netdev - Re: [PATCH] netlink: fix memory leak of dump

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Date:   Mon, 23 Jul 2018 18:34:36 +0800
From:   shaochun chen <cscnull@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        David Miller <davem@...emloft.net>,
        kadlec <kadlec@...ckhole.kfki.hu>,
        "johannes.berg" <johannes.berg@...el.com>, jason <Jason@...c4.com>,
        ktkhai <ktkhai@...tuozzo.com>,
        "lucien.xin" <lucien.xin@...il.com>,
        "xiyou.wangcong" <xiyou.wangcong@...il.com>,
        dsahern <dsahern@...il.com>,
        netfilter-devel <netfilter-devel@...r.kernel.org>,
        tom <tom@...ntonium.net>, netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] netlink: fix memory leak of dump

I have a question: we will try_module_get in __netlink_dump_start(),
but why we need to call try_module_get again in  nft_netlink_dump_start ??

2018-07-23 17:52 GMT+08:00 shaochun chen <cscnull@...il.com>:

> allocate memory in ->start(),  it's not convenient for users.
> if call ->done() isn't ok for clean memory when netlink_dump_start() fail,
> maybe we should have another function ->clean() to clean memory.
>
> 2018-07-23 17:28 GMT+08:00 Florian Westphal <fw@...len.de>:
>
>> Pablo Neira Ayuso <pablo@...filter.org> wrote:
>> > > diff --git a/net/netfilter/nf_tables_api.c
>> b/net/netfilter/nf_tables_api.c
>> > > --- a/net/netfilter/nf_tables_api.c
>> > > +++ b/net/netfilter/nf_tables_api.c
>> > > @@ -5010,6 +5013,22 @@ nft_obj_filter_alloc(const struct nlattr *
>> const nla[])
>> > >     return filter;
>> > >  }
>> > >
>> > > +static int nf_tables_dump_obj_start(struct netlink_callback *cb)
>> > > +{
>> > > +   const struct nlattr * const *nla = cb->data;
>>
>> On-Stack input.
>> I can't see how its wrong, ->start() happens from same context as
>> netlink_dump_start so its valid.
>>
>> > > +   struct nft_obj_filter *filter = NULL;
>> > > +
>> > > +   if (nla[NFTA_OBJ_TABLE] ||
>> > > +       nla[NFTA_OBJ_TYPE]) {
>> > > +           filter = nft_obj_filter_alloc(nla);
>> > > +           if (IS_ERR(filter))
>> > > +                   return -ENOMEM;
>> > > +   }
>> > > +
>> > > +   cb->data = filter;
>>
>> And this replaced the on-stack input with dynamically
>> allocated one, which will be free'd via ->done().
>>
>> > >  /* called with rcu_read_lock held */
>> > >  static int nf_tables_getobj(struct net *net, struct sock *nlsk,
>> > >                         struct sk_buff *skb, const struct nlmsghdr
>> *nlh,
>> > > @@ -5028,21 +5047,13 @@ static int nf_tables_getobj(struct net *net,
>> struct sock *nlsk,
>> > >
>> > >     if (nlh->nlmsg_flags & NLM_F_DUMP) {
>> > >             struct netlink_dump_control c = {
>> > > +                   .start = nf_tables_dump_obj_start,
>> > >                     .dump = nf_tables_dump_obj,
>> > >                     .done = nf_tables_dump_obj_done,
>> > >                     .module = THIS_MODULE,
>> > > +                   .data = (void *)nla,
>> >
>> > You cannot do this.
>> >
>> > nla is allocated in this stack.
>>
>> Yes.
>>
>> > the nla will not be available in the second recv(), it won't be there.
>>
>> Its replaced in ->start().
>>
>> As David pointed out, once ->start() returns 0 we set cb_running, i.e.
>> only after successful ->start() netlink core will call ->dump() again.
>>
>> So I see no problem setting ->data to onstack cookie and then
>> duplicating it to heap via kmemdup in ->start().
>>
>> As far as I can see netlink core offers all functionality already,
>> so we only need to switch netfilter to make use of it.
>>
>> If you disagree please let me know, otherwise I will cook up
>> a patch along this pattern for net/netfilter/*.
>>
>> Thanks.
>>
>
>

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