netdev - Re: [PATCH net v2] cls_u32: fix use after free in u32_destroy

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Date:   Fri, 2 Feb 2018 13:52:55 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>, Li Shuang <shuali@...hat.com>
Subject: Re: [PATCH net v2] cls_u32: fix use after free in u32_destroy_key()

On Fri, Feb 2, 2018 at 6:30 AM, Paolo Abeni <pabeni@...hat.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@...hat.com>
> Fixes: c0d378ef1266 ("net_sched: use tcf_queue_work() in u32 filter")
> Signed-off-by: Paolo Abeni <pabeni@...hat.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!