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

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Date:   Mon, 23 Jul 2018 10:05:18 +0800
From:   shaochun chen <cscnull@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     David Miller <davem@...emloft.net>, pablo <pablo@...filter.org>,
        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

allocate memory in cb->start(), which means passing 'static' variable
through control->data,
then allocate memory in cb->start() according to cb->data (cb->data is
equal to control->data now),
and set the memory back to cb->data which will be used in cb->dump().
It's a bit complicated， please see nf_tables_getset.

2018-07-23 2:09 GMT+08:00 Florian Westphal <fw@...len.de>:

> David Miller <davem@...emloft.net> wrote:
> > From: Florian Westphal <fw@...len.de>
> > Date: Sun, 22 Jul 2018 18:39:25 +0200
> >
> > > 3. change meaning of ->done() so its always called once ->start()
> > >    was invoked (and returned 0), this requires audit of all
> > >    places that provide .done to make sure they won't trip.
> > >
> > > 3) seems to be what Tom intended when he added .start, so probably
> > > best to investigate that first.
> >
> > Hmmm...
> >
> > Any time ->start() succeeds, we set cb_running to true.
>
> Right.
>
> > From that point forward, ->done() will be called at some point at all
> > of the locations that check if cb_running is true and set it to false.
>
> Also right, thanks for pointing this out, I missed fact that netlink
> core restarts a dump after this.
>
> So 3) is already true which means we should try to see if we can move
> all dump-related extra magic into ->start().
>
> Shaochun, can you see if this is possible?
>
> Something along these lines (totally untested), which makes this
> a netfilter fix:
>
> 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;
> +       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;
> +       return 0;
> +}
> +
>  /* 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,
>                 };
>
> -               if (nla[NFTA_OBJ_TABLE] ||
> -                   nla[NFTA_OBJ_TYPE]) {
> -                       struct nft_obj_filter *filter;
> -
> -                       filter = nft_obj_filter_alloc(nla);
> -                       if (IS_ERR(filter))
> -                               return -ENOMEM;
> -
> -                       c.data = filter;
> -               }
>                 return nft_netlink_dump_start_rcu(nlsk, skb, nlh, &c);
>         }
>
>

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