netdev - Re: crash in xt_policy due to skb_dst_drop() in nf_ct_frag6

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Date:   Tue, 16 Oct 2018 10:11:20 +0200
From:   Florian Westphal <fw@...len.de>
To:     Maciej Żenczykowski <zenczykowski@...il.com>
Cc:     Lorenzo Colitti <lorenzo@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Westphal <fw@...len.de>,
        Linux NetDev <netdev@...r.kernel.org>,
        Maciej Zenczykowski <maze@...gle.com>
Subject: Re: crash in xt_policy due to skb_dst_drop() in nf_ct_frag6_gather()

Maciej Żenczykowski <zenczykowski@...il.com> wrote:

I am currently travelling and not able to investigate
until next week.

> commit ad8b1ffc3efae2f65080bdb11145c87d299b8f9a
> Author: Florian Westphal <fw@...len.de>
>     netfilter: ipv6: nf_defrag: drop skb dst before queueing
> 
> +++ b/net/ipv6/netfilter/nf_conntrack_reasm.c
> @@ -618,6 +618,8 @@ int nf_ct_frag6_gather(struct net *net, struct
> sk_buff *skb, u32 user)
>             fq->q.meat == fq->q.len &&
>             nf_ct_frag6_reasm(fq, skb, dev))
>                 ret = 0;
> +       else
> +               skb_dst_drop(skb);

This is only supposed to drop dst of skbs that are enqueued,
i.e. frag6_gather returns NF_STOLEN.

In case skb completes the queue, then that skbs dst_entry
is supposed to be kept, so skb_dst() does NOT return NULL.

Its not supposed to be any different than ipv4 defrag.

> const struct dst_entry *dst = skb_dst(skb); // returns NULL

That is not supposed to happen.