netdev - Re: [PATCH net] net: Prevent invalid access to skb->prev in __qdisc_drop

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Date:   Thu, 29 Nov 2018 15:53:37 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     cpaasch@...le.com, netdev@...r.kernel.org,
        bhole_prashant_q7@....ntt.co.jp, tyhicks@...onical.com
Subject: Re: [PATCH net] net: Prevent invalid access to skb->prev in
 __qdisc_drop_all

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 29 Nov 2018 15:09:18 -0800

> diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
> index 2c38e3d0792468162ee0dc4137f1400160ab9276..22cd46a600576f286803536d45875cd9d537cdca 100644
> --- a/net/sched/sch_netem.c
> +++ b/net/sched/sch_netem.c
> @@ -431,6 +431,9 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>         int count = 1;
>         int rc = NET_XMIT_SUCCESS;
>  
> +       /* Do not fool qdisc_drop_all() */
> +       skb->prev = NULL;
> +
>         /* Random duplication */
>         if (q->duplicate && q->duplicate >= get_crandom(&q->dup_cor))
>                 ++count;

If this works I definitely prefer it to making the entire stack pay the
price to fix this crash.