netdev - Re: [PATCH net] net: netem: fix use after free and double free with packet corruption

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Date:   Fri, 14 Jun 2019 09:40:18 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     David Miller <davem@...emloft.net>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        netem@...ts.linux-foundation.org,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        oss-drivers@...ronome.com, Eric Dumazet <edumazet@...gle.com>,
        posk@...gle.com
Subject: Re: [PATCH net] net: netem: fix use after free and double free with
 packet corruption

On Wed, Jun 12, 2019 at 11:52 AM Jakub Kicinski
<jakub.kicinski@...ronome.com> wrote:
>
> Brendan reports that the use of netem's packet corruption capability
> leads to strange crashes.  This seems to be caused by
> commit d66280b12bd7 ("net: netem: use a list in addition to rbtree")
> which uses skb->next pointer to construct a fast-path queue of
> in-order skbs.
>
> Packet corruption code has to invoke skb_gso_segment() in case
> of skbs in need of GSO.  skb_gso_segment() returns a list of
> skbs.  If next pointers of the skbs on that list do not get cleared
> fast path list goes into the weeds and tries to access the next
> segment skb multiple times.

Mind to be more specific? How could it be accessed multiple times?

>
> Reported-by: Brendan Galloway <brendan.galloway@...ronome.com>
> Fixes: d66280b12bd7 ("net: netem: use a list in addition to rbtree")
> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@...ronome.com>
> ---
>  net/sched/sch_netem.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
> index 956ff3da81f4..1fd4405611e5 100644
> --- a/net/sched/sch_netem.c
> +++ b/net/sched/sch_netem.c
> @@ -494,16 +494,13 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>          */
>         if (q->corrupt && q->corrupt >= get_crandom(&q->corrupt_cor)) {
>                 if (skb_is_gso(skb)) {
> -                       segs = netem_segment(skb, sch, to_free);
> -                       if (!segs)
> +                       skb = netem_segment(skb, sch, to_free);
> +                       if (!skb)
>                                 return rc_drop;
> -               } else {
> -                       segs = skb;
> +                       segs = skb->next;
> +                       skb_mark_not_on_list(skb);
>                 }
>
> -               skb = segs;
> -               segs = segs->next;
> -

I don't see how this works when we hit goto finish_segs?
Either goto finish_segs can be removed or needs to be fixed?

Thanks.