, if
> 
> 
> Nit: double "will"
> 
> 
> > +                        * it's still referenced by some knode
> > +                        */
> > +                       if (ht->refcnt == 0)
> > +                               kfree_rcu(ht, rcu);
> 
> 
> This part looks fine.
> 
> Thanks!

Thank you for the feedback!

I will send a v3 soon, after some testing.

Cheers,

Paolo