netdev - Re: [PATCH net] ip: frags: fix crash in ip_do

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Date:   Thu, 6 Sep 2018 11:23:14 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     ap420073@...il.com
Cc:     David Miller <davem@...emloft.net>,
        Peter Oskolkov <posk@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>
Subject: Re: [PATCH net] ip: frags: fix crash in ip_do_fragment()

On Thu, Sep 6, 2018 at 11:06 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Thu, Sep 6, 2018 at 10:51 AM Taehee Yoo <ap420073@...il.com> wrote:
> >
> > A kernel crash occurrs when defragmented packet is fragmented
> > in ip_do_fragment().
> > In defragment routine, skb_orphan() is called and
> > skb->ip_defrag_offset is set. but skb->sk and
> > skb->ip_defrag_offset are same union member. so that
> > frag->sk is not NULL.
> > Hence crash occurrs in skb->sk check routine in ip_do_fragment() when
> > defragmented packet is fragmented.
>
> Have you tested this patch ?
>
> Moving back ip_defrag_offset is conflicting with the rbnode !
>
> A more correct fix would be to properly clear skb->sk at reassembly.

Something like that :

diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index 88281fbce88ce8f1062b99594665766c2a5f5b74..e7227128df2c8fd54727c234f76043133809bd1e
100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -599,6 +599,7 @@ static int ip_frag_reasm(struct ipq *qp, struct
sk_buff *skb,
                        nextp = &fp->next;
                        fp->prev = NULL;
                        memset(&fp->rbnode, 0, sizeof(fp->rbnode));
+                       fp->sk = NULL;
                        head->data_len += fp->len;
                        head->len += fp->len;
                        if (head->ip_summed != fp->ip_summed)