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Date:   Mon, 17 Jul 2017 14:30:06 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [PATCH net] udp: preserve skb->dst if required for IP options
 processing

On Mon, 2017-07-17 at 22:59 +0200, Paolo Abeni wrote:
> Eric noticed that in udp_recvmsg() we still need to access
> skb->dst while processing the IP options.
> Since commit 0a463c78d25b ("udp: avoid a cache miss on dequeue")
> skb->dst is no more available at recvmsg() time and bad things
> will happen if we enter the relevant code path.
> 
> This commit address the issue, avoid clearing skb->dst if
> any IP options are present into the relevant skb.
> Since the IP CB is contained in the first skb cacheline, we can
> test it to decide to leverage the consume_stateless_skb()
> optimization, without measurable additional cost in the faster
> path.

Also the problem was not skb->dst at all, but fact that 
skb->head can not be freed if IP options need later to be fetched.

( __ip_options_echo() -> 
   sptr = skb_network_header(skb);

... -> use after free



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