netdev - Re: [PATCH net] net: sched: fix possible crash in tcf_action

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Message-ID: <20190921190800.3f19fe23@cakuba.netronome.com>
Date:   Sat, 21 Sep 2019 19:08:00 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        syzbot <syzkaller@...glegroups.com>,
        Vlad Buslov <vladbu@...lanox.com>,
        Jiri Pirko <jiri@...lanox.com>
Subject: Re: [PATCH net] net: sched: fix possible crash in
 tcf_action_destroy()

On Wed, 18 Sep 2019 14:37:21 -0700, Cong Wang wrote:
> On Wed, Sep 18, 2019 at 12:57 PM 'Eric Dumazet' via syzkaller
> <syzkaller@...glegroups.com> wrote:
> >
> > If the allocation done in tcf_exts_init() failed,
> > we end up with a NULL pointer in exts->actions.  
> ...
> > diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
> > index efd3cfb80a2ad775dc8ab3c4900bd73d52c7aaad..9aef93300f1c11791acbb9262dfe77996872eafe 100644
> > --- a/net/sched/cls_api.c
> > +++ b/net/sched/cls_api.c
> > @@ -3027,8 +3027,10 @@ static int tc_dump_chain(struct sk_buff *skb, struct netlink_callback *cb)
> >  void tcf_exts_destroy(struct tcf_exts *exts)
> >  {
> >  #ifdef CONFIG_NET_CLS_ACT
> > -       tcf_action_destroy(exts->actions, TCA_ACT_UNBIND);
> > -       kfree(exts->actions);
> > +       if (exts->actions) {  
> 
> I think it is _slightly_ better to check exts->nr_actions!=0 here,
> as it would help exts->actions!=NULL&& exts->nr_actions==0
> cases too.
> 
> What do you think?

Alternatively, since tcf_exts_destroy() now takes NULL, and so
obviously does kfree() - perhaps tcf_action_destroy() should 
return early if actions are NULL?