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

[Cong Wang <xiyou.wangcong@gmail.com>]

On Wed, Jun 12, 2019 at 11:52 AM Jakub Kicinski
<jakub.kicinski@netronome.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@netronome.com>
> Fixes: d66280b12bd7 ("net: netem: use a list in addition to rbtree")
> Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
> Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.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.