netdev - Re: [PATCH net] net: sched: prevent a use after free

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Date:   Mon, 3 Feb 2020 11:58:47 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>, Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Mohit Bhasi <mohitbhasi1998@...il.com>,
        "Mohit P. Tahiliani" <tahiliani@...k.edu.in>,
        "V. Saicharan" <vsaicharan1998@...il.com>,
        Gautam Ramakrishnan <gautamramk@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH net] net: sched: prevent a use after free

On Mon, Feb 3, 2020 at 12:39 AM Dan Carpenter <dan.carpenter@...cle.com> wrote:
>
> On Sat, Feb 01, 2020 at 11:38:43AM -0800, Cong Wang wrote:
> > On Thu, Jan 30, 2020 at 10:57 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
> > >
> > > The code calls kfree_skb(skb); and then re-uses "skb" on the next line.
> > > Let's re-order these lines to solve the problem.
> > >
> > > Fixes: ec97ecf1ebe4 ("net: sched: add Flow Queue PIE packet scheduler")
> > > Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> > > ---
> > >  net/sched/sch_fq_pie.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/net/sched/sch_fq_pie.c b/net/sched/sch_fq_pie.c
> > > index bbd0dea6b6b9..78472e0773e9 100644
> > > --- a/net/sched/sch_fq_pie.c
> > > +++ b/net/sched/sch_fq_pie.c
> > > @@ -349,9 +349,9 @@ static int fq_pie_change(struct Qdisc *sch, struct nlattr *opt,
> > >         while (sch->q.qlen > sch->limit) {
> > >                 struct sk_buff *skb = fq_pie_qdisc_dequeue(sch);
> > >
> > > -               kfree_skb(skb);
> > >                 len_dropped += qdisc_pkt_len(skb);
> > >                 num_dropped += 1;
> > > +               kfree_skb(skb);
> >
> > Or even better: use rtnl_kfree_skbs().
>
> Why is that better?

Because it is designed to be used in this scenario,
as it defers the free after RTNL unlock which is after
sch_tree_unlock() too.

Thanks.