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Date:   Mon, 10 Feb 2020 22:32:59 +0100
From:   Florian Westphal <fw@strlen.de>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Netdev <netdev@vger.kernel.org>,
        David Miller <davem@davemloft.net>,
        Florian Westphal <fw@strlen.de>
Subject: Re: [PATCH v2 net 1/5] icmp: introduce helper for NAT'd source
 address in network device context
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References: <20200210141423.173790-1-Jason@zx2c4.com>
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Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> On Mon, Feb 10, 2020 at 3:15 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> > +               ip_hdr(skb_in)->saddr = ct->tuplehash[0].tuple.src.u3.ip;
> > +       }
> > +       icmp_send(skb_in, type, code, info);
> 
> According to the comments in icmp_send, access to
> ip_hdr(skb_in)->saddr requires first checking for `if
> (skb_network_header(skb_in) < skb_in->head ||
> (skb_network_header(skb_in) + sizeof(struct iphdr)) >
> skb_tail_pointer(skb_in))` first to be safe.

You will probably also need skb_ensure_writable() to handle cloned skbs.

I also suggest to check "ct->status & IPS_NAT_MASK", nat is only done if
those bits are set.