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

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Message-ID: <8736358wu0.fsf@buslov.dev>
Date:   Fri, 25 Sep 2020 22:45:11 +0300
From:   Vlad Buslov <vlad@...lov.dev>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     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 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.

Regards,
Vlad