netdev - Re: [Patch net 1/2] net_sched: defer tcf_idr_insert() in tcf_action_init

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Date:   Mon, 28 Sep 2020 10:38:21 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Davide Caratti <dcaratti@...hat.com>
Cc:     Vlad Buslov <vlad@...lov.dev>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Vlad Buslov <vladbu@...lanox.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: [Patch net 1/2] net_sched: defer tcf_idr_insert() in tcf_action_init_1()

On Mon, Sep 28, 2020 at 3:14 AM Davide Caratti <dcaratti@...hat.com> wrote:
>
> hello,
>
> On Fri, 2020-09-25 at 22:45 +0300, Vlad Buslov wrote:
> > On Fri 25 Sep 2020 at 22:22, Cong Wang <xiyou.wangcong@...il.com> wrote:
> > > On Fri, Sep 25, 2020 at 8:24 AM Vlad Buslov <vlad@...lov.dev> wrote:
> > > > > +     if (TC_ACT_EXT_CMP(a->tcfa_action, TC_ACT_GOTO_CHAIN) &&
> > > > > +         !rcu_access_pointer(a->goto_chain)) {
> > > > > +             tcf_action_destroy_1(a, bind);
> > > > > +             NL_SET_ERR_MSG(extack, "can't use goto chain with NULL chain");
> > > > > +             return ERR_PTR(-EINVAL);
> > > > > +     }
> > > >
> > > > I don't think calling tcf_action_destoy_1() is enough here. Since you
> > > > moved this block before assigning cookie and releasing the module, you
> > > > also need to release them manually in addition to destroying the action
> > > > instance.
> > > >
> > >
> > > tcf_action_destoy_1() eventually calls free_tcf() which frees cookie and
> > > tcf_action_destroy() which releases module refcnt.
> > >
> > > What am I missing here?
> > >
> > > Thanks.
> >
> > The memory referenced by the function local pointer "cookie" hasn't been
> > assigned yet to the a->act_cookie because in your patch you moved
> > goto_chain validation code before the cookie change. That means that if
> > user overwrites existing action, then action old a->act_cookie will be
> > freed by tcf_action_destroy_1() but new cookie that was allocated by
> > nla_memdup_cookie() will leak.

Yes, good catch!


>
> maybe we can just delete this if (TC_ACT_EXT_CMP(...)) { ... }
> statement, instead of moving it? Each TC action already does the check
> for NULL "goto chains" with a_o->init() -> tcf_action_check_ctrlact(),
> so this if () statement looks dead code to me _ I probably forgot to
> remove it after all actions were converted to validate the control
> action inside their .init() function.

Good point, I think you are right, I will send a patch to remove it.

Thanks!