netdev - Re: [PATCH nf] netfilter: ipv6: fix oops when defragmenting locally generated fragments

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Message-ID: <CANP3RGfCDumc40Tr2gr0KHenQ6x29tWAjhnM+8yk7nK5nNHRFA@mail.gmail.com>
Date:   Tue, 23 Oct 2018 14:04:18 -0700
From:   Maciej Żenczykowski <maze@...gle.com>
To:     Florian Westphal <fw@...len.de>
Cc:     netfilter-devel@...r.kernel.org,
        Lorenzo Colitti <lorenzo@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Linux NetDev <netdev@...r.kernel.org>
Subject: Re: [PATCH nf] netfilter: ipv6: fix oops when defragmenting locally
 generated fragments

On Tue, Oct 23, 2018 at 7:47 AM, Florian Westphal <fw@...len.de> wrote:
> Unlike ipv4 and normal ipv6 defrag, netfilter ipv6 defragmentation did
> not save/restore skb->dst.
>
> This causes oops when handling locally generated ipv6 fragments, as
> output path needs a valid dst.
>
> Reported-by: Maciej Żenczykowski <zenczykowski@...il.com>
> Fixes: 84379c9afe01 ("netfilter: ipv6: nf_defrag: drop skb dst before queueing")
> Signed-off-by: Florian Westphal <fw@...len.de>
> ---
>  net/ipv6/netfilter/nf_conntrack_reasm.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/net/ipv6/netfilter/nf_conntrack_reasm.c b/net/ipv6/netfilter/nf_conntrack_reasm.c
> index 8f68a518d9db..f76bd4d15704 100644
> --- a/net/ipv6/netfilter/nf_conntrack_reasm.c
> +++ b/net/ipv6/netfilter/nf_conntrack_reasm.c
> @@ -587,11 +587,16 @@ int nf_ct_frag6_gather(struct net *net, struct sk_buff *skb, u32 user)
>          */
>         ret = -EINPROGRESS;
>         if (fq->q.flags == (INET_FRAG_FIRST_IN | INET_FRAG_LAST_IN) &&
> -           fq->q.meat == fq->q.len &&
> -           nf_ct_frag6_reasm(fq, skb, dev))
> -               ret = 0;
> -       else
> +           fq->q.meat == fq->q.len) {
> +               unsigned long orefdst = skb->_skb_refdst;
> +
> +               skb->_skb_refdst = 0UL;
> +               if (nf_ct_frag6_reasm(fq, skb, dev))
> +                       ret = 0;
> +               skb->_skb_refdst = orefdst;
> +       } else {
>                 skb_dst_drop(skb);
> +       }
>
>  out_unlock:
>         spin_unlock_bh(&fq->q.lock);
> --
> 2.18.1
>

I don't quite follow how this fixes things, but I'll trust you on it.
(nor do I understand why only 4.9 LTS appears to crash with a null ptr deref)

Thanks for the fix.